Ciria 185

Report 185

London, 1999

The Observational Method in ground engineering:

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principles and applications

Duncan Nicholson BSC MSC DIC CEng MICE Che-Ming Tse BSCACGI MSC DIC CEng MICE MBA Eur Ing Charles Penny BSC CEng FICE MlGasE MiMgt with contributions from

Eur tng Simon O’Hana BSC CEng MICE Ross Dimmock BSC(Hons.)

sharing knowledge

building best practice

6 Storey’s Gate, Westminster, London SW1P 3AU TELEPHONE 0171 222 8891 FAX 0171 222 1708 EMAIL [email protected] WEBSITE www.Ciria.0rg.uk

Who we are For almost 40 years ClRlA has managed collaborative research and produced information aimed at providing best practice solutions to industry problems. ClRlA stimulates the exchange of experience across the industry and its clients, and has a reputation for publishing practical, high-quality information.

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ClRlA offers several participation options that have been designed to meet different needs. These include: 0

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Core Programme membership - for organisations that wish to influence CIRIA’s collaboratively funded research programme and obtain early access to the results. Project funding -for organisations that wish to direct funds to specific projects of interest. Project funders influence the direction of the research and obtain early access to the results. New Books Club - popular with organisations that wish to acquire ClRlA publications at special member prices. Construction Productivity Network - for organisations interested in improving their performance and efficiency through sharing and application of knowledge with others.

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Construction Industry EnvironmentalForum - provides a focus for the exchange of experience on environmental problems and opportunities.

Where we are To discover how your organisation can benefit from CIRIA’s authoritative and practical guidance contact CIR IA by: Post Tel FaX Email

6 Storey’s Gate, Westminster, London SWl P 3AU 0171 222 8891 0171 222 1708 [email protected]

Details are available on CIRIA’s website: www.ciria.0rg.uk

Cover photograph: the Observational Method was used during the application of the New Austrian Tunnelling Method in the Castle Hill segment of the Channel Tunnel project (courtesy Eurotunnel).

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Summary

The Observational Method in ground engineering is a continuous, managed, integrated, process of design, construction control, monitoring and review which enables previously defined modifications to be incorporated during or after construction as appropriate. All these aspects have to be demonstrably robust. The objective is to achieve greater overall economy without compromising safety. The Method can be adopted from the inception of a project or later if benefits are identified. However, the Method should not be used where there is insufficient time to implement fully and safely complete the planned modification or emergency plans.

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The objective of this report is to provide technical and contractual guidance that will lead to increased, more effective and innovative application of the Observational Method. Topics covered include history and definition of the Observational Method, principles and practice, contractual framework and applications.

The Observational Method in ground engineering: principles and applications. Construction Industry Research and Information Association

Report 185

0CIRIA 1999

ISBN 0-86017-497-2

Reader interest

Classification

Design, specification, construction, managers, clients and supervising engineers involved in civil and geotechnical works.

AVAl LAB1LlTY

Unrestricted

CONTENT

Review of available guidance Committee-guided

STATUS USER

Civil, geotechnical and tunnel engineers, construction managers and clients

Published by CIRIA, 6 Storey’s Gate, Westminster, London SWlP 3AU. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, including photocopying or recording, without the written permission of the copyright-holder, application for which should be addressed to the publisher. Such written permission must also be obtained before any part of this publication is stored in a retrieval system of any nature.

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ClRlA Report 185

Foreword

This report is the result of a research project carried out under contract to CIRIA by Ove Amp and Partners in collaboration with Balfour Beatty Civil Engineering Limited.

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Following CIRIA’s usual practice, the research project was guided by a steering group, which comprised: Mr J R Stacey (chairman)

Jubilee Line Extension Project

Dr D Allenby

Edmund Nuttall Limited

Dr J Apted

Hyder Consulting Limited

Dr D R Carder

Transport Research Laboratory

Dr J A Charles

Building Research Establishment

Mr R H Coe

Binnie Black & Veatch

Mr S P Corbet

G Maunsell & Partners

Mr H R Davies

Union Railways Limited

Mr D Lamont

Health and Safety Executive

Mr Q J Leiper

Tarmac Construction

Prof D Muir Wood

University of Bristol

Mr K Nicholls

Wardell Armstrong

Mr A J Powderham

Mott MacDonald Group

Mr D Sharrocks

Costain-Taylor Woodrow JV

Prof J B Burland (corresponding member)

Imperial College of Science, Technology and Medicine.

CIRIA’s research managers for the project were Dr A J Pitchford and Mr F M Jardine. This project was funded by CIRIA’s Core Programme sponsors, the Transport Research Laboratory and by the Construction Sponsorshp Directorate of the Department of the Environment, Transport and the Regions.

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Acknowledgements

CIRIA and the authors would like to thank all the organisations that took part in the interviews or replied to the questionnaires and the following people and organisations (in alphabetical order): h e y Construction Limited Dr Peter Avgherinos (Mott MacDonald Group) Mr Richard Caldwell (formerly Ove Arup and Partners) Dr Mike Francescon (Ove Arup and Partners) Sir Alexander Gibb and Partners Highways Agency Dr Andrew Lord (Ove Amp and Partners) Mr Charles Milloy (Ove Arup and Partners)

Dr Paul Morrison (Ove Arup and Partners) Sir Alan Muir Wood

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Dr Nick O’Riordan (Ove Arup and Partners) Mr Douglas Parkes (Ove Arup and Partners) Dr Brian Simpson (Ove Amp and Partners) Mr Kendrick White (Ove Arup and Partners).

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ClRlA Report 185

Contents

Summary ............................................................................................................................ 2 Foreword ........................................................................................................................... 3 . . . . . Acknowledgements ........................................................................................................... 4

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Boxes ................................................................................................................................ 8 Figures .............................................................................................................................. 8 Tables ................................................................................................................................ 9 Glossary .......................................................................................................................... 11 Abbreviations .................................................................................................................. 15 Notation .......................................................................................................................... 16

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INTRODUCTION .......................................................................................... 17 Background to the project .................................................................. 17 1.1 Benefits and limitations of the Observational Method ....................... 18 1.2 Objectives of the report ...................................................................... 19 1.3 1.4 Report layout ...................................................................................... 19 1.5 Methodology ...................................................................................... 21 The Observational Method ................................................................. 23 1.6

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DEFINITION AND HISTORY ..................................................................... 2.1 Definition ........................................................................................... A brief history of the Observational Method ...................................... 2.2 2.2.1 Early procedures ................................................................... Rationalisation by Terzaghi and Peck .................................. 2.2.2 2.2.3 Wider application ................................................................. Post- 1990 development ....................................................... 2.2.4

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CONCEPTS 30 3.1 Design ................................................................................................. 30 3.2 Risk ..................................................................................................... 32 3.2.1 Risk management and the Observational Method ................32 3.2.2 Definitions ...................................................... ......... 32 3.2.3 Methodology ........................................................................ 32 3.2.4 Risk tolerability criteria ........................................................ 33 3.2.5 Tools and techniques for risk management .......................... 33 Comparison between the predefined design method and the 3.3 Observational Method ....... ............................................................ 34 Ways of applying the Observational Method to uncertainties 3.4 in the ground ....................................................................................... 36 3.4.1 Ground treatment uncertainty ............................................... 38 3.4.2 Geological uncertainty ....................................... 3.4.3 Parameter uncertainty ........................................................... 40 .............40 3.5 Design codes and design conditions ......................... 3.5.1 Development of design codes .............................................. 40 .. 3.5.2 Design conditions ................................................................. 43 .............................. 43 3.5.3 Comments on codes ............... Establishing design values .................................................................. 43 3.6 3.6.1 Serviceability limit state predictions . 3.6.2 Ultimate limit state predictions ............................................ 47 3.6.3 Empirical approach to design ............................................... 48

25 25 25 25 25 26 27

....................................................................................................

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3.7 3.8

3.9

3.10 3.11

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Partial factors of safety ....................................................................... Rapid deterioration ............................................................................. 3.8.1 Ground conditions ................................................................ 3.8.2 Groundwater conditions ....................................................... 3.8.3 Temporary surcharges .......................................................... Construction sequence and programme ............................... 3.8.4 Trigger criteria and trend rate ............................................................. 3.9.1 Trigger criteria ..................................................................... 3.9.2 Trend and rate of change ...................................................... Multi-stage construction trigger values ................................ 3.9.3 Implementation of planned modification ............................................ Value management .............................................................................

.............................................................

TECHNICAL CONSIDERATIONS National and corporate policies .......................................................... 4.1 Corporate and project organisation ..................................................... 4.2 4.3 Design and planning .............................................................. 4.3.1 Desk study ............................................. .......................... 4.3.2 Site investigation .................................................................. 4.3.3 Data interpretation ................................................................ 4.3.4 Initial design ......................................................................... 4.3.5 Final design ........................................................... Construction control and monitoring .................................................. 4.4 4.4.1 Monitoring plan .................................................................... 4.4.2 Site monitoring ..................................................................... Review and implement planned modification .................................... 4.5 Technical and procedural auditing ..................................................... 4:6 Pitfalls in the use of the Observational Method .................................. 4.7

.......................................................

48 49 49 50 50 51 51 51 53 58 59 62

65 68 69 71 72 74 74 78 78 80 81 82 83

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MANAGEMENT CONSIDERATIONS 5.1 Culture ............................................................................................. 5.2 Strategy ............................................................................................. 5.3 Competence ........................................................................................ 5.4 Systems .............................................................................................

86 86 88 89 92

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CONTRACTUAL FRAMEWORK .............................................................. 93 Nature ofthe work ....... ............................................................ 93 6.1 6.1.1 Client’s desi temporary works ................................ 94 Contractor-led temporary and permanent works .................. 94 6.1.2 6.1.3 Engineer-led permanent works with contractor-led temporary works................................................................... 94 Contract and tender considerations .................................. 6.2 6.2.1 Common contract types and the OM ................. 6.2.2 Difficulties and solutions ..................................................... 97 98 6.2.3 Operating environment ......................................................... 6.2.4 Tender strategy ..................................................................... 98 ................................ 98 Value engineering clause .............................. 6.3 Professional indemnity ....................................................................... 99 6.4 Contractual risk ................................................................................ 100 6.5 100 6.5.1 Technical and managerial risk ............................................ 6.5.2 Commercial risk ................................................................. 101 6.5.3 Programme risk .......................... ................................... 102

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TUNNEL APPLICATIONS ......................................................................... 103 Design and planning ......................................................................... 105 7.1 Construction control and monitoring ................................................ 109 7.2 Review and implement planned modification .................................. 111 7.3 Case study - Castle Hill NATM tunnels, Folkestone. UK ...............113 7.4 Design and planning ........................................................... 115 7.4.1 Construction control and monitoring ................................. 118 7.4.2 Review and implement planned modification .................... 119 7.4.3

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EXCAVATION APPLICATIONS 8.1 Cuttings ........................................................................................... Design and planning ........................................................... 8.1.1 Construction control and monitoring ................................. 8.1.2 Review and implement planned modification .................... 8.1.3 8.2 Retaining walls ................................................................................. 8.2.1 Design and planning ........................................................... 8.2.2 Construction control and monitoring ................................. 8.2.3 Review and implement planned modification .................... Case study - A4lA46 Batheaston-Swainswick Bypass. 8.3 Bath. UK ........................................................................................... Design and planning ........................................................... 8.3.1 Construction control and monitoring ................................. 8.3.2 8.3.3 Review and implement planned modification ....................

..............................................................

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138 138 141 142

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OTHER APPLICATIONS ........................................................................... 145 9.1 Ground treatment .............................................................................. 145 Design and planning ........................................................... 145 9.1.1 9.1.2 Construction control and monitoring ....... ................146 Review and implement planned modification .................... 148 9.1.3 Embankment and reclamation works ................................................ 154 9.2 Design and planning ........................................................... 154 9.2.1 Construction control and monitoring ................................. 157 9.2.2 Review and implement planned modification .................... 161 9.2.3 162 9.3 Environmental geotechnics ............................................................... ............................... 163 9.3.1 Design and planning ....................... Construction control and monitoring ................................. 165 9.3.2 Review and implement planned modification .................... 166 9.3.3 9.4 Structures.......................................................................................... 169 Design and planning ........................................................... 169 9.4.1 Construction control and monitoring ................................. 172 9.4.2 Review and implement planned modification .................... 175 9.4.3

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CONCLUSIONS AND WAYS FORWARD 10.1 Conclusions ...................................................................................... 10.2 Ways forward ...................................................................................

REFERENCES

...............................................

............................................................................................................

APPENDIX A1 RECOMMENDED READING

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121 123 123 129 129 130 130 133 133

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179 179 182 183 195

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BOXES Box 2.1 Box 3.1 Box 4.1 Box 8.1 Box 8.2 Box 8.3 Box 8.4 Box 8.5 Box 9.1 Box 9.2 Box 9.3 Box 9.4

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Box 9.5 Box 9.6 Box 9.7 Box 9.8

The requirements of the Observational Method in EC7 ............................ 27 Definitions of robustness ........................................................................... 31 76 HSE risk hierarchy ..................................................................................... The application of the Observational Method to design cuttings in soil at Bay Tree Cottage. Bath ............................................................. 123 Use of staged excavation at Izmit-Adapazari. Turkey ............................ 124 The application of the OM to rock slope excavation ............................... 126 Modification strategy at Limehouse Link. London. using soft props ...... 134 Modification strategy at Minster Court. London. using an earth berm ....136 Treatment of solution features in Upper Chalk at Castle Mall. Norwich 150 Compensation grouting at Waterloo Station. London ............................. 152 An example of a restoration plan for a dioxin-contaminated river in northern England ..................................................................................... 167 Example of using the Observational Method to protect structures affected by tunnelling .............................................................................. 171 Protection of the Mansion House using the Observational Method ........ 173 Strategy adopted for modifying the Foyle Bridge foundation design ...... 175 The remediation strategy at Poechos Dam. Peru ..................................... 176 The strategy adopted for protecting the London Underground 177 Jubilee Line tunnels under the Grand Buildings in London ....................

FIGURES Figure 1.1 Figure 1.2 Figure 3.1 Figure 3.2 Figure 3.3 Figure 3.4 Figure 3.5 Figure 3.6 Figure 3.7 Figure 3.8 Figure 3.9 Figure 3.10 Figure 3.11 Figure 3.12 Figure 3.13 Figure 3.14 Figure 3.15 Figure 4.1 Figure 5.1 Figure 5.2 Figure 7.1 Figure 7.2 Figure 7.3 Figure 7.4 Figure 7.5

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Some potential benefits of the Observational Method ............................... 18 The Observational Method ........................................................................ 22 Simplified example of ground treatment. geological and parameter uncertainties ............................................................................................. 37 Types of soil strength parameters .............................................................. 41 Application of factor of safety to different types of parameter values ....... 44 Ideal relationship between predicted and measured performance. following EC7 ........................................................................................... 44 poor agreement between predicted and measured performance ................45 Comparison of predicted most probable and measured . . characteristic value .................................................................................... 46 Ultimate limit state prediction ................................................................... 47 Brittle and ductile behaviour ..................................................................... 50 54 Defining trigger criteria ............................................................................. HSE discovery-recovery model ................................................................. 55 Traffic light system for an incremental excavation process ...................... 56 Trend and rate of change ........................................................................... 57 Multi-stage construction trigger values ..................................................... 58 Approach based on Peck (1969a) .............................................................. 60 Progressive modification (simplified diagram) ................... An example of traffic light system for a staged excavation ....................... 78 Management considerations ...................................................................... 87 Skills and competence ............................................................................... 91 The application of the Observational Method to the NATM ..................105 Tunnel deformation measurement ........................................................... 108 Management review process for in-tunnel monitoring ............................ 110 Plan of Castle Hill tunnels and landslip ................................................... 113 114 Contractual arrangement .......... ............................................................... ~

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Figure 7.6 Figure 7.7 Figure 7.8 Figure 7.9 Figure 8.1 Figure 8.2 Figure 8.3 Figure 8.4 Figure 8.5 Figure 8.6 Figure 8.7 Figure 8.8 Figure 8.9 Figure 8.10 Figure 8.1 1

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Figure 8.12 Figure 9.1 Figure 9.2 Figure 9.3 Figure 9.4 Figure 9.5 Figure 9.6 Figure 9.7 Figure 9.8 Figure 9.9 Figure 9.10 Figure 9.1 1

Service tunnel Ch . 10555m: deformation monitoring results showing implementation of contingency action to curtail high deformation rate .. 116 Typical tunnel monitoring review procedure ........................................... 118 Service tunnel actual shotcrete thickness, support types and deformation compared to design .................................................................................. 119 Modified running tunnel temporary support design ................................ 120 The Observational Method applied to multi-stage excavation ................122 The design options at Bay Tree Cottage, Bath ........................................ 125 Staged excavation at Izmit-Adapazari, Turkey ....................................... 125 The engineer’s approach to temporary battered slope design ..................127 Recommended factors of safety for new slopes and rainfall with a ten-year return period in Hong Kong .................................................... 128 Prop design at Harris Bank and Trust CO, Chicago ................................. 133 Use of soft props at Limehouse Link. London ........................................ 135 Variation of berm geometry at Minster Court. London ........................... 137 Site location plan at Batheaston-Swainswick Bypass ............................. 137 (a) Original construction sequence (b) Contractor’s alternative construction sequence .................................. 139 Comparison of measured wall deflection with amber trigger value after berm excavation .............................................................................. 143 Occurrence of measured deflection normalised by amber trigger value .. 144 Treatment of solution features at Castle Mall. Norwich .......................... 151 Correlation between embankment lateral deflection and centre-line settlement ................................................................................................. 158 159 Embankment stability control charts ....................................................... 160 Creep effects ........................................................................................... Proposed use of the Observational Method in ground remediation ......... 163 Risk assessment methodology for environmental geotechnics ................164 A restoration strategy proposed for a dioxin-contaminated river in 168 northern England ..................................................................................... Cost of ground treatment to a selected clean-up target level for particular depth of contaminated soil ....................................................... 169 The choice of foundation for Foyle Bridge (a) tender stage. (b) reappraisal stage. (c) final stage ......................................................... 171 Application of the OM at Mansion House. London ................................ 174 Demolition strategy adopted at the Grand Buildings. London ................178

TABLES Table 2.1 Table 3.1 Table 3.2 Table 3.3 Table 3.4 Table 3.5 Table 4.1 Table 4.2 Table 4.3 Table 4.4 Table 4.5 ClRlA Report 185

Development history of the Observational Method ................................... 29 Comparison between risk management methodology and the Observational Method ............................................................................... 33 Comparison of the predefined design method and the Observational Method ............................................................................... 35 Examples of using the OM in ground treatment uncertainties ...................38 Comparison of terms to describe soil parameter values and conditions used in design codes .................................................................................. 42 ultimate limit states in persistent and transient Partial factors . situations (EC7. Table 2.1) ........................................................................ 48 66 Components of the.Observationa1 Method ................................................ Aspects to be considered in national and corporate policies ..................... 68 Matters to be considered in corporate and project organisations ...............70 Examples of monitoring systems ............................................................... 80 Potential misuse of the Observational Method .......................................... 84 9

Table 6.1 Table 7.1 Table 8.1 Table 8.2 Table 8.3 Table 9.1 Table 9.2 Table 9.3 Table 9.4 Table 9.5

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Table 9.6 Table 9.7 Table 9.8

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Difficulties and solutions ........................................................................... 97 Comparison of the OM components with comments in Report on the NATM ................................................................................................. 104 Examples of excavation applications ....................................................... 121 Publications describing experience on retaining wall behaviour .............132 Design conditions adopted for retaining wall design at A4/A46 .............140 Observable parameters for a dewatering system...................................... 147 Typical responsibilities of the engineer and specialist (treatment) contractor ................................................................................................. 148 Application of the Observational Method to the installation of driven piles and stone columns ............................................................... 153 Limits of behaviour influencing embankment design .............................. 155 Factors to be considered when selecting a method of treatment for contaminated sites .................................................................................... 166 Typical monitoring systems for foundation ............................................. 172 Typical monitoring systems for embankment dams ................................ 172 Examples of Observational Method-based strategies for protecting structures .................................................................................................. 176

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Glossary

ab initio (adoption of the Observational Method)

“The intended use of the Observational Method from the inception of the construction phase (Peck, 1969a).

best way out (adoption of the Observational Method)

The application of the Observational Method when ‘Lconstructionhas already started and some unexpected development has occurred, or whenever a failure or accident threatens or has already taken place” (Peck, 1969a).

contingency plan

“A course of action or modification of design, defined in advance, for a foreseeable signijicant deviation of the observational findings from those predicted on the basis of working hypothesis” (Peck, 1969a).

”

Contingency plans are invoked when the monitoring reveals that trigger criteria are likely to be exceeded and safety reduced. design condition

A set of parameters such as stratigraphy (geology), soil properties, water pressure, loading and structural geometry, eg most probable condition.

hazard

A hazard is something that has the potential to cause harm.

mean value

Spatial mean - The arithmetic mean of a set of data with allowance for systematic variation in depth or location. If linear variation with depth (or another parameter) is assumed, the mean may be found by linear regression. Probabilistic mean - A value such that the probability of a higher (or lower) value occurring in the current context is assessed to be 50 per cent. The context may refer to: tests on individual specimens of material or the spatial mean of a parameter over a defined zone; the value of a load at an instant; or the peak value of the load during the lifetime of the structure.

moderately conservative

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A cautious assessment of the value of a parameter, worse than the probabilistic mean but not so severe as a worst credible parameter value. Sometimes termed a “conservative best estimate”.

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more probable

This is a term introduced by Powderham (1994) and is used in association with “progressive modification”. It describes the design conditions that are less conservative than those assumed in a conventional design but are more conservative than the most probable conditions. They denote conditions more probable than in a conventional design.

most probable

A set of parameters that represents the probabilistic mean of all possible sets of conditions. It represents, in general terms, the design condition most likely to occur in practice.

most unfavourable

The worst design condition or set of design parameters that the engineer considers might occur in practice.

Observational Method

The Observational Method in ground engineering is a continuous, managed, integrated process of design, construction control, monitoring and review that enables previously defined modifications to be incorporated during or after construction as appropriate. All these aspects have to be demonstrably robust. The objective is to achieve greater overall economy without compromising safety. The Method can be adopted from the inception of a project, or later if benefits are identified. However, the Method should not be used where there is insufficient time to implement fully and safely complete the planned modification or emergency plans.

plann d modifi ati n

A planned modification is one that is defined in advance of the construction works. The modification can be of design, construction sequence, construction method, etc (see Section 4.5). It can include: 0

a contingency plan applied during adverse conditions (these may range from small modifications to significant contingency plans) a plan to achieve identified benefits.

(While an emergency plan can be in the form of a planned modification, it is site-specific and is a special subject that is not within the scope of this book. Readers should refer to the relevant health and safety regulations for guidance on emergency plans.)

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ClRlA Report 185

predefined design method

In the traditional procurement of projects, a single robust and possibly over-conservative design is fully developed before site work starts on a particular phase. Monitoring is sometimes used, but in a passive way to confirm that design predictions are not exceeded. There is no primary intention to vary the design during construction. In this report, this procedure is referred to as the “predefined design method”. Predefined design includes: design by calculations 0

design by prescriptive measures

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design using load tests andtests on experimental models. progressive failure

This develops when a local failure overstresses the adjacent soil or structure and this in turn fails.

progressive modification

A design based on the more probable condition (see above) with design changes introduced sequentially towards the most probable condition (see above) (Powderham, 1994).

rapid deterioration

Deterioration which occurs so rapidly that there is insufficient time fully to implement the planned modifications or emergency plans, eg brittle failure. Besides quality-control measures, material quality and workmanship, the rate at which deterioration develops also depends on ground conditions, groundwater conditions, temporary surcharge, construction sequence and construction programme.

risk

Risk = hazard severity x likelihood of occurrence (HSE, 1991).

risk-based design

Design based on risk assessment and consideration of risk hierarchy.

robustness

This term is used, depending on context, in the same senses as the two usages below. “Buildings should have an acceptable general structural integrity that permits them to tolerate or avoid damage in the event of abnormal or accidental conditions. Structural form, type of structure and construction are all-important in this context, as are provisions for continuity, ductility, toughness and redundancy. This desirable characteristic of structural systems has come to be known as robustness (IStruct.E, 1990). ”

“A robust design is one where the risk of failure, or of damage, to the structure is extremely remote during its design life (HSE, 1996). ”

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serviceability limit states

States that correspond to conditions beyond which specified service requirements for a structure or structural element are no longer met, eg its durability is impaired, its maintenance requirements are substantially increased or damage is caused to nonstructural elements. Alternatively, such conditions of the structure that may affect adjacent structures or services in a like manner (BSI, 1994a).

trend rate

The change of monitored value with time.

trigger criteria

These are the limits for implementing the planned modifications in the Observational Method. These criteria can be movements or forces or other values derived from the design calculations for different conditions. These criteria may also be based on ground conditions, health and safety considerations, workmanship, quality of materials, construction rate, resource availability, etc.

ultimate limit states

States associated with collapse, or with other similar forms of structural failure that may endanger property or people or cause economic loss (BSI, 1994a). They generally correspond to maximum load-carrying capacity resistance of a structure or a structural part.

value management

The management of the value process throughout a project in order to maximise value, which could include cost, quality, safety and timely completion.

worst credible

A worst credible parameter value is the worst that the designer believes might realistically occur. In the case of a load or a geometric parameter, it is usually a value that is unlikely to be exceeded. In the case of a soil strength parameter, it should be a pessimistic value which is very unlikely to be any lower (Padfield and Mair, 1984). As a guide, it can be regarded as representing the 0.1 per cent fractile (Simpson et al, 1979).

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Abbreviations

ClRlA Report 185

ALARP

as low as reasonably practicable

BS

British Standard

CDM

Construction (Design and Management) Regulations 1994

CHSW

Construction (Health, Safety and Welfare) Regulations 1996

CPT

cone penetration test

DBFO

design, build, finance and operate (contract)

DBOM

design, build; operate and monitor (contract)

DBOT

design, build, operate and transfer (contract)

D&C

design and construct (contract)

EC1

Eurocode 1

EC7

Eurocode 7

ECC

NEC Engineering and Construction Contract (New Engineering Contract, 2nd edition)

EPSRC

Engineering and Physical Sciences Research Council

FOS

factor of safety

HSC

Health and Safety Commission

HSE

Health and Safety Executive

ICE

Institution of Civil Engineers

Mod C

moderately conservative

MP

most probable

MU

most unfavourable

NATM

New Austrian Tunnelling Method

NEC

New Engineering Contract 1st Edition

OM

the Observational Method

QA SCL

quality assurance

SI

site investigation

SLS

serviceability limit states

SPT

standard penetration test

TBM

tunnel boring machine

TRRL

Transport and Road Research Laboratory

ULS

ultimate limit states

VE

value engineering

VM

value management

VP

value planning

sprayed concrete linings

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Notation

effective angle of shearing resistance settlement at the centre of the base of the embankment maximum lateral deflection below the embankment toe (used in Section 9 only) maximum lateral deflection at the embankment toe partial factor effective cohesion compression index swelling index undrained shear strength coefficient of consolidation undrained Young's modulus drained Young's modulus

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coefficient of earth pressure at rest standard penetration test (SPT) N value preconsolidation pressure embankment load embankment load at failure compressive strength of soil or rock peak shear strength residual shear strength design value characteristic value

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Introd uction

1.I

BACKGROUND TO THE PROJECT Engineers have used observations to deal with uncertainties in the ground and to monitor the performance of structures since the early age of civil engineering. Improvements to designs were made by trial and error or as part of a “design as you go” process.

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Today’s predefined design method is generally based on a single design solution that is hlly developed before construction work starts on a particular phase of the development. If used at all, monitoring is employed as a check on whether design predictions are exceeded. There is generally no intent to modify the design and usually there are no contingency plans prepared for critical situations. Peck (1 969a) refers to a quotation from Terzaghi: “Znfacing uncertainties in the ground, two methods have been postulated: either the use of an excessive factor of safe@, or else to make assumptions about the ground in accordance with general, average knowledge. Thejrst approach may be wasteful and the second could be dangerous.” In the late 1940s an integrated process for predicting, monitoring, reviewing and modifying designs evolved with the development of modern soil mechanics theories. These procedures were eventually set out as the “Observational Method” (OM) by Peck in his Rankine Lecture (Peck, 1969a). Over the years, Peck’s principles have been extended by a number of engineers and the OM has also been recognised as a design method in design codes such as Eurocode 7 (BSI, 1995). In the early 1990s there was a noticeable increase in use of the OM in the UK and more interest in research into the Method. This had been prompted by concerns about reducing costs and improving safety and team co-operation. Concern for cost saving - Competitive tendering for design work can lead to simple but over-conservative schemes. However, clients are becoming less tolerant of over-design. The Latham Report (Latham, 1994) suggested that UK practice could be more efficient and that total project costs could be reduced by up to 30 per cent by 2000. There is therefore a need for value-for-money designs. Concern for safety - There has always been a demand for a safer construction environment. With the introduction of the Construction (Design and Management) (CDM) Regulations in 1994 (HSC, 1994), new duties have been placed upon clients, designers and contractors to take health and safety into account and to coordinate and manage the health and safety of a construction project throughout all its stages. Concern for team co-operation - Isolation of the design work from the construction work can result in simple but uneconomical design. It can also lead to confrontation between designers and contractors. Effort can be wasted in enforcing aggressive contracts and developing claims by the contract parties rather than working with a common aim to build the project. However, with the evolution of new contract types, such as the New Engineering Contract (NEC) (ICE, 1995), and the increasing use of contracts such as design and construct (D&C) and design, build, finance and operate (DBFO), the traditional gap between designers and ClRlA Report 185

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,

contractors has narrowed. Also, the emergence both of new business processes, such as the Private Finance Initiative, and of new practices generated by research, such as that sponsored under the Construction as a Manufacturing Process programme, has led to more opportunities opening for use of the OM. Despite increasing use, little has been done to improve the OM definition or to provide guidelines to follow since Peck, although Eurocode 7 (BSI, 1995) provides a framework. Worryingly, different engineers have defined different principles when applying the OM. The ad-hoc or “design-as-you-go” approach to the OM in the past has been castigated by Health and Safety Executive (HSE, 1996). During the preparation of this report, therefore, the research team has sought every opportunity to consult industry in order that the recommendations of the report are practical, workable and satisfy the current health and safety regulations.

1.2

BENEFITS AND LIMITATIONS OF THE OBSERVATIONAL METHOD

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Broadly speaking, the OM is a process in which acceptable limits of structural and geotechnical behaviour are established. In addition, performance predictions, monitoring, review and modification plans, and emergency plans, are fully prepared. The design is checked for robustness before construction starts. During (and after) construction, the results from the monitoring are reviewed against the predictions and robust modifications are introduced where appropriate. The Method offers the potential for savings of time or money, and the monitoring provides the needed assurance concerning safety. Some potential benefits of using the OM are shown in Figure 1.1. Cost savings can be shared among the client, the designer and the contractor through appropriate contractual clauses and each benefits through provision of a better service or product. Achieving value I economy

Greater motivation for the project team

1

construction control andmanagement

__

Figure 1.1

18

1

(

OBSERVATIONAL

METHOD

I\\

7 1 Control of design uncertainties

between designers

I ‘

Some potential benefits of the Observational Method

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It should be noted that the OM must not be used in the following situations: where there is insufficient time fully to implement and complete the contingency and emergency plans 0

where observations would be difficult to obtain or are unreliable.

When the Method is used to control the risk of exceeding an ultimate limit state or where there is a risk to people associated with the use of the Method, a safety case needs to be provided. The risk-control measure must be an explicit part of the safety management system required by the relevant safety regulations, eg CDM. In this context, a “safety case” is defined as a systematic and, where possible, quantified demonstration that an installation or system meets specified safety criteria. Moreover, overall economy can be achieved through the use of the OM only if: thorough, higher-quality site investigations are carried out site investigations are substantially completed and the data are interpreted before the initiation of the Method design cases are analysed to cover all the likely scenarios.

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1.3

OBJECTIVES OF THE REPORT This research project was commissioned by CIRIA in 1995 with the objective of establishing and setting out the principles of the OM and explaining its use in different applications. During the course of the project these were amplified into the following objectives for the report: 0

to establish a definition of the Observational Method

0

to show how to integrate the Observational Method into current design codes and site control procedures to identify limitations of the Method to provide a set of concise technical, managerial and contractual guidelines for designers, constructors and clients

1.4

0

to encourage better geotechnical prediction

0

to help to promote a cultural change in the industry.

REPORT LAYOUT The following paragraphs summarise how the report is structured to meet the above objectives. This opening section describes the methodology adopted for the research, highlights the need for the OM in ground engineering and introduces the key elements of the OM. These elements are discussed separately in following chapters. The definition and history of the development of the OM are given in Section 2. Requirements of the OM, for example, have previously been stated by Peck (1969a) and in EC7 (BSI, 1995). Over the years, the OM has changed from a design-as-you-go method to an integrated and managed design-and-construction process to suit the present requirements of the industry. The meaning of “design” is discussed in Section 3, which also compares the OM and the predefined design method. Section 3 considers all the concepts relating to the use of the OM, including terminology, factors of safety (FOS), rapid deterioration, risk assessment, trigger value, trend rate and implementation of planned modification. These concepts are compared with those in existing standards and codes of practice.

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Implementation of the OM requires a team effort from the designer, constructor and client (client's representative). For the OM to be successful there should be a consistent set of technical requirements for the design and site works. A clear management framework is the key to organisation of the project team and identifying the responsibilities of its members. Both the technical and management frameworks have to be in place when applying the OM. Section 4 is a step-by-step guide to implementing the OM process. The report restates the criteria recommended previously by Peck and EC7, but extends them to cover the requirements of present-day projects. It covers policy and organisational issues, from national to project level. Section 5 describes management considerations. It explains the additional management skill required for the OM when compared with predefined designs and that the management approach at the design and planning stage should be different from that during construction.

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Questions about the sharing of risks and benefits are discussed in Section 6 by examining the advantages and drawbacks of various types of contract when used in association with the OM. Six specific geotechnical applications of the OM are presented in Sections 7, 8 and 9. Certain technical considerations not previously mentioned in Section 4 are also addressed here. Section 10 presents the main conclusions from the research project. It also suggests aspects needing further research. Definitions of the key terms used in association with the Method are shown in the Glossary. In addition to the list of references cited in the report, the Appendix gives sets of references by topic that the research team has recommended for further reading. Most of the projects referred to in this report were constructed before the recent developments of the OM. Therefore, not all the OM elements mentioned in this report can be found in the case histories. Nevertheless, they are good examples of how one or more aspects of the OM have been implemented successfully. Readers should refer to the quoted publications for unabridged descriptions of particular projects.

It should be noted that recommendations made in this report should be used in conjunction with the appropriate British Standards and health and safety regulations and guidance. They are not substitutes for any parts of the existing codes of practice and regulations.

20

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1.5

METHODOLOGY This research had been based on the in-house experience of Ove Arup and Partners and Balfour Beatty Civil Engineering Limited, together with guidance from a steering group. The steering group comprised consultants, contractors, Health and Safety Executive staff and members of research organisations and universities. Most of the case histories quoted in this report are the result of a literature review carried out by the authors.

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Industry was consulted throughout the preparation of this report. The consultation process included: three main elements. 1.

Seminars - Two seminars were organised by the authors on behalf of CIRIA in October 1995 and March 1996, in Newcastle and London, respectively. The objectives of these seminars were to listen to the views of industry on the OM and to report on the progress of the project.

2.

Questionnaires - About 100 questionnaires were sent to engineers worldwide. They were asked about the technical, management and contractual issues regarding implementation of the OM. About one-third of the questionnaires were returned.

3.

Interviews - Nine interviews were conducted by the authors. The interviewees included property developers, consultants, contractors and staff of the Health and Safety Executive.

The results of this consultation process are incorporated in the report.

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1

NATIONAL 8 CORPORATE POLICY

1-1

CORPORATE & PROJECT ORGANISATION

1

,

TECHNICAL& PROCEDURAL AUDITING

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L-

~

1 __

,

DESIGN & PLANNING

A

MONITORING

1 -1

REVIEW 1

L

~

-~

~-

EXCEEDED?

I

Yes

L

w

MODIFICATION (INCLUDING CONTINGENCY PLANS)

Figure 1.2

22

The Observational Method

ClRlA Report 185

1.6

THE OBSERVATIONAL METHOD The elements of the OM are shown in Figure 1.2. Historically, Peck (1969a) identified two situations of use of the OM. They are: 0

from inception of the project, ie ab initio

0

during construction when unexpected site problems develop, ie best way out.

Although the stage at which the OM is introduced into the project is different in each case, the principles and procedures for implementing the OM are identical in both situations. The elements shown in Figure 1.2 should be fully incorporated in either case. These elements are: 1.

National and corporate policy

These are the values and objectives of an organisation and the criteria and principles on which actions and responses are based. National policies include health and safety regulations, design codes and conditions of contracts. Examples of corporate policies include quality management systems and works specifications.

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2.

Corporate and project organisation

This is the establishment of the responsibilities and relationships between organisations and individuals within an organisation or a project team involved in design, construction and monitoring of the work.

3.

Design and planning

These are concerned with data gathering, design, data interpretation, risk assessment and the allocation of resources (eg money, time or effort) in order to achieve objectives and to decide priorities. Planning of the OM must start from the desk study stage in which the type of problem to be encountered is defined. A phased approach to site investigation (SI) is recommended. During the initial phases, the site should be investigated sufficiently such that the range of geological conditions and soil parameters can be established to the particular OM requirements identified in the desk study. So far as reasonably practicable, the SI should preclude meeting unexpected conditions of a safety-critical nature. The final phases of SI can be undertaken during construction and may involve, for example, logging the excavation face. The data interpretation stage involves assessment of the SI information, the range of design conditions likely to be encountered and the corresponding design parameters to be considered. Design cases should cover all the likely scenarios and planned modifications should be such that they can be introduced in time to prevent safety from being reduced to unacceptable levels. A planned modification is one that is defined in advance of the construction works. The modification can be of design, construction sequence, construction method etc (see Section 4.5). It can include:

a.

a contingency plan applied during adverse conditions (these may range from small modifications to significant contingency plans)

b.

a plan to achieve identified benefits.

In the UK, under the Construction (Health, Safety and Welfare) Regulations 1996, planning of emergency procedures is required in all construction projects, including those based on the OM. While an emergency plan can be in the form of a ClRlA Report 185

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planned modification, it is site specific and is a special subject beyond the scope of this report. Readers should refer to the relevant health and safety regulations for guidance on this subject. The cost, the health and safety issues and the risk associated with the construction scheme and the modification options should be assessed. The constructor should also prepare an outline method statement to reflect the above requirements. This is an iterative process in which different options are assessed until a final scheme is chosen and a design - and the corresponding planned modifications - are fully developed. Trigger criteria should also be established. 4.

Construction control and monitoring

This is a planned process of collecting information about construction progress and monitoring involving a variety of checking or monitoring activities. A monitoring plan, monitoring specifications and construction control plan setting out agreed procedures and reporting methods should be developed. The duties and responsibilities of different parties are also specified. During the site-monitoring stage, the instrumentation should be installed, readings taken and monitoring reports delivered to all parties within agreed time limits.

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5.

Review and implement planned modification

This includes management of the review process and the implementation of planned modification if required. The review will be based on information such as the construction progress, geological conditions encountered (face logs) and instrumentation results. These must be presented for review with the relevant prediction. It is the possible to take decisions on modification of design, construction method, workmanship and use of materials. These require planned pro-active monitoring in which timely, reliable and easily interpretable daQ are collected, critically examined by competent people, and their conclusions brought to the attention of the managers authorised to initiate any.necessary planned actions. 6.

Technical and procedural auditing

This is the structured process of collecting independent information on the efficiency, effectiveness and reliability of the management system and drawing up plans for corrective action. Auditing should be carried out to check that the design, monitoring, material quality, workmanship, construction method, review processes and the project quality management system are reaching the correct technical interpretations.

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2

Definition and history

2.1

DEFINITION The Observational Method in ground engineering is a continuous, managed, integrated, process of design, construction control, monitoring and review that enables previously defined modifications to be incorporated during or after construction as appropriate. All these aspects have to be demonstrably robust. The objective is to achieve greater overall economy without compromising safety. The Method can be adopted from the inception of a project or later if benefits are identified. However, the Method should not be used where there is insufficient time to implement fully and safely complete the planned modification or emergency plans.

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The OM concepts in relation to existing design codes are discussed in detail in Section 3. The technical procedures and the management considerations are presented in Sections 4 and 5 respectively.

2.2

A BRIEF HISTORY OF THE OBSERVATIONAL METHOD A summary of the development of the OM is given in Table 2.1.

2.2.1

Early procedures Engineers used observations at the early age of civil engineering because of a lack of design theory. Structures were designed on a trial-and-error basis, but were monitored during construction to understand soil-structure interaction, structural behaviour and improve future designs. Two examples are: 1.

A structural application of the OM associated with the development of masonry arch design in Europe in the Middle Ages (Muir Wood, 1990). An individual buttress was deliberately designed to permit its improvement during construction in response to indications of incipient cracking or instability.

2.

In the late 19th century experienced timbermen monitored the performance of timber struts or mine roof supports by striking them with a sledge-hammer, and would know from the sound of the blow whether or not the member was overstressed (Tomlinson, 1995). It can be assumed that they would have had a predefined contingency plan to provide additional supports at overstressed locations.

The OM requires more formal procedures for the initial design and formulating the contingency plan.

2.2.2

Rationalisation by Terzaghi and Peck The process of predicting, monitoring, reviewing and modifying designs evolved with the development of soil mechanics theories in the mid-20th century. K Terzaghi and later, R B Peck played important roles in formalising the use of this process.

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Terzaghi and Peck (1948) drew attention to the reality of the ground differing from the assumptions based on site investigation results. They warned that if field observations showed the real conditions to be less favourable than the designer had anticipated, the design would have to be modified in accordance with the findings. They also considered that “Design on the basis of the most unfavourable assumptions is inevitably uneconomical, but no other procedure provides the designer in advance of construction with the assurance that the soil-supported structure will not develop ,unanticipated defects. However, if the project permits modfications of the design during construction, important savings can be made by designing on the basis of the most probable rather than the most unfavourable possibilities. The gaps in the available information are filled by observations during construction, and the design is modi3ed in accordance with thefindings. This method of design may be called the Observational Procedure.” (Terzaghi and Peck, 1967)

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The term “Observational Method” was introduced by Peck in his Rankine Lecture in 1969 (Peck, 1969a). He considered that the complete application of the observational method embodies the following ingredients:

2.2.3

a.

Exploration sufficient to establish at least the general nature, pattern and properties of the deposits, but not necessarily in detail.

b.

Assessment of the most probable conditions and the most unfavourable conceivable deviations from these conditions. In this assessment geology often plays a major role.

C.

Establishment of the design based on a working hypothesis of behaviour anticipated under the most probable conditions.

d.

Selection of quantities to be observed as construction proceeds and calculation of their anticipated values on the basis of the working hypothesis.

e.

Calculation of values of the same quantities under the most unfavourable conditions compatible with the available data concerning the subsurface conditions.

f.

Selection in advance of a course of action or modification of design for every foreseeable significant deviation of the observational findings from those predicted on the basis of the working hypothesis.

g. h.

Measurement of quantities to be observed and evaluation of actual conditions. Modification of design to suit actual conditions.

Wider application Between 1970 and 1990 the OM received further recognition and its use was introduced in a wider range of ground engineering operations. De Mello (1977) recommended the OM as one of the design principles for embankment dams. Whitman (1 984) discussed evaluating calculated risk in geotechnical engineering in his Terzaghi lecture. He considered the OM to be a method suitable for updating the probability of a given event as additional information is obtained in geotechnical engineering, ie it can be consider as a form of Bayesian updating. In 1987 the first draft of Eurocode 7 was published and set out the requirements for the OM where a design is reviewed during construction. In the same year, Muir Wood (1987) applied Peck’s principles to a six-step OM for use in tunnelling works. During this period, the OM was applied to other branches of engineering, including coastal engineering (Muir Wood, 1995) and hazardous waste site remedial treatment

26

ClRlA Report 185

(Brown et al, 1990). D’Appolonia (1990) also proposed attending the use of the OM to the operational phases of facilities and processes, this concept of monitored decisions process being a way to handle problems in environmental geotechnics during the postconstruction phases which require contingency planning.

2.2.4

Post-I 990 development The drive for competitiveness renewed interest in the OM in the 1990s. With business processes and new contract forms that minimise the gap between designer and constructor, there are now more opportunities for use of the OM. During this period, several case histories of the OM were published at symposia and conferences such as those organised by the Transport Research Laboratory (TRL) and by the Institution of Civil Engineers (ICE) in the UK. The Highways Agency commissioned TRL to develop a database (Carder, 1995) for establishing the most probable movement and acceptable limits when using the OM to control construction (Card and Carder, 1996). Muir Wood (1990) revisited Peck’s eight ingredients to add to the thinking on the use of the OM in tunnelling, while Holm (1993) and Morgenstern (1994) re-emphasised the advantages of using the OM in environmental geotechnics. Charles (1993) proposed a simplified OM for monitoring and maintaining embankment dams. Box 2.1

The requirements for the Observational Method in EC7

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Clause 2.7 of EC7 (BSI, 1995) states: “Because prediction of geotechnical behaviour is often difficult; it is sometimes appropriate to adopt the approach known as the ‘observational method’, in which the design is reviewed during construction. When this approach is used the following four requirements shall all be made before construction is started: the limits of behaviour which are acceptable shall be established the range of behaviour shall be assessed and it shall be shown that there is an acceptable probability that the actual behaviour will be within the acceptable limits a plan of monitoring shall be devised which will reveal whether the actual behaviour lies within the acceptable limits. The monitoring shall make this clear at a sufficiently early stage; and with sufficiently short intervals to allow contingency actions to be undertaken successfully. The response time of the instruments and the procedures for analysing the results shall be sufficiently rapid in relation to the possible evolution of the system a plan of contingency actions shall be devised which may be adopted if the monitoring reveals behaviour outside acceptable limits. “During construction the monitoring shall be carried out as planned and additional or replacement monitoring shall be undertaken if this becomes necessary. The results of the monitoring shall be assessed at appropriate stages and the planned contingency actions shall be put in operation if this becomes necessary.”

One of the major developments during this period was the publication of the final draft of Eurocode 7 (EC7) (BSI, 1995), which states the requirements for using the OM (see Box 2.1). Similar requirements have also been adopted in the Guide to retaining wall design published by the Hong Kong Geotechnical Engineering Ofice (1993). In the ninth Gtotechnique Symposium in Print, organised by the ICE, engineers from around the world described their experiences of using the OM in modern civil engineering practice and drew attention to the further development of the’Method. Nicholson (1994) highlighted the similarities between the OM and the health and safety management code of practice in the UK. Powderham (1994) introduced the concept of “progressive modification”. He defined the term “more probable” conditions (see ClRlA Report 185

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Section 3), explaining that designs based on the “more probable” conditions could be significantly less conservative than a conventional design, but more conservative than one based on the “most probable” conditions. In the 1990s the OM has been used successfully in conjunction with the value engineering process, by which benefits are shared between clients and contractors in accordance with a value engineering contract (Powderham and Rutty, 1994).

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Research projects also focused on integrating the OM into the business process. The University of Bristol, in association with several industry sponsors, was awarded a grant under the EPSRC Innovative Manufacturing Initiative (IMI) to undertake a research programme entitled “Geotechnical innovation through observation”. The research objectives are to develop potential applications of the OM in order to encourage innovation throughout the process of design and construction, to manage hazards and reduce the risk of unforeseen circumstances and to identify clear business benefits. The Highways Agency, aware of potential advantages of the Method, also commissioned the project “Application of the Observational Method in the construction of road tunnels” with the objective of developing contract forms for specific use with the OM. The concern for safety in tunnel construction also led the ICE to publish a design and practice guide on the NATM for tunnels in soft ground (ICE, 1996a). The Health and Safety Executive reviewed the application of the OM in constructing sprayed concretelined tunnels in London Clay (HSE, 1996). These reports advanced the understanding of NATM design and need to consider face support safety.

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Table 2.1

Year

Development history of the Observational Method

Name

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1948-1967 Terzaghi and Peck

Event Terzaghi and Peck pointed out that as design based on the most unfavourable assumptions is uneconomical, if gaps in the available information can be filled by observations during construction, the design can be modified safely to make savings. They called this method of design the “Observational Procedure”.

1969

Peck

In the ninth Rankine Lecture, Peck introduced the term the “Observational Method” and listed requirements and limitations for application of the OM.

1977

De Mello

Recommended the OM as a design principle for embankment dams.

1984

Whitman

Recognised the use of the OM for managing risks in geotechnical engineering.

1987

Muir Wood

A six-step OM was proposed for use in tunnelling.

Draft Eurocode 7 (EC7)

The draft set out requirements for the use of the OM.

1990

Brown et a1

Proposed to incorporate the OM into hazardous waste site-remediation process.

1990

Muir Wood

Revisited the OM, with further thinking about use of the OM in tunnelling:

D’Appolonia

Proposed to extend the use of the OM to the operational phase of facilities and processes through his concept of Monitored Decisions Process.

1992

TRL

A symposium entitled “Observational Method in design and construction of geotechnical structures” was held.

1993

Hong Kong GEO

Guide to retaining wall design, which recognised the OM as one of the design methods for retaining structures in Hong Kong, was published.

Holm

Advocated the incorporation of the OM into waste restoration study in the USA.

Charles

Proposed a simplified OM for monitoring and maintaining embankment dams.

TRL

A second symposium entitled “The Observational Method” was organised.

Morgenstern

Recognised the advantages of using the OM in environmental geotechnics.

1994

Powderham and Advocated the application the OM in combination with a value engineering clause. Rutty

1995

1996

GCotechnique Symposium in Print

A set of papers was published in GCotechnique under the title “The Observational Method in Geotechnical Engineering”. New concepts such as “progressive modification” by Powderham and integration with health and safety regulations by Nicholson were published.

EC7

The final draft of EC7 was published including the requirements for the OM.

CIRIA

Commissioned the research project “The Observational Method in ground engineering: principles and applications”.

Highways Agency

Commissioned the research project “Application of the Observational Method in the construction of road tunnels”.

TRL

Set up a database of ground and wall movements for establishing the most probable movement and acceptable limits when using the OM.

ICE

Published a design guide: Sprayed Concrete Linings (1vATM)for Tunnels in So) Ground.

HSE

Reviewed the use of the OM in constructing sprayed lined concrete tunnels in London Clay.

EPSRC IMI

The University of Bristol was awarded a grant to carry out a research programme “Geotechnical innovation through observation”.

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3

Concepts

3.1

DESIGN Regulation 2 of the CDM Regulations (1994) defines “design” in relation to any structure to include drawing, design details, specifications and bills of quantities (including specification of articles or substances) in relation to the structure (HSC, 1994). Design is no longer just a set of calculations but also includes: planning of construction matters, eg buildability, construction sequence, material use and safety. As such, this definition of design is in line with the “integration of design and construction” principle of the OM and will be adopted in this report.

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A construction project normally involves three parties: the client, the designer and the contractor. A design can be carried out by the client’s principal designer, ie the engineer or by designers employed by the contractor. In this report, both will be referred to as designers. The term “constructor” will be used to denote any person who carries out or manages construction work. The client’s principal designer normally develops the permanent works design. This is usually developed into a single predefined design or predetermined design, which is put out to tender. The designer may also provide an indication of the construction method. The contractor and the temporary works designer employed by the contractor are responsible for the construction method statement and any temporary works design. This is the traditional contractual division between the engineer and the contractor. In recent years the management contracting and construction management forms of contract have widened this division between the engineer and the contractor. CDM Regulation 13 places additional health and safety duties on the designer to apply the “principles of prevention and protection”. These require hazards to be identified and risks to be assessed. They require the possible construction methods to be examined and the balance of design details versus safety of construction workers to be assessed. Hazards should be eliminated through good design where possible, and where they cannot, risks should be as low as reasonably practicable. The “planning supervisor” and the “health and safety plan” required under the CDM Regulations provide additional links between the design stage and construction method. A definition of the OM developed for this report is given in Section 2.1. A detailed summary of the OM procedure is set out in Section 4. This has been developed from the Peck (1969a) and the EC7 (BSI, 1995) procedures, which are listed in Section 2. The Method requires at least two fully developed designs for the range of conditions foreseen at the site. The construction method has to be sufficiently developed to enable the construction to be changed if trigger criteria are exceeded. The objective is to achieve overall economy without compromising safety. A good understanding of the design process and construction method are essential; this draws together the designer and the constructor. This requires similar risk and cost-benefit studies to be made to those required under the CDM regulations to control health and safety risks so far as reasonably practicable. Most of the existing geotechnical design - eg BS 8004 (1986), BS 8002 (1994a) - do not refer to the OM, but EC7 (BSI, 1995) clauses 2.1 and 2.4 to 2.7 define four 30

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geotechnical design approaches or requirements, which are listed below. These approaches can be used on their own or in combination. 1. Design by calculations. 2.

Design by prescriptive measures.

3.

Design justified by load tests and tests on experimental models.

4.

TheOM.

Numbers 1 to 3 are primarily approaches for validating the proposed design. Design by prescriptive measures, 2 , has much in common with the semi-empirical procedures that have developed from case history experience, eg strut load design envelopes, Peck (1969a). In these cases reliable design calculation approaches are still being developed. The OM (4) can be used where there is a wide range of uncertainty regarding ground conditions and their behaviour. It combines the use of the design approaches (1 to 3) with flexible construction methods. The flexibility in the OM is provided by the integrated process of planning, monitoring, reviewing and timely implementation of planned modifications as shown in Figure 1.2. Box 3.1

Definitions of robustness

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Definitions of robustness have been provided by the Institution of Structural Engineers (1990) and the Health and Safety Executive (1996). The Institution of Structural Engineers (1Struct.E)

The 1Struct.E publication The Achievement of Structural Adequacy in Buildings (1990) states “Buildings are required to be safe during construction and during their intended life. Engineers translate this requirement into the provision of adequate strength and stability. In addition, structures should have a degree of robustness, so that they are unlikely to be sensitive to adverse effects, such as misuse or accident, leading to damage disproportionate to the cause. Robustness cannot be quantified in all circumstances, but in qualitative terms, considerable improvements can be usually be made by careful attention to structural layout, to detailing, to the provision of alternative load-carrying path.” The same publication by the 1Struct.E also states: “Buildings should have an acceptable general structural integrity that permits them to tolerate or avoid damage in the event of abnormal or accidental conditions. Structural form, type of structure and construction are all important in this context, as are provisions for continuity, ductility, toughness and redundancy. This desirable characteristic of structural systems has come to be known as robustness.” Health and Safety Executive (HSE)

The HSE (1996a) publication Safety of New Austrian Tunnelling Method (NATM) Tunnels states: “In engineering design the dimensions, strength and articulation of the structure should be chosen so that the structure can withstand the likely loading. A robust design, which is essential, is one where the risk of failure, or of damage, to the structure is extremely remote during its design life.”

An OM principle is that the design, construction and design modifications all have to be “demonstrably robust”. The Institution of Structural Engineers (1990) and HSE (1996) have provided definitions of “robustness”. These are shown in Box 3.1. This robustness is provided by identifying the hazards and risks and checking that the proposed design can adequately withstand them.

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3.2

RISK

3.2.1

Risk management and the Observational Method Risk management is the process of managing responses and policies to reduce and control risks. It is an integral part of the OM, which concentrates on technical risk. The merits of using the OM to manage ground risks have been discussed by Whitman (1984), Blockley (1994), Chowdhury (1994) and Godfrey (1996). It encourages the idea that not all the risks can be established before the start of construction and that designs and site procedures may be expected to change during construction to control the risks. In this sense, technical risks are only a part of risk management. Risks associated with health and safety, financial and programme time can also be managed. The OM can achieve economy when compared with predefined design, while the most probable conditions are encountered. However, allowance can be made for the most unfavourable conditions which may be worse than those assumed in the predefined design. The essence of risk management is that the potential hazards are identified during the design and planning stage of a project, measures taken to eliminate the risk if at all practical and the residual risks managed and controlled during construction.

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3.2.2

Definitions Risk management now has its own accepted methodologies and there are a few terms that must be understood and used consistently.

3.2.3

1.

A hazard is something that has the potential to cause harm, eg a container of flammable fluid or unknown ground conditions.

2.

Likelihood (or probability) is the frequency with which the hazard will manifest itself, eg once every 100 projects or a 1 per cent probability.

3.

Consequence (or hazard severity) is the outcome of the hazard manifestation, eg serious injury to an employee or .E100 000 overspend.

4.

Risk is the combination of the likelihood and the consequence (risk = likelihood x consequence).

Methodology There are four fimdamental stages of risk management: 1.

Hazard identification: where the hazards are identified and documented in a register via experience of similar projects, brainstorming, structured interviews etc.

2.

Risk assessment: where the likelihood and consequence of each hazard are evaluated and combined to estimate the risk corresponding to each hazard. A risk register is produced at this stage for systematic assessment of the risks.

3.

Risk reduction: where the hazards are eliminated if possible and the risks are reduced by a combination of design changes, procedural changes and additional monitoring.

4.

Risk control: where the risks are monitored, controlled and managed throughout the project. The risks normally diminish as the construction progresses.

These four stages have been compared to the elements of the OM and are shown in the following table based on Nicholson (1994). These are similar to those recommended by HSE for setting standards to control risks (HSE, 1991). Godfrey (1996) has further subdivided the stages into a ten-step procedure for systematically controlling risks. 32

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Table 3.1

Comparison between risk management methodology and the Observational Method ~

Risk management methodology

The Observational Method (see Table 4.1)

Hazard identification

Design and planning deskstudy site investigation 0 data interpretation.

Risk assessment

Design and planning 0 data interpretation initial design 0 final design.

Risk reduction

Design and planning 0 initial design 0 final design 0 trigger criteria.

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Construction control and monitoring 0 monitoring plan site monitoring. Risk control

Construction control and monitoring 0 monitoring plan 0 site monitoring. Review and implement planned modification Technical and procedural auditing

3.2.4

Risk tolerability criteria The acceptability of financial risks is always subjective and has to be determined on a project-by-project basis. HSE provide guidelines on the tolerability of human risks and the CDM regulations go far in ensuring that risks to humans are properly assessed and controlled. Technical risk tolerability is largely governed by: codes of practice, eg EC7

3.2.5

0

published damage criteria, eg BRE

0

public perception.

Tools and techniques for risk management CIFUA Special Publication 125 Control of risk: a guide to the systematic management of riskfrom construction (Godfrey, 1996) devotes a section to,risk managers’ tool boxes and provides guidance about several risk assessment and management techniques that range from the very basic to the highly sophisticated. The selection of the appropriate technique depends upon the complexity of the problem, the nature of the criteria and the available resources. However, all the techniques follow the methodologies set out in Section 3.2.3 above. The term “robustness” has been defined by the Institution of Structural Engineers (1 990) and HSE (1996); see Box 3.1. The CDM Regulations require risk assessments to

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identify potential hazards during and after construction. The robustness test process involves assessing these hazards and checking that the design caters for them in an acceptable manner.

3.3

COMPARISON BETWEEN THE PREDEFINED DESIGN METHOD AND THE OBSERVATIONAL METHOD

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In the traditional procurement of projects, a single robust and possibly over-conservative design is fully developed before site work starts on a particular phase. Monitoring is sometimes used, but in a passive way to confirm that design predictions are not exceeded. There is no primary intention to vary the design during construction. In this report, this procedure is referred to as the “predefined design method”. Current contractual frameworks often appear to maximise the separation between the design “product” and the construction “process”. However, the designer cannot now ignore the link required under the CDM regulations. The OM, on the other hand, requires the designer to consider the range of foreseeable conditions. Designs are developed for this range and construction modification strategies designed before work starts on any particular element of work. Planning is important to ensure that the modification can be implemented with sufficient speed to avoid the development of failure conditions. Monitoring is essential and is used in an active way to provide the data for the review stage. At this stage the monitoring results are compared with the predicted trigger criteria. Planned modifications - or, if required, emergency plans - can then be introduced if appropriate. Making arrangements for dealing with emergencies in construction sites is a legal requirement under the Construction (Health, Safety and Welfare) Regulations 1996 (HMSO, 1996). A comparison of the process for the predefined design and the ab initio OM is shown in Table 3.2. In the event of unforeseen conditions developing on site during construction, the best way out application of the OM can be introduced into either method.

If there is little uncertainty about the ground, there will be no need to follow the OM, as there is no proactive monitoring and planned modification. However, if there is great uncertainty, the predefined design method could lead to a solution that may be unsafe or later prove to be unnecessarily expensive. The OM can take account of the monitoring results and provide a safer and more economic solution, if appropriate, on certain types of project.

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Table 3.2

Comparison of the predefined design method and the Observational Method

The predefined design method

The Observational Method

Normally one set of soil parameters: eg moderately conservative or characteristic values (EC7) -but may do parametric study.

The range of foreseeable soil parameters is considered: eg most probable and most unfavourable.

One design and one set of predictions based on limited construction method considerations.

Two or more designs and construction methods are sufficiently developed to include predictions for trigger criteria.

A construction method option may be outlined sufficiently for the design to be progressed. This is subsequently developed by the contractor in the method statement

A flexible construction method statement is developed that can incorporate design changes and modification strategies. It is often developed jointly by the contractor. and the designer.

Monitoring limited to checking that predictions are not exceeded.

Comprehensive and robust monitoring that is regularly reviewed as the basis for management and design decisions.

Predictions unlikely to be exceeded. Therefore, construction programme is not constrained by monitoring results. If predictions are exceeded then unforeseen conditions have developed and the work may need to stop while the problem is resolved.

The design, construction method and construction programme may be changed depending on the review of monitoring results.

Management of construction, monitoring, interpretation and implementation of modification plan or emergency plan are required. The monitoring system must be sensitive enough to allow early discovery of a rapidly deteriorating condition. The modification plan must be rapidly implemented to ensure that the limiting trigger criteria are not exceeded.

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Emergency plans are needed to control failure.

Emergency plans must be introduced in accordance with the Construction (Health, Safety and Welfare) Regulations 1996. This can be achieved as an extension of the OM trigger criteria beyond the serviceability limit state to ensure that failure does not cause injuries.

The OM may be initiated at this stage in its “best way out” format.

It may be that the best way out OM can be introduced to overcome unforeseen ground conditions.

35

3.4

WAYS OF APPLYING THE OBSERVATIONAL METHOD TO UNCERTAINTIES IN THE GROUND The nature and degree of uncertainty in the ground vary from site to site. The complexity of the proposed construction and the adjacent environment may also introduce uncertainty to the selection and use of design parameters.

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The principal objective of the OM is to apply sufficient resources to prevent uncertainty in the ground leading to unacceptable levels of risk. In this report, the uncertainties are classified under the following three headings: Ground treatment uncertainty

A range of ground treatment techniques has been developed to improve different properties of the ground. The designer often identifies specific performance requirements for the treatment that meets the needs of the proposed structure. The effectiveness of the treatment is monitored and reviewed during the treatment and modifications are implemented where necessary to meet the specification.

Geological uncertainty

Where there are complex geological and hydrological conditions the ground may vary unexpectedly between boreholes. At the design stage, a conceptual model of the geological conditions is developed. In this OM application, modifiable design solutions are developed for the range of conditions. During site works the actual ground conditions are determined. The appropriate design solution is then selected to suit the actual conditions.

Parameter uncertainty

Uncertainties in knowledge of the ground and modelling its behaviour mean that it may not be possible to assess ground parameters accurately. The OM involves the development flexible and robust designs for a range of parameter values. During construction the monitoring results are reviewed and the design is modified as appropriate. This is the OM process identified in EC7. There may also be limitations in the numerical methods used for the design analysis. However, these are considered to be small when compared with the uncertainties in the ground parameters in the analysis.

Tunnelling in urban areas provides an example of the three ways in which the OM can be applied to ground uncertainty - see the simplified example in Figure 3.1. Ground treatment using compensation grouting might be used with a view to limiting amounts of settlement to selected building damage criteria. Face support involves geological uncertainty. It may be necessary to adopt horizontal boreholes and face logging to establish local ground conditions so that appropriate support can be selected. However, first it is must be established that “brittle” face failure will not occur and that there will be sufficient time to implement contingency and emergency plans fully. The sprayed concrete lining convergence monitoring provides data for the review of parameter uncertainty.

36

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Crack Without Compensation grouting, excessive settlement leading to cracking.

Acceptable level of Settlement

Settlement without compensation grouting

__---

’

/

/ / /

/

/

/

/

/

/

/

/

’”

-

-

-

/ ,I’

,*’ ,,’

G ROUND T R EATM ENT UNCERTAINTY settlement control using compensation grounting

Final grouting

0

0

Preconditioning ,,,’

Initial grouting

’//////// Convergence

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NATM Tunnel

///

__---

’//////// PARAMETER UNCERTAINTY convergence monitoring lining pressure

Figure 3.1

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GEOLOGICAL UNCERTAINTY -tunnel face logged - variable ground - scour holes - greasy backs - buried channel, etc.

Simplified example of ground treatment, geological and parameter uncertainties

37

Ground treatment uncertainty Ground treatment operations provide the engineer with the opportunity to improve the ground to a standard required by the design. Examples of ground treatment where the OM is used to monitor, review and modify the treatment are summarised in Table 3.3. Table 3.3

Treatment

Concurrent monitoring Direct

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Vibrocompaction

Examples of using the OM in ground treatment uncertainties

Post-treatment monitoring

Field testskrials

Modification

pressuremeter SPTICPT

plate or zone test

additional treatment or closer grid spacing

zone tests

additional treatment

Indirect

input energy

Dynamic compaction

surface settlement

SPTICPT pressuremeter

Grout injection for strength

volume pressure flow rate

areas pressuremeter SPT

regrout using tube h manchette or closer grid

Permeation grouting to reduce permeability

heave seepage

volume pressure flow rate

permeability test

pump test

regrout using tube h manchette or closer grid

Jet grouting for strength and permeability reduction

temperature groundstructure displacement

slurry density . drilling properties

slurry strengths SPTICPT core

trials

increase grid or reduce spacing revise process

Compensation grouting

grout volume heave

displacement

grouting trial

additional stages of grouting

Driven piles

pile set driving analysis

heaveluplift

trial piles

redrive or additional pile

Dewatering

pump rates piezometer pressures

settlement

preliminary pumping tests

additional walls or larger pumps

redrive after set-up with CAPWAP

There can be uncertainty as to whether: 0

the treatment method is appropriate to achieve the required standard

0

the quantity of treatment needed to meet the design standard is the most costeffective option.

Initial field trials and tests are often used to check that the treatment method works. The following types of monitoring have been identified to check the performance during and after treatment: 0

concurrent monitoring - direct and indirect post-treatment monitoring.

Examples are given in Table 3.3. OM procedures are readily applied to the treatment operations. The objective is to use the monitoring results to confirm that the treatment has met the specified standard and, where necessary, to modify the treatment (process) design by increasing or decreasing the amount of treatment. ClRlA Report 185

The monitoring should take place as the treatment work progresses to record its effectiveness. Where possible, this monitoring should provide measurements that are directly related to the performance specification. Examples of direct monitoring for ground treatment include:

0

compensation grouting

tilts measured by electrolevel and settlements by precise levelling

vibro compaction

vibrator energy inputs

drivenpiles

driving record and pile-set or pile-driving dynamic analysis

Examples of indirect monitoring for ground treatment include: 0

tube B manchette grouting

grout injection volume, pressures and flow rates

dynamic compaction

surface settlement

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Where direct monitoring is not available for the assessment of the degree of improvement achieved by the treatment, more reliance is placed on a programme of index tests carried out after each round of treatment. These may include SPT, CPT pressuremeter and laboratory tests or samples. Load tests or field trials are used to develop parameter correlations with the index tests. Performance criteria are often developed from these tests. If at the review stage the ground treatment has not achieved the specified performance, a decision can be taken to modify the treatment. Usually this would involve additional rounds of treatment, eg

0

3.4.2

compensation grouting

additional grout injection from tubes B manchette

vibro compaction

additional treatment points on a closer grid

driven piling

redrive or install more piles.

Geological uncertainty Geological uncertainty results from limitations of the site investigation, such as exploratory hole depth and spacing, sampling frequency and testing regime in relation to the variability of the local geology. Decreasing the borehole spacing may reduce the risk that inevitably remains. Three geological uncertainty conditions are important for OM applications: 1.

Complex geological conditions, eg zones of faulting, variable and deep weathering, heterogeneous strata and contaminated sites.

2.

Geological conditions in which there may be unexpected natural and man-made features, eg solution features in karstic formations, scour hollows in London Clay, buried channels, old mine workings, old foundations:

3.

Geological conditions that determine permeability and drainage paths, eg where heavy rainfall or a burst water main could cause a rapid change in water pressure.

Construction schemes and design modifications are prepared for the anticipated range of geological conditions before the start of site works. These are applied appropriately as the ground conditions are revealed during construction. Water-bearing strata can be critical to the viability of the OM because of the possible development of rapid failure conditions (eg resulting from higher than expected water pressures, softening or out-wash and collapse of sandy strata). There may be insufficient ClRlA Report 185

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time to install contingency systems such as dewatering or grouting to overcome a groundwater problem.

3.4.3

Parameter uncertainty Accurate prediction of geotechnical behaviour depends upon reliable estimates of ground parameters for use in design. There are difficulties, however, in extrapolating from small-scale tests to the behaviour of a mass of soil and to representing the construction sequence. Thus, for example, construction monitoring often shows measured deformations to be well below those calculated. Nevertheless, our understanding of soil mechanics is improving, as are the numerical and analytical prediction methods. The limitations of sampling and laboratory testing are understood and many soil parameter correlations have developed from case histories and large-scale field tests. There is a need, however, continuously to revise and extend these ground parameter correlations.

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The OM methodology developed by Peck (1969a) and set out in EC7 (BSI, 1995) deals mainly with handling parameter uncertainty.

3.5

DESIGN CODES AND DESIGN CONDITIONS

3.5.1

Development of design codes The history of the development of the OM is summarised in Section 2. It is important to understand changes to methodology and parameter value definitions over the past 30 years to help in appreciation of OM literature and how best to integrate the OM into current codes. A summary of the terms used to describe design conditions and parameter values is given in Table 3.4. 1960s

Codes were based on permissible stresses and lumped or global factors of safety. Peck (1969a) used the terms “most probable” and “most unfavourable” to describe his range of soil conditions. An interpretation of these is illustrated in Figure 3.2.

1970s

Structural codes such as CP110 (BSI, 1972) introduced limit state design methods. These involve designing for the most critical limit state and checking that other limit states will not be reached.

A limit state is a condition beyond which a structure no longer satisfies the design performance requirements. Thus, ultimate limit states (ULS) are associated with the onset of collapse, danger or serious damage. Serviceability limit states (SLS) correspond to conditions beyond which specified service requirements for a structure or structural element are no longer met, eg doors not closable. These include the effects of settlement and cyclic loading. A partial factor approach was introduced where a design value (&) is a material property value obtained by dividing the characteristic value (&) by a partial factor (y,,,). For the serviceability limit state y, = 1.O. The characteristic values (Xk)are the material property values having a prescribed probability of not being attained in a hypothetical unlimited test series, eg the 5 per cent fiactile of concrete cube strength test results (no more than one in 20 cubes would have a lower strength) - see Figure 3.2. 40

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t

I I

Figure 3.2

I

.E I

I I

Types of soil strength parameters

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Geotechnical engineers found it difficult to work with the ideas of characteristic values, particularly of strength.

1980s

In the 1980s, additional guidance was introduced in the form of CIRIA Report 104 (Padfield and Mair, 1984) and BS 81 10 (BSI, 1985). CIRIA Report 104 also introduced “moderately conservative parameter values”, which are defined as being worse than the probabilistic mean but not as severe as the worse credible. This value is sometimes termed a “conservative best estimate”. It does not have a precise material property specification value (see Figure 3.2 and Table 3.4). CIFUA Report 104 and BS 81 10 also use the term “worst credible parameter”, meaning the worst value that the engineer believes might occur in practice. Simpson et al (1979) considered this value to represent the 0.1 per cent fractile. It may be similar to the most unfavourable condition of Peck, see Table 3.4.

1990s

BS 8002 (BSI, 1994a) developed the moderately conservative values in terms of representative or conservative values. It also identifies “most likely” values, which are considered to be similar to the most probable parameter values - see Table 3.4. Powderham (1994) discusses the development of the OM and introduced the term “progressive modification” as well as the concept of a “more probable” value - see Table 3.4. Eurocode 1 (BSI, 1994b) was introduced as a basis for design. It recommends the 5 per cent fractile for characteristic strength properties and a 50 per cent fractile for characteristic stiffness properties. For geotechnical design, Eurocode 7 (BSI, 1995) applies these definitions to ground properties as follows. (Clause numbers below refer to EC7.)

Characteristic values (cl 2.4.3) for soil and rock properties are based on laboratory and field tests with conversion factors to allow for testing scale,

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fissuring etc. These characteristic parameters are also to be selected as cautious estimates of the value affecting the occurrence of the limit states. In a more detailed “application rule”, EC7 suggests that the cautious estimate is equivalent to a set of parameter values with a 5 per cent probability that a worse value would govern the occurrence of a limit state. (Here “govern” implies that this adverse value would occur in a sufficient quantity and disposition to be the dominant value of the parameter as a limit state occurred.) As all the partial factors used in serviceability limit states calculations are 1.O, it is expected that 5 per cent of the monitoring results may exceed this limit state. The definition can also be extended to the most probable set of parameters, where not more than 50 per cent of the measurements exceed this state. Partial material factors (cl 2.4.2 (15)) have been introduced for three

geotechnical design cases. Limit values for foundation movements (cl 2.4.6 (1)) are the values at

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which ultimate or serviceability limit states are deemed to occur.

Observational Method soil parameters and conditions (material properties)

Source

Most probable Observational Method (Peck, 1969a)

Most unfavourable Most unfavourable

Most probable

CPllO (1972)

Characteristic

CIRIA Report 104 (1984)

Moderately conservative (conservative best estimate)

Worst credible

Worst credible

BS8110 Part 2 (1985)

Expected or mean (for concrete modulus)

Characteristic

Eurocode l(1994)

Mean for stiffness (50% fractile)

Characteristic for strength (5% fractile)

BS8002(1994)

Most likely

Representative or conservative values

Observational Method (Powderham, 1994) Eurocode 7 (1995)

42

Moderately conservative

I

Most probable

1

More probable

I

Moderately conservative

Most unfavourable

Characteristic cautious estimate

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3.5.2

Design conditions Peck (1969a) refers to most probable and most unfavourable “conditions”. These conditions comprise sets of relevant soil and groundwater parameters. Terms such as “most probable” and “most unfavourable” apply to both parameters and conditions. Eurocode 7 (BSI, 1995) refers to these conditions as “design situations”. The code requires that detailed specifications for design situations are developed and that each situation shall be verified to be sure that no relevant limit state is exceeded. These are considered with the appropriate loading conditions. The process of identifying conditions or design situations has much in common with the CDM regulations, which require that risk assessments are made and that checks are carried out in order that a robust design be explicitly developed.

3.5.3

Comments on codes

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Table 3.4 compares the soil parameter classes adopted by different guides and codes. Modern codes identify two of the three classes, one of which is moderately conservative. While Peck (1969a) identified the two parameter conditions of “most probable” and “most unfavourable” (these are shown in Table 3.4 in relation to other codes), he did not use the “moderately conservative” class. This makes it difficult to relate the different design approaches. For consistency in hture, it is recommended that the OM uses: 0

“most probable” and “moderately conservative” conditions for deformation and load predictions - serviceability limit states (SLS) designs “most unfavourable’’ conditions for ultimate limit states (ULS) designs and robustness check during risk assessment.

3.6

ESTABLISHING DESIGN VALUES The design values for ultimate limit state (stability) calculations are chosen so that the probability of failure will be acceptably small. They should take into account uncertainties inherent in that part of the design to which they are applied. The CIRIA,Report 104 approach to a design value is taken as:

Design Value= Soil Parameter Valuex

I Factor of Safety

The EC7 (BSI, 1995) definition of the design value for a ground property is:

Design Value= Characteristic Value x

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Partial Factor

43

Using a parameter value distribution such as shown in Figure 3.2 to select the same design value by the two approaches means that different factors of safety are identified, as shown in Figure 3.3. The normal distribution curves shown in these figures are for illustration purposes. The moderately conservative value is defined by a zone. The selection of a specific value within this zone depends on the type of foundation and the ability of the soil and foundation to redistribute local inequalities of load. ?Fmp I

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F m u F m C I

1

1 I I

1 1 I

-

I Design value =

i

+ I I I

I

I

I

I Fmp = Factor of safety on most probable I parameter value

I

.E

t t 1 in 1000 1 i n 2 0

Soil parameter value x (1 I Factor of safety )

Fmc = Factor of safety on moderately

t 1 in2

Soil Strength Parameter Results

b

Figure 3.3 Application of factor of safety to different types ofparameter values

fi I I

I 1

Predicted most probable value

"Ideal" distribution of measured deflections

Figure 3.4

44

Predicted EC7 ,/Characteristic value

Ideal relationship between predicted and measured performance, following E c7

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3.6.1

Serviceability limit state predictions - For serviceability conditions, such as crack width control in reinforced concrete,

Eurocode 7 (BSI, 1995) adopts characteristic values as the design values - ie design values - are derived using a partial factor of 1.O. Therefore, movement predictions made using these characteristic values should result in cautious estimates of the measured movements (statistically implying that 5 per cent of the measured movements may exceed these predictions). This is illustrated in Figure 3.4, where the distribution of measured deflection is consistent with the prediction based on characteristic values.

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Measured most probable value

I

Measured Characteristic V: e Predicted EC7 characteristic value (very cautious from Figure 3.4) I

I

P I

Factor of ignorance

Figure 3.5

I I

Poor agreement between predicted and measured performance

Experience with some types of structure, such as retaining walls, has shown that field measurements are often less than predictions, as illustrated in Figure 3.5. In the example it shows, the predicted characteristic deflection is a “very cautious estimate”. The term “factor of ignorance” has been used to illustrate the difference between this “predicted characteristic deflection value” and the “measured characteristic value from performance” that corresponds to the measured 5 per cent fractile. Using EC7, it is possible to back-analyse and reassess “cautious estimates” of characteristic parameters that give calculated deflections corresponding to the measured characteristic value. This is the progressive modification approach in the OM, discussed in Section 3.10. Uncertainties in the ground and lack of accurate parameter correlations often mean adopting “very cautious estimates” for the characteristic values in the predefined design method. When planning to use the OM, it is helpful to consider deformation predictions based on “most probable” parameter sets. These predictions may be about 30 ClRlA Report 185

45

per cent less than the “characteristic” predictions, but will depend on the specific problem (see Figure 3.6). These “most probable” predictions can also be used in the assessment of amber trigger criteria (see Section 3.9). During construction, comparisons can be made between the “measured characteristic value” and the “predicted most probable value”, as in Figure 3.6. In this example it can be seen that there is reasonable agreement between the two values. In the progressive modification approach the “calculated most probable” prediction could then be used in the assessment of the revised cautious estimate (characteristic value) conditions.

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For the example illustrated in Figures 3.5 and 3.6 the predicted characteristic value has been taken as the starting point in the OM progressive modification approach.

Figure 3.6

46

Comparison of predicted most probable and measured characteristic value

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3.6.2

Ultimate limit state predictions

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Eurocode 7 (1995), see Table 3.5, identifies three sets of partial factors to apply when assessing the ultimate limit state. Cases A and C in Table 3.5 involve applying partial factors to the characteristic values of the ground. These factored parameters (ie design values) are similar to the most unfavourable conditions. A most unfavourable deformation analysis made using these factored values directly provides one assessment of the ultimate structural forces, moments and shear forces (partial factor on actions being 1.O). This deformation analysis also provides the deflections and curvatures associated with these ultimate conditions. It is therefore possible to compare deformation predictions for this ultimate condition with the most probable and characteristic design value predictions of deformation. This is illustrated in Figure 3.7. These ultimate limit state deformations provide useful input into the predicted maximum values for the red condition, see Section 3.9, and the development of emergency plans. Simpson et a1 (1 979) noted that the frequency of occurrence of the ULS is about 1O-6. In addition, it is unnecessarily conservative to consider all parameters and loadings at their worst credible values at the same time.

f\

Distribution of measured deflections

/

7 Figure 3.7

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limit state characteristic

Ultimate limit state I

I I I

I I I I I I

I I

I I I

I

5%

I I

I I

Ultimate limit state prediction

47

Table 3.5

Partial factors - ultimate limit states in persistent and transient situations (EC7, Table 2.1)

Case

Actions

Ground properties

Permanent

Variable

tan$'

c'

C"

4.

Unfavourable

Favourable

Unfavourable

Case A

1.00

0.95

1.so

1.10

1.30

1.20

1.20

Case B

1.35

1.oo

1.50

1.00

1.00

1.00

1.00

Case C

1.00

1.oo

1.30

1.25

1.60

1.40

1.40

* Compressive strength of soil or rock.

3.6.3

Empirical approach to design

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The empirical approach to design involves: 0

previous experience in similar ground conditions

0

previous experience of similar structures, including monitored and interpreted performance of the system

0

monitoring of proposed system during construction modification plans based on a review of the records.

The Peck strut force envelope (Peck, 1969a) is an example of monitoring and empirically interpreting maximum (conservative) strut forces for deep excavations. These envelopes are derived from case history experience for different ground conditions and have recently been revised by Twine and Roscoe (1997) to include additional case history data. These envelopes provide characteristic prop forces for use with EC7 for design by prescriptive measures. However, it is likely that the most probable values are considerably less than the envelope. Peck (1969a) quotes the Harris Trust excavation in Chicago where the most probable prop forces were about two-thirds of the design envelope values. In the Chicago example, the OM was used. The modification plan was to install additional struts where loads greater than the most probable values began to develop. The empirical approach is often used in tunnelling. Barton and Grimstad (1994) refer to the Norwegian Geotechnical Institute (NGI) Q-system for rock mass quality, which is related to tunnel size and support system selection, eg shotcrete and rockbolts. This is an example of an empirical approach applied to complex rock mechanics problems.

3.7

PARTIAL FACTORS OF SAFETY The factors of safety used in the ultimate limit state calculation depend on: the soil parameter value used, eg characteristic or worst credible (see Figure 3.4) 0

the design case - eg EC7 identifies three different design cases, under which different partial factors are used (see Table 3.5) the risk and severity of failure - EC7 identifies three geotechnical categories for assessing risk, but does not give separate factor values (at present).

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The different factors of safety that are used for slope stability analysis in Hong Kong are shown in Figure 8.5. These illustrate the link between factor of safety and consequence or hazard severity. Another example is the guideline for designing spoil heaps and lagoons published by the National Coal Board (1970), which recommends the use of different factors of safety for design in accordance with the risk categories. The OM should not be used if at ultimate limit states there would be significant risk to people. In these cases, it may not be possible to evacuate people from the construction site or from the adjacent properties. Risk to life implies rapid deterioration of conditions and there may not be sufficient time to implement modification or emergency plans. Risk assessment is an important tool for assessing the required factor of safety. The OM provides a means to reduce the likelihood of a hazard occuning.

3.8

RAPID DETERIORATION The OM operates most effectively when conditions deteriorate only gradually to the design limit states. This enables monitoring records to be reviewed so that there is time after discovery for the modification plan to be implemented. The faster the deterioration rate the greater the requirement for continuous monitoring with immediate review and modification plan implementation.

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Besides quality-control measures, material quality and workmanship, the rate at which deterioration develops also depends on: ground conditions groundwater conditions 0

temporary surcharges construction sequence and programme.

3.8.1

Ground conditions Different types of soil exhibit different stress-strain behaviour. Ductile and brittle soil behaviour is illustrated in Figure 3.8. Ductile behaviour is associated with reducing soil modulus prior to the peak strength with little reduction in the post-peak strength. Brittle behaviour is associated with near-constant soil modulus at strains up to the peak strength and a rapid loss of strength with strain past the peak strength and then a gradual reduction to residual strength after the peak strength. Additional information on brittleness is provided by Bishop (197 1) and Chandler (1 984). Brittle behaviour often leads to progressive failure. This is associated with the transfer of stress to the adjacent soil, which in turn becomes overstressed, leading to progressive and rapid development of an extensive failure. The OM should not be used where these conditions are anticipated or when the sudden failure cannot be contained locally to arrest the development of progressive failure.

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zp= Peak shear strength z, = Residual shear strength

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Strain Figure 3.8

3.8.2

Brittle and ductile behaviour

Groundwater conditions Rapid deterioration can also result from a sudden generation of high pore-water pressures as a result of 0

rainfall

Rapid pore-pressure increases as a result of heavy rain are the cause of many of the slope failures in Hong Kong. Motorway cutting slope failures in the UK are examples of slower seasonal effects.

0

rapid shearing

Examples include liquefaction of loose saturated soils or where mud flow conditions develop.

0

burst water mains

A main road in the Central District of Hong Kong was closed for several weeks when a burst water main led to the collapse of an adjacent sheetpile wall.

Permeable wall systems such as soldier or contiguous pile walls allow water pressures behind the wall to dissipate naturally and therefore are self-compensating. However, the seepage into the excavation or tunnel can erode and transport the adjacent soil, which may cause local settlement and collapse of adjacent ground.

3.8.3

Temporary surcharges Temporary surCharges can rapidly apply unexpected loads to a structure. The scope of the OM can be extended to assess and monitor these conditions. This surcharge problem can also be encountered with predefined design method. However, the monitoring and reviewing framework is not present in a predefined design, and therefore, modifications cannot be implemented to combat this effect in time.

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3.8.4

Construction sequence and programme The construction sequence can be used to control the rate of deterioration in two ways. 1.

Using the multi-stage construction process.

In these cases the factor of safety gradually reduces as each stage is added. Predictions of movements can be made at each stage to identify trigger values for forces and deformations. An example is embankment construction on soft clay in a series of lifts where the monitoring results are reviewed before the next lift is started. Other examples include excavations made in a series of stages or levels. Where necessary, planned modifications are implemented. 2.

Using the incremental construction process.

An example is the tunnel face during NATM tunnelling work. The tunnel is advanced in

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increments, keeping a relatively constant factor of safety at the tunnel face as the work progresses. In these cases, it is difficult to assess predictions for intermediate stages and rates of change of movement are important. Other examples include large single-stage excavations. Planned modifications include smaller excavations. These provide increased three-dimensional support and speed up the construction cycle, which limits the extent of rapid deterioration.

3.9

TRIGGER CRITERIA AND TREND RATE

3.9.1

Trigger criteria Trigger criteria are the limits for implementing the planned modifications in the OM. They can also be used, if necessary, for implementing emergency plans. These criteria can be values of forces or movements, which can be assessed by calculation or by empirical procedures for the range of conditions considered in an OM design. They can also be based on ground conditions, workmanship or material quality where large uncertainties exist. Figure 3.9 summarises some factors that need to be considered when defining trigger criteria. Card and Carder (1996) also provide advice on suitable movement trigger limits to allow the OM to be implemented for the design and construction of highway retaining structures. Their advice is based on the results of a TRL study of wall movements where retained cuttings and cut-and-cover tunnels were being constructed for highway schemes (Carder, 1995). Trigger criteria are developed from an understanding of the time it takes to implement the recovery cycle. This includes discovery time, review and decision-making time and the modification implementation time. The time available depends on the rate of deterioration (as discussed in the previous section). The HSE (1996) introduced the Discovery-Recovery model for tunnelling work. This is illustrated in Figure 3.10. It shows the level of risk against time. The health and safety risk to individuals should always be within the acceptable risk level for the project. The likelihood of failure may depend upon the rate at which risk increases. If the rate of risk development is fast or there are no readily identifiable precursors of failure (see Section 3 . Q there will be little warning and little prospect of preventing a major failure. Moreover, the greater the initial level of project risk, the less time there will be for taking action. Early discovery of the adverse situation allows effective recovery

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measures to be put in place through contingency planning. Early discovery is crucial in providing sufficient time to enable a recovery to be made. It also reduces the levels to which risk may rise (see Figure 3.10). Primary and secondary monitoring systems are usually installed on OM projects, see Section 4.4. The primary system is simple and controlled and reviewed by site staff. It is used for routine monitoring, and the results are checked against the trigger criteria. The secondary system provides additional data for the designer. It also acts as a check and as a back-upxsystem and supplement to the primary system.

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The green-amber-red traffic light system is often used on OM projects to represent the condition of different parts of the work. As an example, Figure 3.1 1 shows the application of traffic light system to the incremental construction process used for the NATM tunnelling. This figure is based on the HSE’s Discovery-Recovery model. Specific definitions are developed to suit each project, but the following principles may be used: Green condition

The monitoring/review process shows that the monitored values are less than the amber trigger value (see Figure 3.1 1). The construction work is in a safe condition.

Amber condition

The amber condition is reached when the monitoringlreview process discovers results exceeding the amber trigger value or trend rate. The amber trigger is set from a consideration of: 0

most probable predictions the duration of the decision-making period (see Figure 3.1 1).

During the decision-making period, the monitoring frequency of the primary instrumentation system is increased to provide checks on data. The results of the secondary instrumentation system are also reviewed in more detail and additional readings taken. Some minor modification plans may be put in place depending on the specific site strategy. Red condition

The red condition is reached when the red trigger value is exceeded. At this point the planned modification must be mobilised to ensure that the relevant (normally serviceability) limit state is not exceeded. The start of the red condition depends on the implementation time for the modification (see Figure 3.1 1).

The trigger criteria for the OM traffic light systems depends on the project type and site location. By way of example, two types of sites are considered: 0

greenfield site The serviceability limit state of the proposed structure will be the main concern. For some low-cost structures, such as small embankments, the ultimate limit state may be considered where people’s safety is not at risk. built-up area

52

The serviceability limit state of existing adjacent structures will often impose tighter movement criteria. A set of damage criteria, such as those by BRE (Burland et al, 1977), should be agreed with the owners of adjacent properties.

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Emergency condition

3.9.2

The Construction (Health, Safety and Welfare) Regulations 1996 (HMSO, 1996) make it a legal requirement to make arrangements for dealing with emergencies at construction sites. The OM traffic light system could be extended to incorporate emergency plans, but should an emergency arise the situation has gone beyond application of the OM, ie unacceptable damage would be taking place or about to take place. However, the available monitoring systems may still be of value in coping with the emergency by helping in the recovery of control.

Trend and rate of change

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Apart from understanding the limits of behaviour of the project structure and adjacent buildings, the project team need to develop an appreciation of the trend (or pattern of movement) and the rate of change of monitored values with time. This is particularly important for incremental construction processes, such as NATM tunnels, where there are no intermediate-stage trigger values. An abnormal trend may occur because of 0

bad workmanship or poor materials

0

insufficient support

0

unforeseen ground conditions.

Monitoring convergence in NATM tunnels is an example of this approach. Figure 3.12a shows an average convergence curve derived from the monitoring results in earlier sections of a tunnel or from case histories. This convergence curve represents the “most probable” condition. Once this curve has been established, the convergence measured in subsequent tunnel sections can be compared with this benchmark. Assuming that the rate of construction is maintained, any abnormal trend can be identified immediately. Figure 3.12b shows the convergence curve from another section of the tunnel where large convergence is developing, eg due to unfavourable ground conditions. The initial gradient of this convergence curve is steeper than that of the “most probable” condition in Figure 3.12a. Extrapolation of this curve at the initial stage indicates that the serviceability limit state would be exceeded unless the construction sequence is modified. Early discovery and timely review of this abnormal trend will provide sufficient time to implement the planned modification fully before the serviceability limit state is exceeded. In this case, the planned modification is to close the lining earlier (see Figure 3.12b).

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Trend and rate of change

57

3.9.3

Multi-stage construction trigger values The construction sequence can often be a multi-stage process. Predictions of monitored values, eg deflections, can be made for each stage based on “most probable” and “characteristic” (moderately conservative) parameters or conditions. These are shown in Figure 3.13, with the corresponding traffic light conditions. Monitored values for two scenarios are plotted on Figure 3.13. If these plot in the red condition, such that they would lead to the SLS being exceeded, the planned contingency will need to be introduced. If these values consistently plot in the green condition, then opportunities for cost savings can be made through the introduction of planned modifications.

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58

Multi-stage construction trigger values

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3.10

IMPLEMENTATION OF PLANNED MODIFICATION In 1969, Peck stated the eight ingredients of the OM, which are listed in Section 2.2.2. Two of these ingredients are as follows: c.

Establishment of the design based on a working hypothesis of behaviour anticipated under the most probable conditions.

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d. Selection of quantities to be observed as construction proceeds and calculation of their anticipated values on the basis ofthe working hypothesis.

However, Powderham (1994) noted the difficulties in implementing a design based on the most probable conditions. He commented: “The base design noted in [Ingredient] (c) is derived from a working hypothesis of anticipated behaviour. This has a strong conceptual element. The process will involve the selection of less conservative design parameters than normal but also require judgement of the more unquantij?ablefactors. Typically these will include non-linear, three dimensional and time dependent effects. The evaluation of such factors may be usefully aided by powerful analytical tools such as finite element or finite difference methods. However, analysis cannot supplant judgement. Possible modes of failure -particularly those of a sudden or brittle nature or those which could lead to progressive collapse - must be assessed carefully. Thus it is not simply a question of analysis based on certain geotechnical or structural parameters. Indeed, it is a fundamental element of the Observational Method to overcome the limitations of analysis by addressing actual conditions. In this context [Ingredient] (c) may present diflculties associated with the term most probable. The most probable conditions will necessarily embrace the more unquantifiable factors noted. Although it is essential to assess the potential influence of such factors, to implement directly a design based on any beneficial effect relating to them may be inappropriate or unacceptable. Some margin of conservatism is always necessary. Careful judgement is necessary to balance the measures to assure safety with the potential savings. ”

Implementation of planned modifications based on Peck’s approach (1969a) and by “progressive modification” (originated from Powderham, 1994) are both discussed in this report. The “progressive modification” approach is preferred. Peck’s approach is only recommended where previous case-history data are available on comparable ground conditions and a multi-stage construction sequence is planned. Furthermore, the design and construction team should have a clear understanding of the objectives, key criteria and individual and collective responsibilities. In today’s health and safety environment, the course of action for the designer has to be set in terms of H&S law; and this means a risk-based approach taking account of what is reasonably practicable. If the cost difference between the “most probable” and “most unfavourable” sets of circumstances is small but the health and safety disadvantage is considerable, the designer should choose the criteria that take sufficient account of the need to avoid foreseeable risk and combat risks at source.

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1. Approach based on Peck (1969a) This recommends that construction starts with a design based on the “most probable” conditions with a contingency plan prepared for every foreseeable significant deviation (see Figure 3.14). In multi-stage construction processes, such as embankments and excavations, the initial stages have high factors of safety and hence the initial risk of failure is low. The factor of safety reduces as the embankment height or excavation depth increases. The monitoring results are reviewed and compared with the predictions/trigger values based on the most probable and moderately conservative conditions. If necessary, the planned modifications can be introduced before the critical stages of construction are reached. This method can be usefd if it is anticipated that the most probable predictions are thought to be conservative and likely to be similar to the actual moderately conservative/characteristic values - see Section 3.6.1. ’

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Construction commences with a design based on the most probable conditions. If interpretation of instrumentation results indicates that the performance is satisfactory, proceed with the MP design as planned.

2.

During the initial stage of construction, the monitoring record may show that the performance of the design is approaching the limit of the acceptable level of risk.

3.

Mobilise resource to implement planned modification, based on MU conditions. Hence, the level of risk is lowered back to the original level.

Figure 3.14

60

b

Approach based on Peck (1969a)

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2.

Progressive modification

In light of the difficulties in directly implementing designs based on Peck’s most probable conditions many designers (Ikuta et al, 1994) have started construction with predefined design parameters. Later, they have reviewed performance and upgraded the parameters by back-analysing the performance and rerunning the predictions. Powderham ( 1994) defined the term “progressive modification” for this approach. Powderham considered that it might be more appropriate to base the initial design on “more probable” conditions rather than “most probable” conditions. The term “more probable” is used here to denote conditions that are between “moderately conservative” and “most probable”. In a later paper, Powderhamand Nicholson (1 996) considered that when applying progressive modification, it may often be appropriate to start with predefined design parameters, ie with moderately conservative (characteristic) parameters since these are understood, and a design produced for these parameters in order to develop cost comparisons.

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Construction begins with a design based on the Mod C conditions.

2a. During the initial stage of construction, monitoring record interpretation indicates that the design is conservative. therefore the actual risk is lower than that initially anticipated. This provides an opporiunity for optimising the design. 3-4. Progressively modifying towards a revised Mod C value. 2b. During construction, the monitoring record might show that the present design is approaching the limit of the acceptable level of risk. Therefore, resource should be mobilised to implement the planned modification (based on MU conditions). Hence, the level of risk is reduced back to the initial level.

Figure 3.15

Progressive modification (simplified diagram)

Use of this approach is shown in Figure 3.15. Construction starts with characteristic or moderately conservative conditions. The results from the initial phases (or previous parts) of excavation can then be reviewed. Comparisons can be made with most probable predictions (see Figure 3.15). If a large proportion of the monitoring data plots below the initial most probable line it is possible to redefine the characteristic design ClRlA Report 185

61

parameters, as discussed in Section 3.5. These revised parameters are used to redefine and economise on the construction sequence - see Figure 3.15. This modification process can be repeated several times. Here, design changes can be reviewed and implemented sequentially. Each change can be initiated from a position of known safety through an incremental step of monitored and established safety. The balance between kisk and cost can thus be reviewed and adjusted appropriately (Powderham and Nicholson, 1996). This approach is preferred where the design and construction team has limited experience of the OM or where incremental construction is proposed, eg tunnelling.

VALUE MANAGEMENT

3.11

Value management (VM) is the management of the value process throughout a project in order to ensure maximum "value" is delivered. Both VM and the O M have as their primary objective elimination of unnecessary or avoidable cost, while meeting other project objectives of time and safety. The value criteria of different parties involved in the project are as follows:

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The client would like:

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full information to be made available at the right time and in a form that can be understood, so that properly informed decisions can be made concerning any issues related to the O M

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time a n d o r money savings to accrue to the client from the introduction of a solution more directly related to the conditions encountered during construction (ie OM)

e

the contract sum and the contract period to be reduced or not to be exceeded

e

there to be no reduction in the quality of the end product should design and construction methods change

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the designers, contractors and specialist subcontractors to co-operate in the client's interests towards the best solution

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no additional risk to be transferred from other parties to the client to remain in control of the decision process safety.

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The consultant designedengineer would like: unrestricted collaboration of all the parties involved an open-book policy enabling full and proper details to be made available by the contractor on which to base reports and decisions any additional costs arising from OM-related duties to be reimbursed the duty of a consultant to act fairly between the client and the contractor not to be compromised no unacceptable risk to reputation or professional indemnity cover safety. The checker would like: all costs arising from iterations of the checking process to be reimbursed no additional risk to professional indemnity cover safety. 62

ClRlA Report 185

The Contractor would like: to maintain or improve safety levels the expected profit element in the contract not to be diminished in any way by use of the OM

0

cash flow to be fully maintained in relation to the continuing cost of the works sufficient incentive to make it worthwhile to proceed with the OM

0

no additional risk to be transferred to the contractor or specialist subcontractors without fair compensatory payment.

The insurer would like: .

no material change in the matters covered by the insurance .policies taken out by the parties in respect of the project proper notice and details of changes if changes do become necessary, to enable policies to be amended and new premiums agreed

0

safety.

The adjoining owner would like: 0

no additional risk of damage emanating from any OM proposal

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no additional nuisance the original period of interference with the use and enjoyment of the property not to be exceeded all proper notices to continue to be given and appropriate time for response allowed safety. Sir Michael Latham (1994) in his report entitled Constructing the Team reviewed the procurement and contractual arrangements in the UK construction industry. He made 30 recommendations, high among which was the need to target reduction of real construction costs by 30 per cent by 2000. Latham did not explain how this 30 per cent cut can be achieved, other than by pointing out that there is considerable scope for improving the efficiency of the UK construction industry and therefore lowering the prices that clients have to pay for buildings. Latham considered [Clause 7.3 in his report] that “. . .better collaboration in design between consulting engineer and specialist couldproduce a target saving of 20%’. The OM promotes this collaboration between designer and constructor. A brief explanation of the value management process follows, before relating it to the OM (see Section 6 for full description).

Value Management (VM) = Value Planning (VP) + Value Engineering (VE)

a. Value planning (VP) In this first step, value is planned into the project. It is necessary to establish to whom value is to be delivered and what that person or group perceives “value” to be. Often this relates to the client’s perception of value. It is necessary to determine from a client’s briefing which items he perceives as value and to list them as the value criteria. These are W h e r refined into the needs - those criteria that are needed for the success of a project - and the wants - those characteristics the client would like but which are not essential for the execution of the project.

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Once determined, these value criteria are sorted into a hierarchy sometimes known as a value tree. If all the criteria of the roots of the tree are delivered, all higher level values will be satisfied. A speculation phase often involving a brainstorming session will check the value logic and then determine how (ie by what schemes or solutions) such value might be delivered. The scheme options are then assessed and weighed against the value criteria in the subsequent evaluation phase. From this evaluation the preferred scheme giving best overall value, ie satisfying the most value criteria, is identified and a design based on this solution developed. In current practice, clients often do not consider the value issues at the concept stage and thus do not initiate a value strategy which would lead to use of the OM where appropriate. If the client’s value criteria include cost reduction or personnel safety improvements, use of the OM will provide a viable option.

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b. Value engineering (VE)

The value engineering (VE) part of the process then starts. VE is simply defined as the systematic identification and elimination of unnecessary cost. In this process the required function of each element, or at least the most costly and most frequently used elements, of the proposed design solution are analysed. Again, the speculation or brainstorming mechanism is mobilised to determine whether any alternative solutions can be found that can provide the required functions at a lower cost. Such functions can include durability, aesthetics and safety, and may contain several of the original value criteria. These alternative solutions are then priced and analysed against the value criteria (often including a life-cycle costing calculation) in order to determine for each selected function that is studied the most acceptable solution. Sometimes minimum capital expenditure is the overriding criterion. At other times, it is minimum risk or minimum whole-life cost. The complementary nature of VM and the OM makes a powerful combination, as both: require a reasonably sophisticated client able to appreciate the potential gains and willing to introduce and support these processes to a successful conclusion 0

involve circumstances that provide less certainty of cost outcome at the time of construction award than traditional lump sum contracts encourage good teamwork during the construction phase

0

have the potential to provide significant cost savings to the client

0

require effective conditions to be incorporated in the contract to ensure successful results

0

require monitoring and audit need experienced and capable management of the process.

Although both systems can operate in isolation the OM functions particularly well when an effective VE clause is in place. This will permit the introduction of new aspects of the OM as and when they arise during a contract, provided the net benefits can first be demonstrated. When adopted, the VP described above will identify and indeed initiate the potential benefits in using an OM solution. A VE clause will then enable the benefits to be achieved in practice. The VE clause enables the savings to be shared in an agreed manner between the client and the contractor, an example being on the Limehouse Link, London Docklands (Powderham, 1994). The principles to be included in an effective VE clause that will help realise the OM benefits are covered further in Chapter 6.

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Technical considerations

This section of the report integrates the concepts of the OM into a process. The main components of the OM are discussed under the following headings: 0

policy

national and corporate

0

organisation

corporate and project

0

design and planning

desk study site investigation data interpretation initial design final design

construction control and monitoring

monitoring plan site monitoring

review and implement planned modification

review and implement planned modification (or emergency plans)

auditing

technical and procedural.

0

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0

These main components of the OM, shown in the flowchart in Figure 1.2, have been further subdivided into project activities in Table 4.1. Peck’s eight ingredients’ (Peck, 1969a) and the OM .framework stated in EC7 (BSI, 1995) are also shown in Table 4.1. HSE (1 996) commented on the use of the OM in NATM tunnelling. These comments compare well with the OM components (see Table 4.1). The limitations and the circumstances under which the OM should not be used have been stated in Section 1.2 and have been discussed in Section 3.8. Some of the potential inadequacies will also be discussed in Section 4.7. By incorporating both Peck’s and the EC7 requirements into the overall process, Table 4.1 shows their requirements put the emphasis on project design activities. However, experience has demonstrated that a suitable operating environment - one in which the definition and limitations of the OM are well understood and suitable specifications and contract arrangement are adopted - is at least as important. The OM components in Figure 1.2 and in Table 4.1 follow the sequence of work adopted for the design and implementation of present-day projects. This can be seen as a step-by-step guide of implementing the OM ab initio. In the “best way out” situation, however, a sequential application of the OM may not be feasible. This may necessitate the project team starting to apply the OM at another of the components in Figure 1.2. For example, the project team may choose to initiate the OM with “design and planning” (examining previous failure records and assessing the feasibility of carrying out remedial works based on the OM) before creating the operating environment (advising the client about the principles of the OM and the implications of its use on contract and fees). However, in principle, there should be no difference in the procedure of implementing the OM in both situations. All components of the OM in Table 4.1 and Figure 1.2 must be fully implemented in both situations. This section deals with technical considerations specific to the OM. The discussions highlight issues in each of the components shown in Figure 1.2. Recommendations made should be used in conjunction with appropriate BS and HSE regulations. ClRlA Report 185

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4.1

NATIONAL AND CORPORATE POLICIES Policies are used to convey the objectives of an organisation or a project. They are the criteria on which actions are based. For the OM to be implemented successfully, suitable policies should be established at both national and corporate levels. Some of the policy matters related to the OM are shown in Table 4.2. Nationally there should be design codes or design guides that define the OM and its proper use. At the corporate level there should be OM-orientated contracts, supported by appropriate specifications and training and education programmes. On the basis of policies such as these the project team will be able to: 0

interpret the client’s requirements and translate them to the product through an established rationale

0

set consistent performance standards through specifications

0

achieve economy of design through optimisation of design parameters achieve health and safety standards

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control the risks through a risk-based approach, which involves hazard identification, risk assessment and risk control through planned modification 0

plan the resources required

0

achieve an equitable sharing of risk, responsibilities and benefits manage progress, cost and quality of the design and construction works.

National policies

Definition and use

Design codes/guides/ national standards

what is the OM? what are its requirements? what are its limitations? when should the OM not be used? How do OM design procedures differ from those in the existing design codes? How does selection of the OM parameters and use of factors of safety compare with those in conventional practice?

Safety regulations, legislation

The responsibilities of the client, designer and contractor under the CDM Regulations 1994 and CHSW Regulations 1996.

A systematic way for identifying and eliminating risks and for managing those which remain.

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Corporate policies

Quality assurance

Identifies the requirements of the client, with the purpose of the desigdconstruction process resulting in output that conform to those requirements and can be produced cost-effectively.

Knowledge

A thorough understanding of safety critical aspects of design and construction is required (HSE, 1996).

Risk-based approach

A risk-based approach to design and management is required. Considerable skill and care at all stages are required. The complexities should not be underestimated (HSE, 1996).

Contracts

The contract and specification to provide: sharing of risks fairly between client and contractor sufficient flexibility in design changes is granted to the contractor performance-based contract sharing of benefits from the use of the OM fairly between the client and the contractor.

cost

Latham’s recommendations (Latham, 1994)

value management Specifications

Research and development

need for flexibility should be performance-based Areas that merit consideration for research would include:

risk prediction and management brittle and ductile modes of response in soils and structures -contractual conditions -long term performance of permanent works -data collection reliability and management.

4.2

CORPORATE AND PROJECT ORGANISATION Organisation is the process of structuring a corporation or a project team. It involves establishing the responsibilities within the corporation or the project team. The key matters are shown in Table 4.3 overleaf.

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Table 4.3

Matters to be considered in corporate and project organisations

Corporate

Competence

Project

Control method

recruitment and placement procedures systems to provide information and instruction, training and support (communications effort needed to meet these needs) arrangements to provide competent cover for staff absence, particularly for staff with critical responsibilities training (HSE, 1991)

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~~~

4.3

definition of project performance standards who is responsible? What for? When? With what expected result? level of supervision line of communication

Risk-based approach

A risk-based approach to design and management is required. Considerable skill and care at all stages is required. The complexities should not be underestimated (HSE, 1996).

Human factors

Awareness of the fact that the OM is heavily dependent on avoiding human failure (HSE, 1996).

Resource planning

The OM is a resource-intensive process needing thorough planning if safety is to be secured.

Previous experience

Previous experience in the OM and in the nature of the works must be taken into account (HSE, 1996).

Management style

Top-down management style to drive the OM - ie the top management leads the implementation.

Securing co-operation

Participation, commitment and involvement at all levels are essential to achieve all the OM objectives.

Communication

Regular meetings between key personnel for timely processing of information so that appropriate actions are taken.

DESIGN AND PLANNING This component is about gathering data, identifying hazards, assessing risks and producing a robust end-product that gives value for money. It is also about the process o f delivering the product through the strategic use of resources applied systematically to the following activities: deskstudy 0

site investigation data interpretation

0

initial design final design.

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4.3.1

Desk study At the feasibility stage of a project, a desk study is normally carried out to assist the appraisal of different engineering schemes. Information obtained from the desk study should aim to secure as much data as possible about the following: 0

geology and stratigraphy

0

soil types

0

hydrology and groundwater regime

0

seismicity

0

existing structures (tunnels, piles, basements)

0

any existing analyses of possible failures.

The scope and objectives of a typical desk study are described in codes of practice such as BS 5930 (BSI, 1981a). Information from adjacent sites can provide extra information on the variability of the soil and its properties. A desk study carried out for an OM project has eight specific objectives.

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1. To determine what is known about the site, and to help decide to what extent and by what method (phased, or in parallel with construction) it should be further investigated. 2.

To identify the type of construction problems that could be encountered, and to consider the range of construction options available.

3.

To establish the acceptable limits of behaviour, eg allowable deformation of the adjacent structures. It is important to obtain information about adjacent properties. In due course, discussions with owners of adjacent property and their consultants will be needed.

4.

To identify the geological hazards and define the following design conditions: 0

most probable (MP)

0

moderately conservative (Mod C) most unfavourable (MU).

To assess the potential risks associated with each of the above conditions and define the acceptable level of risk.

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5.

To assess the feasibility of using the OM based on the understanding of the soil properties, eg brittleness (which causes rapid deterioration).

6.

On the basis of the knowninformation, to assess if the degree of uncertainty is large enough to merit the use of the OM.

7.

On the basis of the desk study, to assess the feasibility of different engineering schemes (using the predefined design method or the OM) in conjunction with a value engineering review.

8.

To assess the cost and benefits of implementing the OM. The OM is cost effective in projects with large uncertainties.

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The main sources of data are previous case-history studies in similar ground conditions. Back-analyses of movements will be useful for establishing the most probable design parameters. For example, TRL (Carder, 1995) was commissioned by the Highways Agency to establish a database of ground and wall movements that could be used in assessing the most probable movements and acceptable limits when using the OM. This database records the pattern of ground and wall movements measured at sites where different construction techniques and construction sequences have been used. The appendix to this report lists case-history references where the OM has been used and examples can be found of the application of the OM for different ground conditions. During the desk study stage, the client should be made fully aware of the circumstances under which the Method is to be adopted, and the likely cost significance of the most probable/most unfavourable scenarios. The designer should assess the following matters for discussions with the client: the options of design by the OM and by the predefined method, and how each is used to handle risks the increased design fees that can be expected for the OM design approach because of the increased scope of work the scope for use of the OM on different parts of the project, and the procedure to be adopted for planned modification the potential cost savings (material costhime) that should stem from use of the OM

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0

the expenditure on site investigation, design, planning, monitoring etc associated with using the OM.

The above discussions are applicable to both “ab initio” and “best way out” situations. Additional information to be studied for the “best way out” application of the OM includes: 0

record of construction progress of the failed work

0

stratigraphy and groundwater conditions at the location of failure any back-analyses of the failure.

4.3.2

Site investigat ion Site investigation carried out specifically for the OM should obtain information for the MP, Mod C and MU design conditions of the project. The importance of site investigation has been described in other publications (ICE, 1992; Site Investigation Steering Group, 1993). These publications concluded that without proper site investigation the ground remains a hazard, presenting a financial risk to the project and a risk to health and safety. The standards for site investigation in the UK are documented in BS 5930 (BSI, 1981a). The methods for laboratory and in-situ tests are described in BS 1377 (BSI, 1990). These should be referred to and are not discussed in detail in this report. Sufficient site investigation must be carried out to ensure that, so far as is reasonably practicable, there is no likelihood of meeting unexpected conditions of a safety-critical nature. In tunnels, where it has not been reasonably practicable to carry out sufficient site investigation from the surface and there is a significant likelihood of encountering geological hazards whose location it has not been possible to determine, the design should specify methods for finding them ahead of construction. For example, it may not be reasonably practicable to locate all unconformable features in London Clay. These may be located by drilling near-horizontal exploratory holes from within the tunnel.

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In some situations, it may be more practical to identify the range of ground conditions likely to be encountered. The following are some examples. 1. Wide variability of ground and groundwater conditions within the site. 2. Presence of variable geological features, eg solution features in chalk.

3. The presence of structures that cannot be removed until construction begins (or listed buildings that cannot be demolished). 4.

Existing adjacent structures the foundation and structural details of which are either not available or not known.

In such a case, if unexpected conditions of a safety-critical nature cannot be excluded, they must be presumed to be present.

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Peck (1969a) considered that “exploration [should be] sufficient to establish at least the general nature, pattern and properties of the deposits, but not necessarily in detail”. This does not imply that the amount of site investigation can be reduced in comparison with that for a predefined design. It means obtaining a sound appreciation of the range of geological conditions and potential engineering behaviour of the ground materials. Modification plans should be developed for these. More detailed exploration of specific aspects of the ground and their variation is undertaken as the construction work proceeds. / The HSE (1996) recommends that ground investigation should be substantially completed before construction starts. When this is,impractical, it could be staged during construction. The following two strategies should be considered. 1.

Phased site investigation

This is particularly usehl on sites where there is no previous information available. An initial phase of SI is undertaken with limited scope of work upon which preliminary designs are based. Subsequent phases of investigation focus on those geotechnical parameters that are key for particular design conditions (MP, Mod C and MU) so that the cost and safety of the designs and planned modification can be clearly estimated. 2.

Site investigation in parallel with construction

The final phase of investigation may be planned such that it can be run in parallel with the site preparation or bulk excavation. Sufficient time should be allowed for information to be fed back into the design process. These investigations could be: additional boreholes or in-situ tests from ground level probing in advance in the location of the proposed works logging faces exposed by excavations. The later the investigation, the less time is available for a designer to plan ahead. Good management is required to co-ordinate and review information gained during the construction works and to implement the modification in accordance with the planned strategy. This point is illustrated by the example of the Vermilion Dam described by Terzaghi and Leps (1960). The pattern of stratification at the site was so intricate that it was impractical to determine all the essential geological features of the subsoil in advance of construction. Therefore, a desk study of the regional geology and geological history of ClRlA Report 185

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the area was carried out. A few boreholes were drilled. Assumptions were made during the design and these were checked and modified according to findings during construction. Additional site investigation was also carried out during construction. Terzaghi commented that exploratory work in excess of what had been performed at that site would not have furnished any vital additional information to the knowledge of the site before construction. However, the engineers in charge made provision for detecting the errors in the assumptions before it was too late to modify the design during construction.

4.3.3

Data interpretation In this stage, site-specific information is reviewed and interpreted. The objectives of this stage are: 1. To assess the MP, Mod C and MU conditions based on site investigation data including: a.

stratigraphy

b.

groundwater conditions, pore-water pressures, permeability

c.

soil strength parameters, compressibility, etc.

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It will probably be necessary to back-analyse similar structures in comparable ground conditions to obtain and confirm the most probable parameters for the proposed method of analysis. 2.

To identify geological hazards and other uncertainties. This should involve a critical appraisal of all activities during construction. Adequate hazard identification requires a complete understanding of the construction sequence and of potential (ie realistic) failure mechanisms.

3.

To decide whether it is feasible and cost-effective to implement the OM in part or all of the project. A cost-benefit analysis may be useful.

The data should be kept under constant review and be reinterpreted as more detailed information becomes available. The objective is to maintain up-to-date predictions of the ground likely to be encountered. A geotechnical report should be produced by the end of this stage. The level of detail varies with the nature of the project. Design codes such as EC7 (Clause 2.8) contain recommendations for information to be incorporated in a geotechnical report. Readers can refer to this for details.

4.3.4

Initial design This is also known as the scheme design stage. This includes not only the design calculations, but also the method statement, construction plan, programme, resources and the procedure of implementing planned modifications and emergency plans etc. Hence, both designer and constructor should be involved, if possible. This is discussed in Section 3.

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The objectives of this stage are: 1. To reconfirm the client’s requirements and expectations. 2. To assess options through value engineering (VE). 3.

To carry out preliminary designs for at least two sets of design parameters: MPandMU,or 0

ModCandMU.

The design cases should cover all the likely scenarios to be encountered. A riskbased approach is recommended (HSE, 1996). 4.

To decide the procedure and resources for implementing the planned modification (see Section 3), eg by (i) Peck’s procedure, or (ii) progressive modification. A planned modification is one defined in advance of construction works. It can include: 0

a contingency plan applied during adverse conditions (these may range from small modifications to significant contingency plans)

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a plan to achieve identified benefits. Such modification can be of design, construction sequence, construction method, etc, see Section 4.5. In the UK, under the Construction (Health, Safety and Welfare) Regulations 1996 (HMSO, 1996), planning of emergency procedures are required in all construction projects, including those based on the OM. Readers should refer to the relevant health and safety regulations for guidance on this subject. 5.

To identify the observations to be monitored and to consider options for monitoring schemes.

6.

To make a VE assessment - cost assessment of the above designs, the corresponding modifications and the associated monitoring: material and labour costs of each option 0

0

cost of installing instrumentation, monitoring and interpreting instrumentation results cost of changing from one scheme to another and to identify possible alternatives.

Several recent publications can be consulted: Creating value in engineering (ICE, 1996b)

CIRIA Special Publication 129: Value management in construction - a client’s guide (Connaughton and Green, 1996)

-

CIRIA Special Publication 122: Waste minimisation and recycling in construction (Guthrie and Mallet, 1995).

7.

To assess the buildability of the design. HSE (1996) considers that designs that take account of the ease of construction, or “buildability”, will greatly facilitate the achievement of quality during construction. This will lead to a better product and improved safety.

8.

To decide on either progressing a predefined design based on Mod C assumptions or continuing to develop the OM.

9. To interact with the constructor concerning the above requirements and to develop the associated preliminary method statement. 10. To take full account of matters affecting health and safety as required in the legislation.

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10. To review the hazards and carry out risk assessment. The risk should be managed on the basis of the risk hierarchy, see Box 4.1. The hierarchy reflects risk elimination and risk control by the use of physical engineering controls and safeguards, which are more reliably maintained than those which rely solely on people. Therefore, the design has to be robust so that it is safe at all times for the range of hazards previously identified. Where a range of control measures is available, it will be necessary to weigh the relative costs of each against the of degree of control each provides, both in the short and long term. In making decisions about risk control, it will be necessary to consider the degree of control and the reliability of the control measures along with the costs of both providing and maintaining the measure. Box 4.1

HSE risk hierarchy

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The HSE (1992) publication entitled Management of health and safety at work Clause 27 states the following risk hierarchy principles for risk assessment. a.

If possible, avoid the risk completely, by using alternative methods or materials.

b.

Combat risks at source, rather than by measures that leave the risk in place, but attempt to prevent contact with the risk.

C.

Wherever possible, adapt work to the individual, particularly in the choice of work equipment and methods of work. This will make work less monotonous and improve concentration, and reduce the temptation to improvise equipment and methods.

d.

Take advantage of technological progress, which often offers opportunities for safer and more efficient working methods.

e.

Incorporate the prevention measures into a coherent plan to reduce progressively those risks which cannot altogether be avoided and which takes into account working conditions, organisation factors, the working environment and social factor. On individual projects, the health and safety plan will act as the focus for bringing together and co-ordinating the individual policies of everyone involved.

f.

Give priority to those measures that protect the whole workforce or activity and so yield the greatest benefit.

9.

Employees and the self-employed need to understand what they need to do, eg by training, instruction, communication and risk assessments.

h.

The existence of an active safety culture affecting the organisations responsible for developing and executing the project needs to be assured

With reference to NATM tunnel design, the designs will have predicted certain ranges of surface settlement, ground stresses and in-tunnel deformations within which the works are expected to lie. It is necessary to demonstrate that the risk of one or more trigger criteria being exceeded is effectively zero or acceptably small. The OM allows risk to be managed more efficiently. Having control over risk means not over-designing, thus reducing costs while maintaining safety. This cannot be achieved, however, unless competent engineers who are aware of the limitations of the design and the availability of alternative contingencies can substantially increase monitoring, interpretative effort and review management. Whether progressive modification or Peck’s (1969a) approach is chosen for implementing the planned modification will depend almost entirely on the quality of the site investigation data. The decision will also be affected by such factors as experience of the design and construction teams, application of the NATM to specific site and ground conditions, availability of relevant plant, manpower and materials, and confidence of the client. The choice of excavation equipment or method should also be identified in broad outline so that its effect on construction sequences can be established, eg whether roadheader, excavator, or drill and blast.

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4.3.5

Final design The objectives of this stage are: 0

to define trigger criteria for the project structure and the adjacent structures

0

to produce final calculations for the MP, Mod C and MU conditions to confirm the method of dealing with planned modification and emergency procedures and the strategy for mobilising the resources

0

0

to confirm that all hazards (brittleness, progressive failure etc) are identified and mitigation measures are put in place to produce a final set of information which can be costed, tendered and h i l t .

A robust design is essential (see Box 3.1). The design for each element should be hlly developed before construction of that element begins. The dimensions, strength and articulation of the structure should be chosen so that the structure can withstand the likely loading.

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The,information required from the designer at this stage includes: 0

a base design with planned modification options

0

trigger criteria

0

required resources

0

contract

0

specificationsldrawings.

The constructor is required to produce the following: 0

method statement

0

resource programme

0

risk assessment.

Apart from these, under UK health and safety regulations, contractors must also produce H&S plans (CDM Regulations 1994) and emergency plans (CHSW Regulations 1996). Readers should refer to these regulations for details of the requirements. The steps set out in Section 4.3.4 are repeated for finalising the design in light of any new investigation findings, W h e r interpretation of data and new requirements from the client. Calculations are used to predict the likely effect on adjacent structures. The results should be reviewed with structural engineers, clients and adjacent property owners such that the acceptable limits are established. The health and safety regulations must also be considered. The concept of trigger criteria is described in Section 3.9. Apart from setting a trigger value, an appreciation of the rates of change and trends of data should also be developed (see Section 3). Rate of failure development must be reliably estimated. Planned contingency actions have to be capable of completion before the level of risk becomes intolerable. This will first require an analysis of the time that will be available from discovery and the time that will be required to effect recovery. Conservative estimates should be made of the time required to identify the need for, and to implement, contingency actions. Trigger criteria may have to be set at levels below the serviceability limit. A traffic light system, as shown in Figure 4.1, can be used to represent construction conditions.

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PLANNING

Identify next excavation stage

I

Excavate next stage

I

I

Monitor deflection and report excavation progress and geology encountered

I

Review against trigge criteria

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I

Extra readings

Review contingency with engineer

Figure 4.1

Replace excavated soil immediately

implement contingency and/oremer enc lans

An example of a traffic light system for a staged excavation

On the design of embankment dams, De Mello (1 977) considered that instrumentation should be included as an item of design decision of practical significance to embankment dams. However, heglso raised two issues regarding the use of instrumentation and trigger criteria. First, interpretation of instrumentation results should not be confined only to a presupposed model of theoretical behaviour, because this will inhibit a truly flexible visualisation of unsuspected problems. Second, if instrumentation is to be of value to the project, it is fundamental to establish prior criteria for the green, amber and red traffic signals for decision on action incumbent as the observed results exceed anticipated limits and tolerances. It is essential too to confirm whether factors of timing and rates of phenomena under observation will permit timely intervening action, and to possess a realistically planned strategy of control and correction of any undesirable performance.

4.4

CONSTRUCTION CONTROL AND MONITORING

4.4.1

Monitoring plan The strategy behind the monitoring plan is to enable the designer to obtain sufficient information to help to establish the validity of the original design and to implement planned modification or emergency plans where necessary. Monitoring provides not only an indication of the onset of unforeseen circumstances, but also a system of early

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discovery and confirmation that appropriate quality standards have been met. The objectives of the monitoring plan are: to identify the observations that are needed to define the instrumentation system to define construction control procedures, reporting methods and the frequency of monitoring to establish duties and responsibilities within the project team, in particular the actions to be taken in the event of the trigger criteria being approached or unexpected adverse trends being recognised in the results to produce construction control instructions and guidance, eg construction control manuals or charts, for the use of site personnel to produce trend data for later comparisons and refinement of design.

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The following points should be considered when developing the monitoring plan. 1.

Ensure that the monitoring plan is simple enough for site workers to understand it and to carry out the monitoring procedures. Terzaghi (1948a) reported the monitoring procedures for a slope. To make sure that the observational data would be digested and recorded adequately, he issued detailed instructions to staff on site. All data were to be presented graphically and be tabulated in specified formats. Essentially these were construction control charts.

2.

Adopt simple formats for presentation of the observations.

3.

Relate the extent of monitoring to the design cases.

4.

Consider the construction sequence.

5.

Use proven technology for measurements and recording.

6 . Check critical observations using separate and different systems of measurement. 7.

Set tolerances for the trigger criteria to take into account questions of the accuracy and precision attainable by both the measurement system and the method and assumptions used for the establishment of the trigger criteria.

8. Provide protective measures for the instrumentation, or allow for redundancy in the number of instruments in case of damage. 9.

Bring the site personnel into the development of the monitoring plan to give them a sense of ownership. Ensure that competentltrained personnel are available.

10. Learn from previous case histories and failures. A monitoring system can be considered as having two levels of entry, called primary and secondary levels. Primary monitoring system used by site team to check results are acceptable against the trigger criteria simple robust readily checkable accuracy appropriate for use rapid to read and interpret directly related to the triggers.

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Secondary monitoring system used to check the performance of the primary monitoring as a back-up system becomes important for the engineer’s interpretation if the amber trigger criteria are exceeded may be more complex than the primary system identify ground behaviour not necessarily directly related to the triggers. Selection of the appropriate primary and secondary monitoring systems will depend on the project requirements. Some examples of manual and electronic monitoring systems are given in Table 4.4. Table 4.4

Examples of monitoring systems

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Manual

I Electronic

survey levelling tape or extensometer demecgauge torque wrench standpipe piezometer crack width monitoring

electro-level fibre optics strain gauge load cell pneumatic/vibrating-wire piezometer

inclinometer magnetic extensometer strain gauges video system

electro-level earth pressure cells load cells

Forbes et a1 (1994) discussed the use of such strategy in the monitoring of the Mansion House during construction of Docklands Light Railway City Extension using the OM. The primary instrumentation system used electro-levels mounted on the basement walls, external faces and masonry columns. Secondary instrumentation included spatial survey, global precise levelling, water-level gauges, vibrating-wire gauges and load cells.

4.4.2

Site monitoring The objective of site monitoring is to collect relevant data so that comparisons can be made with trigger criteria at the review stage. Activities at the site monitoring stage are: 0

installing the instrumentation

0

collecting the intended data

0

reporting the results

0

recording construction progress

0

recording actual ground conditions.

Monitoring schemes should be based on thorough hazard analyses of all possible failure mechanisms, no matter how unlikely.

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4.5

REVIEW AND IMPLEMENT PLANNED MODIFICATION The objectives of this stage are to: 0

pull together all required information

0

review monitoring results against pre-determined criteria

0

carry out planned modification to effect quality improvements or implement emergency plans if necessary.

The frequency of reviewing the monitoring data should be determined from the analysis of potential failure and recovery patterns so that adverse trends and events are identified in time. Abnormal trends may reflect the need for changes to: 0

design construction method and sequence

0

monitoring method

0

management and quality assurance (QA) system

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health and safety systems and the health and safety pladfile 0

recovery and emergency arrangements

0

personnel.

Data from instruments should be properly assessed and actions recorded. All information ought to be checked and processed by experienced staff who are able to detect abnormal trends and who are aware of the relevant trigger criteria. Data interpretation procedures should be established in advance so that confusion and delays in reaching critical safety decisions can be avoided. Ad-hoc arrangements are inappropriate. The frequency of review should be agreed in advance. Interpretation is likely to involve several different disciplines within the project team. Regular formal meetings involving client teams, designers, constructors, material suppliers and those who may be affected, would constitute an appropriate way of managing the decisions. Procedures for implementing planned modification are discussed in Section 3.10. Timely processing of information is crucial to the quality and safety of the modification. The site data required for performance reviews are as follows: 0

actual ground conditions compared with predicted ground conditions

0

construction progress in relation to planned progress

0

measured performance versus calculated values and trigger values interpretation of data including trend rates.

The Discovery-Recovery model introduced by HSE (1996) should be considered (see Figure 3.10). A satisfactory discovery mechanism relies on early detection of adverse changes in risk from the information available. This requires planned proactive monitoring in which timely, reliable and easily interpretable data are collected, critically examined by competent persons, and the conclusions passed to an appropriate level of management for any necessary planned actions to be triggered. The more frequent and risk-directed the proactive monitoring, the greater is the opportunity for early discovery. The probable rate of development of adverse changes and the time needed to effect recovery are factors that should be taken into account when considering the monitoring frequency.

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The recovery mechanism comprises decision making and action. Management structures that minimise the time taken to reach decisions and to implement contingency plans are crucial to controlling the level of risk. As stated by the HSE (1996) the longer the recovery, the greater the risk that will arise and the more likely a major accident. Importantly, the recovery process should not cause the risk to rise to an unacceptable level. This form of analysis should be undertaken during the iteration process used during design and should be verified before construction commences. Readers should also refer to the appropriate standards and safety regulations, eg CHSW Regulations 1996 for details of requirements for implementing emergency plans.

4.6

TECHNICAL AND PROCEDURAL AUDITING All control systems, one of which is the OM, tend to become slack over time. This calls for the OM to be audited at a frequency agreed by all parties in the project. The aim of auditing is to provide an independent assessment of the validity and reliability of all the components of the OM, see Figure 1.2. The OM audit complements the health and safety audit and quality audit. Depending on the situation, this can be carried out internally or by a third party.

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Two aspects of the OM should be audited: 0

technical

design, workmanship, quality of data collected etc

0

procedural

how the works are carried out.

Auditing constitutes a feedback loop for the OM, as shown in Figure 1.2. Auditing supports monitoring by providing the project team with information on the implementation and effectiveness of strategies and performance standards. It also provides a check on the reliability, efficiency and effectiveness of the system and provides information to the project team for maintaining and improving the OM components (HSE, 1991). Technical and procedural auditing, and also monitoring (Section 4.4), can be carried out as part of quality control and quality assurance (QA). With reference to NATM tunnels construction, the HSE (1996) considers that the achievement of quality is essential if a project is to be completed successfklly and that QA can play an important part in an overall system of risk management and control. For example, QA procedures could be applied to:

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0

material supply by external suppliers where interruption in supply could have safety implications

0

establish inspection, testing, monitoring and review protocols.

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4.7

PITFALLS IN THE USE OF THE OBSERVATIONAL METHOD If not careful, the OM can easily be discredited by misuse. Peck (1985) warned that: “The Observational Method, surely one of the most powerful weapons in our arsenal, is becoming discredited by misuse. Too often it is invoked by name but not by deed. Simply adopting a course of action and observing the consequences is not the Observational Method as it should be understood in applied soil mechanics. Among the essential but often overlooked elements are to make the most thorough subsurface investigations that are practicable, to establish the course of action on the basis of the most probable set of circumstances and toformulate, in advance, the actions to be taken ifless favourable or even the most unfavourable conditions are actually encountered. These elements are often d$jcult to achieve but the omission of any one of them reduces the Observational Method to an excusefor shoddy exploration or design, to dependence on good luck instead of good design. Unhappily, there are far too many instances in which poor design is disguised as the state of the art merely by characterising it as an application of the Observational Method. ”

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On the following pages Table 4.5 shows how the OM might be misused.

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Table 4.5

Potential misuse of the Observational Method

OM component

Potential inadequacies

Definition

misunderstanding the definition wrongly attributing the term to work

Desk study

0

Site investigation

0

0

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0

poor understanding of the concept of risk and hazard; inadequate analysis of hazards failure to anticipate all possible failure mechanism, their likelihood of occurrence and potential consequences of failure. Therefore the necessary design will not be fully developed inability to turn the risk assessment into effective risk-control and risk-management measures lack of appreciation of the level of risk that would be considered tolerable no sensitivity analysis to enable the relative importance of differing risks to be judged effects on adjacent structures (limits of behaviour) not adequately considered design focused solely on the final integrity of structure, without examining the intermediate stages of construction inadequate consideration of the timescale of failure occurrence

Final design

failure to produce a robust design failure to consider buildability. Inadequately thought-out method statements resulting in impractical procedures and Construction methods being proposed that subsequently have to be changed during construction failure to assess the sensitivity of design elements to variations in construction

Monitoring plan

poor choice of significant observations ad-hoc instrumentation monitoring put forward as the OM failure to set criteria against which to monitor trigger level or trend rates not considered, or when considered not correctly set

Site monitoring

lack of clarity in the objectives of the monitoring failure to analyse and present the data in time misuse and abuse of instrumentation monitoring limited to checking that design assumptions are reasonable data presentation format not appropriate for subsequent performance review

0

0

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failure to anticipate probable and unfavourable conditions failure to consider the influence of progressive failure as a consequence of soil brittleness behaviour lack of detailed ground investigation to carry out necessary stability assessment failure to define the range of unfavourable conditions

Data interpretation

Initial design

failure to anticipate unfavourable conditions failure to understand the limit of behaviour of the adjacent structures

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Construction

Review and modification

poor understanding of the concepts of risk and hazard; Inadequate analysis of hazards failure to anticipate all possible failure mechanism, their likelihood of occurrence and potential consequences of failure inability to turn the risk assessment into effective risk-control and risk-management measures lack of appreciation of what level of risk would be considered to1erab1e over-reliance on observations during construction without adequate design and planning in advance of construction a

a

a a

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Auditing

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a

insufficient time to implement design modifications in emergency situations failure to consider how to discover emerging trends and to make sure that recovery through remedial action can be taken within the time available poor appreciation of rates of change and trend data inadequate arrangements to review monitoring data and reach appropriate decisions inadequate challenge functions built in to review critical design, construction and safety issues

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5

Management considerations

There is more interaction between designers and constructors on OM projects than in ones of predefined design. This interaction needs management and co-ordination. The commitment of the members of the project team and their willingness to “own” and solve problems are therefore of critical importance. With reference to NATM tunnel construction, HSE (1996) considers that where failures have occurred, poor management has been a significant contributory factor. Good management is therefore of particular importance to OM projects. The OM should not be initiated unless the technical issues have been considered thoroughly and an effective management system has been set up. This section highlights some management considerations in the implementation the OM, which are additional to those normally required for predefined designs. They are discussed under the four categories shown in Figure 5.1, which are: culture strategy competence

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systems. These have been based on interviews and questionnaires sent out during the research. In addition, reference has been made to the following publications: Successful health and safety management (HSE, 1991) Safety of New Austrian Tunnelling Method (NATM) tunnels (HSE, 1996) Sprayed concrete linings (NATM)for tunnels in sojiground (ICE, 1996a) The Observational Method in geotechnical engineering (ICE, 1996c). The above publications provide useful guidelines or examples of project management based on the OM and are recommended for further reading.

5.1

CULTURE Culture, in this context, encompasses the following facets: the attitude of the project team members towards the aim of achieving quality, safety and project cost optimisation a clear perception of hazards and risk and the adoption of a risk-based approach to management the appreciation of the meaning of “design” in the context of the OM and the CDM Regulations the willingness of the team members to recognise and manage the risks and to face and solve problems together the willingness to break down boundaries between different sections of a project team. Project team members put their effort into supporting each other rather than having an attitude of “them versus us” the understanding within the project team or the corporation of the OM requirements and limitations the willingness to adopt an integrated design and construction approach to projects

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0

0

the willingness to invest in research and development that can enhance the buildability, quality and safety of the works the willingness to accept new business processes such as partnering the enthusiasm to educate non-OM users and to train them to adopt proper procedures for implementing the OM the willingness to learn from past collapses and successes.

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CULTURE

Quality Health and safety Value management Risk management OM requirements and limitations Co-operation Integrated deisgn and construction Research and development Training and education

Considerations

Clear communication Reliable and timely information gathering Reliable and timely information processing Reliable and timely information reviewing Auditing - OM procedures and requirements - quality - health and safety

Contract Risk-based control Team building Resource planning

COMPETENCE

Skills Knowledge Experience

Figure 5.1

Management considerations

A good OM culture starts with the commitment of senior management, who will seek opportunity to initiate the Method where appropriate, lead the change and champion successes of the OM. A sustainable OM culture within a corporation or a project team relies on its policies in: 0

quality

0

health and safety education and training

0

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research and development in areas such as risk management, brittle and ductile modes of response in soils and structures, contractual conditions, data collection etc (Powderham and Nicholson, 1996).

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Good communication, externally and internally, will promote the OM culture. Communication should be targeted at all levels of the corporation and project team. Before the start of the site works, for example, it is helpful to hold meetings or workshops with people at all levels, from foremen and technicians through to senior management. Good communication also includes documenting experience of successful use of the OM and making this information available to future projects.

5.2

STRATEGY In order to attain maximum benefit from the OM, the objectives of the project should be defined and the means to achieve these objectives by using appropriate strategies should be planned, in the areas below. a.

Contract

Section 6 shows that not only the contract form, but also the tender procurement strategy (eg competitive tender, negotiated contract), payment method (lump sum, remeasurable) and the operating environment (eg partnering) can have a strong influence on the success of the OM. The inclusion in the contract of a value engineering, or similar incentive, clause will be useful. This will allow the benefits obtained through the use of the OM to be shared among all parties involved in the project.

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Advice on a contractual framework for the OM is given in Section 6. b. Risk-based control strategy

The management strategy adopted in the OM should be one of risk-based control (HSE, 1996). This means that hazards and risks should be taken into account, starting from the project’s inception, including the possibility of high consequence-low probability events. It should not simply focus on routine risks arising from work activities. Risk assessments must be carried out. Any limitations imposed by designs on construction sequences and programmes ought to be clearly understood. Safe working methods and contingency and emergency plans should be developed. Proper management of risk can save time and money. A guidance.note issued by the Treasury’s Central Unit on Procurement, Managing risks and contingency for works projects (HM Treasury, 1993), explains the project finance aspects of risk management and provides an insight into the procedure for planning and incorporating the cost of contingency measures into the project. Like the risk-management procedures for health and safety discussed in Section 3, the guidance note emphasises the importance of constantly reviewing and updating the results of risk assessments. This should be reflected in appropriate updating of the contingency measures and their cost allowances.

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c.

Team building

The integrated design and construction nature of the OM means that it requires an integrated project team of personnel from different organisations to work together, within the designer’s or the constructor’s organisations. The following points should be noted. 1.

The staff from the designer’s and constructor’s organisations should be similar in authority and seniority to lessen barriers to communication or approvals for action.

2.

The structure of the team will be affected by contractual arrangements. A complicated contractual framework leads to a correspondingly complicated project team, with more effort being required for effective communication and cooperation (HSE, 1996).

3.

Design and construction (including site monitoring) are closely interwoven on OM projects. The OM is an interactive process that binds permanent and temporary works and their designs together. Difficulties will arise if this is not recognised in the way contracts for services and construction are let.

4.

Good buildability requires co-operation between designers and constructors. Constructors should understand the design and the designer should know what problems designs may pose for constructors (HSE, 1996).

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d, Resource planning OM is a resource-intensive process. The resources required for the “most probable” (or “moderately conservative”), together with “most unfavourable”, construction schemes should be planned. Consideration should also be given to there being substitutes on standby for staff holding critical responsibilities (who may be absent because of sickness or holiday leave). Specific attention should be paid to the reliability of delivery of the construction materials required for modifications in emergencies.

5.3

COMPETENCE The HSE (1 996) defined competence as the possession of the necessary skills, knowledge and experience for the successful completion of a job function or task to an acceptable standard. Skills, knowledge and experience are crucial to the success of the OM. The HSE (1991) discusses the methods to secure competence in achieving health and safety standards. Of particular relevance to the OM are the following:

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0

thorough recruitment and placement procedures

0

the provision of information and training

0

arrangements for supervised on-the-job experience

0

the availability of competent cover for staff absences.

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It is best to consider competence at three stages. a. Design and planning The manager should bring together a balanced team that combines design soundness, innovative thinking and construction ability to identify modifications to the construction process. A balance has to be struck among cost savings, risk and control measures. Alternatively, there may be a balance between using the predefined design method and OM options. Additionally, all managers, supervisors and operatives should be familiar with the specification requirements, approved methods and procedures for the work, the site safety plan and contingency and emergency arrangements. The HSE (1996) considers that previous experience will include knowledge and understanding of: available technologies and their limitations previously successful approaches to design, construction and monitoring site-specific information such as geology and existing land-uses

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people and organisations with the experience, expertise, shlls, resources and competencies to undertake the works. b. Construction During the construction stage, the manager has to keep the site team diligent and alert and co-ordinate the construction work, the monitoring and the review (including the design engineer in the team). Also, the works must be supervised by experienced site personnel during installation and inspected throughout the monitoring period. c.

Review For the timely avoidance of critical hazards, it is essential that experienced engineers review and act upon the monitoring records. The ICE (1996a) considers that good NATM tunnelling practice requires supervisory staff from both contractor and engineer to meet and review the results of both visual inspections and recorded instrumental monitoring every day. Deficiencies in quality of workmanship or adverse trends in the monitored information should be noted and appropriate action, and the timing of such action, decided upon. The manager should make sure that, when required, modifications identified by the review are rapidly communicated to the construction team, who should already have been briefed on potential modification.

Therefore, a variety of skills - commercial, conceptual, analytical (particularly for identifying hazards and assessing risk), administrative, social and interpersonal - are vital in successfully managing the OM. Competence of the staff should be assessed before assigning them to any post in the OM team. Blessington and Lloyd (1994) considered the various levels of skills in relation to the recurrence of problems during work. Implementing the OM called on the f i l l range from unskilled workers to researchers, as shown in Figure 5.2.

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1

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5.4

SYSTEMS HSE (1996) considers that to secure safety through the OM, the system should be able to: define acceptable performance limits for the purpose of serviceability and safety devise monitoring strategies that are able to detect adverse events sufficiently early develop contingency and emergency arrangements capable of successful implementation put them into effect should the need arise. Furthermore, three systems are equally essential. a.

Communication system

The line of reporting and giving instructions must be clear. Decisions must be made at an appropriate management level. All members should think through what they aim to accomplish and make sure that their associates know and understand their aims. This is particularly important for the OM because interaction between designer and constructors is more frequent than in predefined designs. It should be noted that complicated contractual conditions may complicate the team structure and hamper inter-organisation and intra-organisation communication. This should be avoided in OM-based projects.

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b. Information gathering, processing and reviewing systems Timely discovery of unacceptable level of risk and timely implementation of planned modification, or the emergency plan if required, is at the heart of the OM. Good management of information gathering, process and reviewing systems is therefore essential. The following points should be considered: reliable monitoring system operated by competent staff 0

fast computers for the timely processing and presentation of data

0

back-up personnel and instrumentation visual display of the monitoring results for all key personnel

0

ability to recognise the onset of failure and to implement contingency and emergency procedures in a timely manner.

c.

Audit system

As discussed in Section 4, technical and procedural audits of OM projects can be carried out as part of a project’s quality assurance or health and safety audits. An auditing system is required to check that prescribed procedures have been carried out. The audit system can be tailored to meet the project needs in quality, health and safety and OM requirements. This audit system should be linked not only to construction quality such as workmanship or material, but also to the full OM process including:

all critical matters related to health and safety achievement of all design criteria that are critical to the integrity of the works quality of information and quality of output programrqe and budget of work reliability of the supervisory and checking procedures - must take into account the possibility of human failure reliability of the monitoring system reliability of the project’s discovery-recovery mechanism understanding of the work procedures by individuals, ie competence.

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6

Contractual framework

Where there is little uncertainty, use of the OM will provide little, if any, benefit. The client has to decide whether certainty of outcome, however costly in time or money, is more important than minimising both expenditures through the OM.

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For the OM to thrive there must be a contractual framework that is conducive to its operation. Such a framework will not be put in place unless the primary participants, particularly the clients, are aware of the net benefits that are possible through adoption of the OM. It is most unlikely, however, that the form of contract will be chosen on account of the OM. When opportunities for the OM arise after a contract is in place, the success of the Method depends on the extent to which it can function within the existing contractual constraints. Case histories show that many OM projects were operated under both traditional and design-and-construct contracts: The essential difference when adopting the OM in a traditional contract rather than a design-and-construct contract is that right from the start the parties have to agree to allow the design of the temporary works (and possibly parts of the permanent works) to be changed during the course of construction. This may lead to the parties agreeing to a change in the contract sum and a change in the completion date. While most construction contracts provide for variations or remeasurement, the mechanisms are often cumbersome; and the cost and time outcome in many standard contract forms is left to be resolved at a later date. Such a change in the fundamental approach - which can so affect the contract sum and the contract period - can only succeed in the fullest sense in a non-adversarial contract environment. It cannot be operated effectively without the agreement, and the understanding, of all the parties. It is worthwhile to set down the possible expectations of the parties in respect of the outcome of the application of the OM, because of this fundamental change. The general purposes for which they go into the enterprise (irrespective of the contract forms and construction methods) remain the same. The client wants to obtain a first-class project on time and within budget. The other parties want generally to improve their knowledge and experience, to gain satisfaction, peer acclaim and recognition sufficient to attract more work, while at the same time making a reasonable profit. An ideal contract framework among the various parties would accommodate all the above expectations; but there is no such utopia. Of the options available to the client listed below, it is unlikely that any will form an ideal contract framework for OM without some amendment to the form of contract itself.

6.1

NATURE OF THE WORK In order to appreciate and focus on the contractual issues related to the OM, it is necessary to understand the nature of the work involved. Whereas a contractor’s alternative design to the permanent works has to be adopted by the client’s designer, this is not the case for temporary works. However, the client’s designer may have to approve the proposed OM to be sure that it is safe and will not adversely affect the permanent works.

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In principle, the OM falls into the general category of temporary works. It relates to the method of working and is a means of installing the permanent works. The OM benefits arise from a flexibility to adjust to conditions as uncertainty reduces. The temporary works often take place at the time of most uncertainty. It is often not easy, and perhaps not possible, to determine from the as-built permanent works that the OM has been used. Responsibility for the OM thus lies with the party responsible for the temporary works or the method of working. The three primary contract options are discussed below.

6.1 .I

Client’s designer-led temporary works A permanent works designer should assume a method of installation and consider temporary works and associated risks. The designer needs to state any permanent works design or management criteria that the contractor is expected or required to meet when installing the works in order not to put the works or personnel at risk. There may be mandatory criteria to be followed by the contractor or simply advice concerning the limitation or expectation of the permanent works design to be taken into account by the contractor in execution of the installation. The options open to the contractor are likely to be limited in this case. In the event of this category of permanent works design being sensitive to the method of installation, t h s should be clearly communicated to, and fully understood by, the contractor. Preferably, it should be spelled out in the contract documents. If the OM is to be used this will certainly be the case.

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6.1.2

Contractor-led temporary and permanent works In a design-and-construct environment responsibility for both the temporary works and the permanent works lies with the contractor. The manager of the desigdconstruction interface has to make sure that, as any OM design develops, the actual temporary construction method is compatible with, and does not jeopardise, the permanent works. That gives the opportunity to achieve an optimum solution that maximises buildability without putting other site personnel at risk.

6.1.3

Engineer-led permanent works with contractor-led temporary works This option requires co-operation between the client’s principal designer, ie the engineer, and the contractor, because the engineer may have to change the design to accommodate the benefits of any OM that the contractor wishes to use. In this case it is not uncommon for the contractor to negotiate a split of the benefits, if the contract has not covered this possibility (eg with a value engineering clause).

6.2

CONTRACT AND TENDER CONSIDERATIONS

6.2.1

Common contract types and the OM In principle, all construction works require a client who pays, a designer who designs and a constructor who constructs. Procurement options vary in so far as who employs the designer, who controls and manages the processes, how and when tenders are obtained, how the work is paid for, and how the final cost is assessed. The options therefore vary the allocation of responsibilities and the placing of risk. In practice, it is unlikely that the form of contract is determined by the OM, rather that the method will need to operate under a range of contracts. Common contract types relevant to the OM can be broadly grouped under one or other of the categories’ described below.

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1. Management contracting This involves overlapping design and construction in which the client appoints designers and a management contractor. The management contractor, who is paid a fee, appoints to each work package a works contractor, whose final contract sum is assessed by variation or remeasurement. Adoption of the OM in these circumstances can lead to difficulties in obtaining the agreement of all parties to use the Method and to disruption of the usual workmg arrangements.

2.

Construction management

Here, there is overlapping design and construction. The client appoints designers and a construction manager, and appoints each works contractor direct. A works contractor’s final contract sum is assessed by variation or remeasurement. This has similar problems to the management contracting solution, but the client is more directly involved.

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3.

Traditional

In this contract type, which includes ICE 5th, 6th, GC/Works/l (Edition 2), JCT etc, the client directly employs a designer. The client employs the contractor by a separate contract. More often than not representatives of the designer also supervise construction of the works to check that they comply with the specification. Changes and assessment of final contract sum are dealt with by variation or remeasurement. In these circumstances, any savings resulting from use of the OM will accrue to the client only through reduced measurement. Difficulties may arise if: a.

at the time of tender development, the contractor is unknown

b.

the successful contractor, when later appointed, is insufficiently skilled to execute the Method.

The OM will work under this type of contract, but with difficulty. The designer may wish to adopt the Method to achieve an economical design, but may find that the contractor, being either unable or unwilling to use a process that so directly controls the programme and method of working, frustrates those intentions. Designers, therefore, tend not to design the works around the OM. Historically there has been little incentive for a contractor to participate in a method that seems to require more sophisticated management and control, the purpose of which is to save the client, not the contractor, money. Certainty of planning, material quantities, labour resources etc is not possible. The Method requires flexibility. It essentially involves operational risk and, thereby, time and cost risk. It also requires teamwork between designer and constructor, which the traditional contract does not encourage. This contract is the least favourable for successful execution of the OM. Nevertheless, it is possible to succeed if the contractor can either be persuaded to adopt the Method or has initiated it with the incorporation and incentive of a value engineering clause as described in Section 3.1 1. With willing participants, the OM has been successfully operated under a traditional form of contract. 4.

Design and construct

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staff or consultants) and overlaps design and construction. Tenders are usually on a lump sum contract basis, with the contract sum only adjusted by agreement between client and contractor. This option can include the procedure whereby the consultant carries the design forward to a certain stage for the client, and then works for the contractor to complete the design (ie by novation of the engineer’s agreement). This solution can include novation of the client’s initial design y d the contractor employing the client’s designer to complete the detailed design, including the OM if appropriate. Design-and-construct contracts are intrinsically more amenable than other forms to inclusion of the OM. They allow a contractor to team up with a consultant at the time of tender and to offer the client a more cost-effective solution. If the design-and-construct contract is for a fixed lump sum, the contractor will retain the saving. The client benefits through a lower tender price, as the contractor, realising at the time of tender that use of the Method might lead to savings, would discount the tender price to win the work.

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There are several important factors in the design-and-construct environment. 1.

The freedom of the design-and-construct contractor to incorporate the OM in the design solution.

2.

The ability to undertake value engineering without contractual restraints.

3.

An improved contract interface between designer and constructor.

4.

Co-ordinated design and construction teamwork, in which team members share the same primary project objectives.

5.

The clear lines of communications between the workforce at the site where the OM is being applied and the design office.

6 . A single party taking responsibility for the OM. 7.

The flexibility to optimise between buildability and design security.

These factors assist effective execution of the OM. Contractual disincentives and positioning do not become handicaps. Probably the most important factor is that it makes close co-operation between the detail designer and the front-line constructor possible. With good co-operation, the reaction time between observation and improvement or correction can be fast enough to maximise savings and minimise risk. Design-and-construct forms of contract are not without problems. More often than not the client’s adviser has prepared the feasibility study and produced an outline design for the purpose of seeking tenders. That adviser - who quite properly has an influence in assessing the tenders - might not have the wisdom, knowledge and experience to assess objectively a tender that contains an OM solution. Consequently the tender is likely to be unsuccessful. The adviser could also have an auditing role and, therefore, possibly restrict the design process. The real problem for a contractor who wishes to pursue the OM in a design-andconstruct environment arises when the client has strict approval requirements on the contractor’s design or an independent check is required. The client, having accepted an offer for a lump-sum price and off-loaded the risk, has no incentive to help the contractor through a prompt or sympathetic approval system. The client’s approval consultant or an external checker has even less interest. 96

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To allow for the possibility of later adoption of the OM, it makes sense to provide an effective value engineering clause in all contract forms (applying to designers, checkers and constructors). Risk is another important aspect of the design-and-construct contract form. In this type of contract, the contractor has to work with a designer who not only understands the Method, but also can optimise it and is willing and confident enough to support the resulting design solution with professional indemnity cover. To do so, the designer should have a full understanding of the construction management capability as well as technical skills. The design-and-construct contract encourages team members to appreciate each other’s skills and contributions.

5.

Other types of contract

Some forms of contract involve all or some of funders, concessionaries, operators, commissioners and demobilisers. These include options such as Design Build Fund and Operate (DBFO), Design Build Operate and Transfer (DBOT) and Design Build Operate and Monitor (DBOM). For the purpose of this report and the OM, these can all be considered similar to the design-and-construct form of contract.

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6.2.2

Difficulties and solutions The difficulties and solutions encountered in traditional and design-and-construct contracts are shown in Table 6.1. Table 6.1

Difficulties and solutions

Contract type Traditional eg ICE Sth, 6th, JCT, GCIWorksll

I

Difficulties Contractor not willing

Solutions introduce VE clause introduce co-operation in tender enquiry and contract scope persuade client to accept non-fixed price and programme introduce partnering.

VE clause: define submission and approval enable contractor to initiate a proposal clarify the various responsibilities of the parties determine savings and split. Third party not willing Design-and-construct eg D&C, DBFO

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client approval consultant Category 3 checking in-house concedblock.

provide incentive involve from start strong interface manager strong management support flexibility.

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6.2.3

Operating environment 1. Partnering The adoption of a partnering agreement or culture, preferably involving all parties related to or affected by the OM, can only have a positive effect. It can operate in tenders with any of the forms of contract listed in Section 6.2.2. Although perhaps expressed in a formal agreement, it identifies a relationship rather than contractual criteria. In an environment where the parties are working together to deliver the optimised solutions (in terms of cost, risk, programme, environment etc) in which the gains and the losses are shared, the OM can deliver its maximum potential. The risks and rewards are shared. 2.

Value management

The complementary nature of value management and the OM is discussed in Section 3.1 1. The inclusion of a VE clause in the contract has beneficial effects on the OM contract environment. The key features in a VE clause are discussed in Section 6.3.

6.2.4

Tender strategy

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The following influence the degree of success of the OM: Contract type

The merits of different contract forms in relation to the OM have been discussed in Section 6.2.1.

Procurement method

A contract can be let by competitive tendering, negotiating, pre-negotiated tendering and competitive negotiated.

Payment method

Each party naturally wishes to maximise its financial position through adopting the OM. Examples of commonly used payment mechanisms are fixed price, remunerable, target and cost plus.

Operating environment

The potential for co-operative working in a partnering situation can lead to realising many of the success factors. Value management encourages innovation and drives out avoidable unnecessary cost.

Different tender mechanisms can enable factors such as competitiveness, potential for added value, flexibility etc, which are conducive to the adoption of the OM. These factors vary with projects. It will be useful to assess these factors in relation to each of the above four items before selecting a suitable tender strategy, eg by an assessment matrix, for ranking each item systematically.

6.3

VALUE ENGINEERING CLAUSE In order to obtain the value through the value management process described in Section 3.1 1, a value engineering clause should be agreed by the parties. The key features of a value engineering clause to enable success of the OM are:

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1.

The right of any party to raise a cost-saving or a time-saving proposal, such as an OM solution.

2.

Demonstrable net benefit to the client. ClRlA Report 185

3.

Contract programme not being unacceptably compromised.

4.

The parties having a share of the net saving.

5.

The level of contractual risk taken by each party being reflected in the percentage split of the savings.

6.

,

Fixed submission and approval periods.

7.

The client being able to reject any proposal.

8.

Reasons being given for the rejection of any proposals.

9. A resubmission being permitted provided it adequately addresses the reasons for any previous rejection.

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A prudent client wishing to have the facility to adopt, if the opportunity arises, an OM approach within a traditional contract could arrange for the contracts of all the parties involved to reflect this possibility. At the time of tender, the client could ascertain which consultants and contractors would be willing to adopt such changes during the contract and could make the selection on this basis. The following points can be considered:

6.4

1.

The client’s incentive lies in initiating the OM before tender.

2.

The contractor shares benefit if a value engineering clause is used regardless of whether a traditional contract or a design-and-contract is adopted.

3.

A subcontractor who is significantly involved can also participate by agreement with the contractor in incentive savings.

4.

The designer’s and the checker’s incentives are often overlooked. Sometimes they receive additional fees for the additional OM-related work. For the consultant these additional fees represent a significant percentage of the total fee, but they are generally nominal in relation to the savings achieved through adoption of the OM. Designers and checkers may also consider that the experience of working in the OM enhances their reputation and prospects for future OM-related work.

5.

Other affected parties - third parties - such as those owning land or property adjacent to the works, are rarely party to the contract. They include local authorities, local environmental officers and local residents and can be hard to convince that there will not be adverse effects from the Method. The more astute of them endeavour’to receive some portion of the benefits; if they do not they can create a serious, if not fatal, blocking mechanism.

PROFESSIONAL INDEMNITY In none of the cases in Section 6.2 does either the contractor or the designer have any ongoing responsibility issues related to the adoption of the OM once the permanent works are completed, because this is only a temporary state. The designer is not normally paid to take risks on behalf of the client and is unlikely to do so as there is no incentive -taking any abnormal risk could affect professional indemnity cover and perhaps livelihood. However, a sophisticated designer, confident of being able to operate the OM, will see opportunity for both client and designer to gain in reputation, and to obtain further commissions, by successfully introducing and supervising the execution of the OM into the works at reduced cost. In this scenario, all the financial benefits of the Method will accrue to the client. The designer will educate the technical representative of the design firm’s insurance company to obtain a constant - if not reduced - insurance premium.

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The OM can manage unforeseeable conditions better than conventional design can. An insurance company will be interested almost exclusively in the cost to it of the worst case scenario. The OM of progressive modification, from “moderately conservative” to the “most likely” state, in steps from one demonstrably robust solution to another robust solution, provides a low-risk case, well below the worst-case scenario using the traditional construction approach. The insurer’s valuation of worst-case scenario is thus reduced, as failure should affect only the last step from the previous robust situation.

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The OM is only for short-term work, so the insurer takes a view, based on the reputation of the designer and the contractor, that the risk can be handled without change to the project premium. The premium will have to cover the activities of both designer and contractor. When the OM is adopted as a “best way out” option it often constitutes a significant change from the original design and construction philosophy. It is likely to have a different, though not necessarily more onerous, risk rating. Such a change has to be brought to the attention of the project insurer, which then may take, or try to take, the opportunity to raise the project premium. A contract that indicates from the outset that the OM will, or may, be used will not require later modification or increase in the insurance premium. If this has not been done and the OM is required as the “best way out”, the insurance company will have to be convinced that there is no increase in risk. Under appropriate conditions, the engineer should be able to convince the insurance agent that the OM is a least-risk option and therefore ought to attract a lower premium, because the level of monitoring and control is higher and a contingency plan is in place. Phased modification, the engineer may argue, is more controllable than the sudden unexpected change or failure that can occur with traditional methods.

6.5

CONTRACTUAL RISK Risks can be considered under three categories that generally require mitigation andor insurance consideration: 1. Technical and managerial risk

2. Commercial risk

3. Programme risk. Technical and managerial risks arise from undertaking any Construction activity including the OM. Commercial and programme risks are a consequence of adopting the Method.

6.5.1

Technical and managerial risk Procedures must be in place to identify hazards and to assess and reduce the technical and managerial risks in construction. One aspect of non-adversarial contracts is the attempt to minimise the risk to all parties so far as practical, ie the fundamental premise is that equitable solutions will be found by agreement at the time, which will preclude wasting large amounts of time and money in subsequent disputes and litigation. The greater the risk placed on any one party the more adversarial that party is likely to become to defend its position and transfer the result to other parties. The traditional view (that one should place the risk with the party best able to handle it) may well still be sensible, but that will not prevent adversarial attitudes developing when the risk

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becomes reality, and all the parties start to reread their contracts. If the will to reach fair and amicable agreement is strong, there should be little problem in solving risk issues that might arise out of applying the OM on any project. Use of the OM reduces the possibility of a significant calculation error leading to a serious or catastrophic failure. Management control is more significant when implementing the OM compared with construction of predefined designs. The OM offers the greatest certainty that the structure, the ground and adjacent buildings are behaving as intended. Unexpected changes can be picked up early through monitoring. Influence of external checker

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An experienced and competent checker is essential. Where there is an external checker without experience of the OM, the checking engineers will need to be given a sufficient understanding of, and competence in, the Method to give them confidence to accept the design proposed. The checker may have no feelings of ownership of the OM proposed and little incentive to assist its incorporation. Because the external checker’s attitude is influential, their position and views should be determined before they undertake a substantive commitment to the OM. If it is positive, the Method can be progressed with confidence. In an extreme case, it might be necessary to find an alternative checker with the capability and experience to check a particular part of the works. As the OM gains in acceptance, it is expected that the majority of designers will become familiar with the Method, so delays in the external checking will be reduced.

6.5.2

Commercial risk Any idea that the client can obtain all the potential gains of the Method in time and cost while passing all the risk to the contractor is an illusion. Whether encouraging the contractor to offer an OM option at time of tender or specifying the OM in the tender enquiry, just as in a non-OM contract, the client is never rid of all the other consequences of late delivery or cost overrun if, for example, the contractor should default. The best riskheward option lies in partnering, as described in Section 6.2.3. The progressive modification approach provides the least commercial risk, ie the option with most commercial control. A conservative approach has a high cost with little or no risk of cost change. A moderately conservative approach does not eliminate the possibility of a high cost, but just reduces its likelihood. The uninitiated sometimes perceive the OM to be a risk to project availability, either in time or cost, so they hesitate to adopt it, but the OM still provides the minimum cost option that adequately meets the actual ground conditions. A contractor cannot expect to win work by adopting a highly conservative approach. This will not survive bidding against a contractor who has adopted a progressive modification OM approach. Not adopting the OM is thus a commercial risk in itself. The contractor who is not prepared to take financial risk in a competitive market is ultimately driven out of it. Where the OM is an option, it is becoming one of the mechanisms for survival. Some clients choose to initiate and control the OM from project inception, taking the risk of uncertainty of cost outcome in order to retain 100 per cent of the benefits. In this event the OM is designed into the contract instructions and, if a lump sum rather than a

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101

remeasured contract is requested, the client pays the cost of risk assessed by the contractor in the tender price. A client wishing to pay the actual cost of the risk will use a remeasurable form of contract. However, a contractor proposing a change to the OM in a traditional remeasured contract is often expected to provide a lump sum price for the change. The client thus achieves certainty of cost and the contractor, judging the level and cost of works that may have to be undertaken, takes the risk of gaining or losing in relation to the estimate. On the other hand, the party responsible for the OM accepts that if less favourable ground conditions occur the profit will be reduced and will balance that possibility against the chance of increasing profit if the ground conditions are more favourable.

6.5.3

Programme risk By its very nature, the OM is of benefit only when the conditions are variable and design can be modified as the variability reduces.

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As the extent of the works is not fixed there is also uncertainty about the time needed to execute them, although programme scenarios can be determined for the worst potential for time lost or best potential for time saving. A judgement has to be made from experience as to the allowance made in the contract or tender programme. The view taken on programme risk will depend on its level of importance and often on the extent of liquidated damages or bonus to be applied per week of impact on the programme.

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7

Tunnel applications

This section covers the use of the OM for tunnels. It does not include cut-and-cover work, which is in Section 8. Following desk studies and site investigation, a better understanding of the ground through which the tunnel is likely to be driven allows the designer to choose a possible type of tunnel and consider a tunnelling method, eg: 0

completely unlined

0

partially lined - rockbolts and limited shotcrete Norwegian Tunnelling Method based on the Q-System (Norwegian Geotechnical Institute) New Austrian Tunnelling Method (NATM) hand-dudtimbered headings

0

shield-dnven - segmentally lined (bolted or expanded)

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machine-driven (including by earth pressure balance TBM) - segmentally lined (bolted or expanded) thrust bore or pipe jack 0

pre-vault support method.

The primary lining of the New Austrian Tunnelling Method (NATM) has been chosen to illustrate the OM applied to tunnel design and construction. The primary lining is defined as the initial ground support system. As an incremental support tunnelling system (Muir Wood, 1990), NATM can be a versatile way to review and, where necessary, to vary face and lining support measures byplanning the results of observations into the design process. It should be stressed that the feedback obtained from the execution of one fully developed design is then used to produce another equally robust alternative. The application of the results of observations must not be ad hoc (HSE, 1996). By definition, it would not be the OM. The HSE (1996) commented on the use of the OM in NATM tunnelling, and these comments compare well with the OM components described in this report (see Table 7.1). There are some situations where the .OM is inappropriate, as in long single tunnels with homogeneous geology. Here, the advantages of a TBM-driven, continuous production process dictates that the same segmental lining be used throughout, the lining being designed for the worst-case conditions found along the tunnel alignment. The variation between “most probable” (MP) and “most unfavourable’’ (MU) conditions defined in Section 3 would be too small to justify planned design modifications. In some situations (eg urban soft ground environments) fully observational NATM is considered inappropriate, not possible or of limited benefit (ICE, 1996). For these situations the support design should be predefined, with no intention of it being altered, and monitoring restricted to confirming predicted performance. NATM requires the deliberate choice and observation of trigger values and trends during construction (usually based on convergence or stresses) and their comparison against design predictions, ie the application of the OM.

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103

Table 7.1

Comparison of the OM components with comments in Report on the NATM (HSE, 1996)

HSE Report on the NATM (1996)

ClRIA (1997)

QUALITY -the achievement of quality is essential to successful completion.

NATIONAL AND CORPORATE

USE OF THE OM - the OM will be unacceptable in certain locations. KNOWLEDGE - a thorough understanding of safety-critical aspects of design and construction is required. A risk-based approach to design and management is needed. Considerable skill and care at all stages are required. The complexities should not be underestimated.

CORPORATE AND PROJECT ORGANISATION

COMPETENCE - competency of the team is crucial and should be assessed. ~

PREVIOUS EXPERIENCE -previous experience must be taken into account.

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DESIGN AND PLANNING

I

HUMAN FAILURE - the Method is heavily dependent on avoiding human failure. Desk study

Desk study.

Site investigation

Sufficient to establish that there is no likelihood of meeting unexpected conditions of a critical nature.

Data interpretation

Worst case and sensitivity analysis to assess likely behaviour over the range of uncertainty.

Initial design

A robust design is essential. An integrated approach should be taken to the design of permanent and temporary works. Design should consider the whole Construction process. Design should ensure that adequate warning of failure will be given. Design should be fully developed before construction begins.

Final design

Designs that take account of the ease of construction or buildability will greatly facilitate the achievement of quality during construction and this will lead to a better product and improved safety. A final review of designs and specifications for buildability should be made before they are released for construction. And as EC7.

CONSTRUCTION CONTROL AND MONITORING

Monitoring plan

The designer should determine the monitoring regime and contingency action. Monitoring should identify possible failure mechanisms and be based on thorough hazard analysis. Rates of failure should be estimated and limits set. And as EC7.

Site monitoring

REVIEW AND IMPLEMENT PLANNED MODIFICATION TECHNICAL AND PROCEDURAL AUDITING

104

As EC7. Two principal objectives: monitor design and monitor construction in a planned way. Multi-disciplinary review teams. Frequent and timely review.

Review and implemenl Design modifications should only be used to enhance a robust design. And as EC7. planned modification There should be emergency procedures to manage risk from developing incidents where control is apparently being lost. Not discussed in relation to the OM.

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7.1

DESIGN AND PLANNING The NATM design process is summarised in Figure 7.1.

n

F

-' I---' SURFACE AND SUBSURFACE MONITORING

ANALYSIS DESIGN

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h

6

EARTH

I

I

DRAWINGS MONITORINGPLANB TRIGGERS

YES

I

RE-CHECK OBSERVATIONS INCREASE MONITORING MODIFY SUPPORT ALTER EXCAVATION SEQUENCE

CONTINUE WORKS AND

NO

IMPLEMENT CONTINGENCY PLAN

I

Figure 7.1

STOP TUNNELLING ETC

I

The application of the Observational Method to the NATM

1. Uncertainties The design of underground support systems has more uncertainty and can pose greater risks than most other ground engineering problems. They require especially good design and careful construction control. The theoretical tools available are at best very simplified models of some of the processes that interact to control the stability of the excavation. Analytical and numerical design methods together with empirical classification systems putting, at the same time, heavy reliance upon engineering judgement and experience are used to arrive at design solutions. These facts associated ClRlA Report 185

105

with the inevitable dearth of information about most of the ground along and around the tunnel (it is not practical to drill boreholes frequently along the whole length) make application of the OM particularly apposite and an extremely useful tool for identifying hazards and containing risk. The uncertainties can be either:

0

Geological uncertainties

the ground conditions normally vary along the route of the tunnel. Hence, it is necessary to identify the range of ground conditions and prepare a solution for each type of ground condition in advance of construction

Parameter uncertainties

uncertainties in selecting design parameters to calculate the load acting on the lining.

2. Limits of behaviour The critical criteria relating to trigger values will vary from project to project. In some cases the serviceability limit state of sensitive surface buildings might determine settlement triggers; in others, greenfield conditions should allow the support lining (in terms of its ultimate limit state) to drive the types of observation to be made.

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Most designs are carried out using limit state principles. These require that the limits of behaviour are defined in advance and acceptable factors of safety are applied to the resulting design. This means considering: 0

Ultimate limit state

lining andor ground failure

0

Serviceability limit

state of the lining (waterproofing, crachng, durability) of nearby services, buildings or other tunnels.

3.

Topography and geology

Material properties of tunnel support systems are well known and variations fall within narrow ranges. However, the main construction material through which the tunnel is driven, the ground itself, is much less precisely known and can be subject to wide variation along the length of the excavation. The interpretation of data from the site investigation should address these aspects by choosing values for parameters ranging from the MU to the MP. Depending on the type of ground, the following might be considered for MUIMP assessment: a.

geological horizons, joint patternshets, faults

b.

cohesion (c’) and friction (#)

c.

stiffness (E)

d.

strength (laboratory sample and in-situ mass)

e.

earth pressure coefficient at rest (KO)

f.

permeability (sample and mass), water presencehlows

g.

time-dependent effects.

4.

Geometry

The dimensions of the tunnel will be determined by: internal cross-sectional area and slope required for its purpose 0

106

excavation profile determined by tunnelling method and lining design.

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5.

Design

The following five key stages are typical for NATM designs. 1.

Apply suitable ground classification and estimate of percentages of different rock mass types. Develop soil/structure model for the range of ground conditions identified.

2. Design robust support measures (including contingencies) and construction sequences for each rock mass type using a combination of empirical, analytical and numerical methods.

3. Establish types of trigger criteria, design instrumentation programme specifying type and number of instruments, monitoring and data-processing procedures. 4. During construction, log the excavated faces by recording the geology and structure of the ground, assess geotechnical observations and the behaviour of the installed support system, monitor construction quality and back-analyse.

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5.

Review the design and adjust support measures and construction sequences as required.

The intention is to activate the load-bearing capacity of the surrounding ground by controlled deformation and observation of the rock or soil mass and its support system (Fermer, 1938 and Pacher, 1964). While NATM is particularly suited to variable ground conditions and complex multi-junction tunnel arrangements, it is important that all schemes are designed for failure mechanisms that are ductile. Brittle mechanisms leave insufficient time for the monitoring systems to give warning of impending disaster (discovery). In general this is not a problem in underground excavations, because both the ground and the lining move into plastic modes of deformation relatively quickly when the whole tunnel/ground mass is considered (see Figure 7.2). However, sudden local variations in the ground can produce rapid movement of material, for example sections of rock mobilised on unfavourable, intersecting and possibly water-lubricated joint planes (eg “greasy-backs” in London Clay), dislodged boulders in glacial moraines or rapidly varying sedimentary deposits. The design should address separately the two support cases, namely those of the recently completed ring and the face stability. The highest risk is associated with the face (HSE, 1996), and sufficiently robust face support has to be designed, with unpredicted events being dealt with by preplanned contingency measures. Junctions are also high-risk areas. The completed detailed design of the primary lining will include: 0

a design statement giving all assumptions, and listing codes, standards, specifications and reference texts the preparation and checking of calculations that analyse the ground/lining structure in terms of the hoop, shear and bending moment stresses against which to design the support measures risk assessment

ClRlA Report 185

0

surface and adjacent structures settlement assessments

0

production of construction drawings giving alignment, construction sequences, general arrangements, elevations and cross-sections of the tunnels to be constructed

0

production of geotechnical monitoring drawings illustrating the type, position and frequency of reading of surface, subsurface and in-tunnel instrumentation

0

contingency plan and emergency plan.

107

VERTICAL DISPLACEMENT W

s

I

0.0 IW z

J

E

w

v)

I

HORIZONTAL DISPLACEMENT II

I? L Y

0.0

tW

-1

LONGITUDINAL DISPLACEMENT W

0

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2 2 0.0 _1

W

z

z I3

0 I-

TIME

b

VECTOR PLOT ILLUSTRATING INITIAL AND FINAL POSITIONS FOR EACH MONITORING POINT VECTOR SCALE EXAGERATED

Figure 7.2

Tunnel deformation measurement

For tunnels driven through varying ground a range of designs are prepared. The MU design solution for a particular rock mass classification could be the MP solution for the next “worse” ground condition. In this way the number of alternative designs can be kept to a minimum and each in turn can become a contingency solution. The preparation of all construction details on computer-aided design (CAD) systems substantially aids the ability to change detail quickly during the construction process. 6 . Trigger criteria and trend rates The choice of trigger criteria should be closely linked to the contingency plan and be defined in such a way that there is no ambiguity about their purpose or about what course of action should be taken if they are reached. One way is to associate the design 108

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serviceability limit state with a trigger value that moves crown deflections from a green to an amber zone and the ultimate limit state with an amber to red zone trigger (Figure 7.3). Moving into the amber zone might trigger, for example, a doubling of the monitoring frequency. Where a constructor has not been appointed, and thus his contingency response time is unknown, the designer must either impose his assumed response time or must change his design to reflect the appointed constructor’s response capability. While absolute values for crown settlement and convergence can be established as triggers in this analytical way, the similar determination of an acceptable rate of ground deformation for control purposes is virtually impossible for most ground conditions. Nevertheless, plotting movement with time reveals much about how the ground and the lining supporting it are behaving.

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Stabilisation of the tunnel can be confirmed by observing the speed with which deformations reduce. An experienced NATM engineer should know when the shape of the curve is showing instability (see Figure 7.2). This knowledge can be secured only after heavily instrumented sections of the initial trial tunnel have been analysed. A pattern of behaviour for that particular size and geometry of tunnel will become apparent in the ground conditions that apply only to that project. The typical deformation curves for a deep tunnel being blasted through tectonically stressed massive granite will be very different from those for a shallow tunnel excavated in clay. To attempt to put limiting gradients to these curves would suggest a recipe for monitoring that does not recognise the large number of variables giving rise to any particular curve shape. The successful assessment of trend rate is dependent on previous experience and the early acquisition of tunnel monitoring data coupled to other relevant records such as construction sequence, material strengths and geological conditions.

7.2

CONSTRUCTION CONTROL AND MONITORING

1

Figure 7.3 shows the management review process for a typical in-tunnel monitoring process. The chosen instrumentation should be both relevant and available. Sufficient resources will have to be allocated to the estimated frequency of in-tunnel, subsurface and surface monitoring. Attention to simplicity and the importance of not building-in superfluous data-gathering at this stage cannot be overstressed. It is very easy to over-specify, resulting in on-site data overload and loss of focus on the important trigger criteria. Back-up monitoring and the ability to use it must be available, eg tape extensometers in place of electronic distance measuring. The day-to-day collection of monitoring data should be carried out thoroughly and strictly in accordance with the design. Readings obtained must be passed promptly to the NATM engineers responsible for their interpretation. Daily review meetings must be held involving key design and monitoring personnel to confirm and agree the conclusions reached by the NATM engineers (see Figure 7.3).

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109

MONITORING

MONITORING

INFORMATION

RESULTS TO ONSITE ENGINEERS FOR REVIEW

,-YE~--YES

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VISUAL IhSPECT Oh ACTiVATE COhTlhGENCY PLAN ACTIVATE EMERGENCY PLAN

7

INFORMATION TO THE CLIENT FOR REVIEW

I

1

PROPERTIES

I DISCUSS WITH CONTRACTORMANAGERS DESIGN TEAM CLIENT

I -

I IMPLEMENT ACTION

REVIEW THROUGH MONITORING

YES

Figure 7.3

110

I1

Nod CONTROLLED?

,

EMERGENCY IMPLEMENT PLAN

1

Management review process for in-tunnel monitoring

ClRlA Report 185

The organisation and management of responsibilities on site is the key to the successful implementation of NATM. It must be recognised as an essential component and staffed accordingly with competent individuals familiar with the design. In urban conditions the co-ordination effort required between different disciplines can be quite complex and onerous. Communication links should be robust, well tested and fully backed up. Where possible, output collection should be automated and displayed in real time for analysis. The need to collect data from the following types of instrument might be required: precise levelling tape extensometer bolts electronic survey targets radial and tangential hydraulic stress cells in-tunnel extensometers surface extensometers inclinometers electrolevels.

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In addition, quality-control tests on materials and shotcrete lining thickness will be required, including: 0

cores for shotcrete strength, density and modulus of elasticity

0

steel-reinforcement certificates (laftice girders, mesh, rockbolts) shotcrete mix design shotcrete constituents.

It is worth noting that obtaining cores from the works increases the knowledge about the shotcrete beyond that achievable with cubes, thus reducing the uncertainties associated with the placement of this potentially more variable material when in place. The mining industry has developed its own monitoring and control procedures. Guidance on the use of rockbolts to support roadways in coal mines and the monitoring systems is given by HSC (1 996).

7.3

REVIEW AND IMPLEMENT PLANNED MODIFICATION Depending upon which OM design approach has been adopted, the results of the monitoring will be applied to bring the design closer to its most probable solution or trigger implementation of the predesigned contingency measures. This review and interpretation of the results is fundamental to the OM process when applied to the NATM. The immediate assessment of observations is carried out to confirm tunnel stability and safety. Beyond that all the data are used to: 0

compare with design estimates and predictions

0

carry out back-analyses and refine calculation models and input parameters

0

0

if possible, modify the design by altering the support measures, advance rates and treatment methods refine and adjust the monitoring requirements.

In the event that triggers are reached and monitoring reveals patterns of ground or behaviour of the structure outside those predicted by the design, contingency measures must be available to contain any unpredicted event. Such measures might range in ClRlA Report 185

111

impact from a marginal increase in monitoring frequency, to ground treatment, additional support measures or even total replacement of the original excavated volume. Examples of possible action are to: focus on geology - more logging and probing increase monitoring frequency andor accuracy slow down advance rates reduce excavation size before applying support protect ahead (spiling, sheeting) alter shape of excavation increase support measures - thicker andor stronger shotcrete, heavier reinforcement, more frequent arching andor rockbolting initiate or improve drainage install emergency propping carry out ground treatment, eg grouting, jet grouting and ground freezing (the treatment might be to improve the stability of the tunnel or to meet other risks such as protection of buildings against damages that might also affect the tunnel)

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backfill excavation, eg with foamed concrete. Whether the design approach chosen follows the “most probable”, “moderately conservative” or the “most unfavourable” route, some form of contingency plan will be required. The course of action for the designer has to be set in terms of health and safety law, and this means a risk-based approach taking account of what is reasonably practicable. Thus, if the cost difference variously between MP and MU is small but the health and safety disadvantage is considerable, the way forward is for the designer to choose the criteria that take sufficient account of the need to avoid foreseeable risk and combat those risks at source. A design based on “most probable” parameters will need to be accompanied by more of the above contingencies worked up to a maximum state of design readiness than one based on “most unfavourable” conditions for which only emergency measures will be necessary. The final contingency plans have to be carehlly developed and contained within clearly identified procedures such that measures which are required to be implemented promptly are understood by all. Responsibilities should be spelled out explicitly and changes updated regularly. The costs of the final contingency plans should be recalculated to check the Method’s continued economic viability.

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7.4

CASE STUDY - CASTLE HILL NATM TUNNELS, FOLKESTONE, UK This case history has been based on Penny et a1 (1991) with additional interviews of key personnel in the project team. It provides an example of NATM tunnelling using the Observational Method. However, some of the recent developments in the OM are not included in this case history.

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The Castle Hill NATM tunnels comprise one 4.8 m internal diameter service tunnel and two 7.45 m internal diameter running tunnels, each approximately 600 m long. The tunnels were constructed by roadheader excavation, shotcrete primary support and a secondary in-situ concrete lining. These tunnels form a section of the Channel Tunnel Land Drives connecting the UK terminal to a 500 m long cut-and-cover section of tunnels at Holywell. Figure 7.4 below shows the layout.

Figure 7.4

Plan of Castle Hill tunnels and landslip

Castle Hill is formed of the Lower Chalk with the tunnels being bored through its base encountering the Chalk Marl, Glauconitic Mar1 and the underlying Upper Gault Clay. A landslip dating back to the last glacial period is present on the western flank of Castle Hill through which the three tunnels pass. Before construction, extensive toe-weighing and drainage measures were implemented to stabilise the slip on which the tunnel portal structure sits. The development of NATM for this section of the Channel Tunnel was realised under a design-and-construct arrangement, with the client acting as an observer. Of the many reasons for adopting NATM, the main factors were: 0

uncertainty of ground conditions safe construction within the mass of the hill and landslip

0

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ease of constructing enlarged sections of tunnels and laybys for large tunnel plant

113

variability of temporary support measures to suit encountered ground behaviour leading to a less conservative approach 0

potential for additional access points for drainage galleries within the landslip drainage facility to improve ground stability programme flexibility and time saving of approximately 10 per cent over the segmental tunnel lining approach, accompanied by quicker start-up time

0

construction cost savings of approximately 15 per cent.

The contract was essentially design-and-build. The client, Eurotunnel, employed a joint venture contractor comprising five main UK construction companies. Eurotunnel retained Sir William Halcrow and Partners as a professional adviser, which, among other duties, performed the role of an independent design checker. The contractor, Transmanche-Link JV,was split into engineering and construction departments, and employed the Mott MacDonald Group as the desig; consultant. The general contract arrangement is summarised in Figure 7.5.

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The contract contained an optimisation clause that permitted the contractors’ engineering team total freedom to select the tunnelling solution that would be both technically and economically appropriate, safety being the paramount consideration.

I -

I

CONTRACTOR TRANSMANCHE-LINK JV (T.M.L.) UK Team: Balfour Beatty Tarmac Costain Wimpey Taylor Woodrow

DESIGN CONSULTANT MOTT MACDONALD GROUP

NATM DESIGN SUB-CONSULTANT

I

I.L.F. CONSULTING ENGINEERS (AUSTRIA)

DESIGN and BUILD ORGANISATION

Figure 7.5

114

I

Contractual arrangement

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7.4.1

Design and planning 1. Deskstudy The alignment of the tunnels was restricted to that shown in Figure 7.4, due to the location at the base of Castle Hill of the A20 road to Dover, and the constrained location of the UK terminal between the foot of the steep escarpment of the South Downs and the M20 motorway. The tunnels were situated in the most northerly position within the hill to avoid instability of the southern slope of Castle Hill. Vertical alignment was determined by the maximum gradient from Shakespeare Cliff to the Terminal. Geotechnical data were available from boreholes and trial pits within the Castle Hill area investigated in the 1970s. Studies clearly identified the Pleistocence landslips and led to toe-weighing and the bored pile portal to the UK terminal area. 2.

Site investigation

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The several different phases of site investigation for the Castle Hill tunnels comprised: Date

Description of investigation

1968

11

boreholes

1972

2

boreholes

7

trial pits

1974

9

boreholes

1987

10

boreholes

6

trial pits

6

standpipes

3

inclinometers

15

standpipes for construction monitoring

17

vibrating wire piezometers for construction monitoring

14

pneumatic piezometers for construction monitoring

7

inclinometers for construction monitoring

1988

In addition to the extensive site investigation, and before selection of the tunnelling method, a 2.5 m i.d. trial tunnel was driven by the NATM and segmental lining techniques at the same horizon as the proposed Running Tunnel North. Apart from identifylng the location and the support characteristics of the two tunnel linings, the tunnel allowed study of the landslip interface and the stand-up time of the Gault Clay. 3.

Design

Resulting from observations of the trial tunnel and factors listed in Section 7.1, the NATM option was chosen. Primary linings had a design life of one year based on the construction programme for the installation of the in-situ concrete lining. All design procedures were produced by the contractor. Three different temporary support classes were established for three tunnel sections. These were based essentially on the overburden and the potentially more problematic ClRlA Report 185

115

sections of tunnel through the slip zone - see Figure 7.4. Each section varied in shotcrete thickness, advance length and the presence of forepoling (spiles or injection pipes). Rockbolting was designed for all three tunnels. Construction sequences initially based on heading and bench cycles were identified for all tunnels. Secondary linings were designed for the greatest overburden load and were constructed of unreinforced in-situ concrete between 250 mm and 300 mm thick. The landslip area required movement joints and reinforcement near to the slip plane. A geotextile fleece and waterproof membrane was fixed to the shotcrete before the final concrete works to allow free drainage of groundwater behind the in-situ lining. 70 ROOF

-------- ss2 ss1 ..”.” D ROOF O \D \ 2 - - _ __ - - __ HDI

D 60

HDI

................ DDI

i ss2

ss1 1

--- DD2

50 h

E

v

8

0

c

40

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U J c

F

;30 C 0

m

4-4

.-v) a 20

10

rate of HDI 20

10

,

0

-1 0

-20

-30

4 0

Values in mm

Figure 7.6

Service tunnel Ch. 10555m: deformation monitoring results showing implementation of contingency action to curtail high deformation rate

Geotechnical plans and sections were prepared by the NATM consultant, indicating in-tunnel monitoring cross-sections for the three tunnels. Two types of section were designed. Deformation monitoring sections comprised tape extensometer readings and precise levelling of the crown and sidewall points. These sections were regularly spaced throughout the tunnel drive, but more closely at both portal zones, and were designed to monitor the structural behaviour of the shotcrete linings (see Figure 7.6). The second type was referred to as a main monitoring section. These were installed less frequently, but in areas of concern such as in the portal areas, and the more complex tunnel geometry with the cross-passage towards the centre point of the tunnel length. These sections monitored structural performance, soil-structure interaction and the ground behaviour around the tunnels. The choice of in-tunnel monitoring devices was based on tried and tested equipment that was robust enough to function in a tunnel environment.

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The main instruments within a main monitoring section are listed below: tape extensometer bolts installed in the shotcrete to monitor relative tunnel lining deformations Glotzl hydraulic earth pressure cells installed at the shotcrete-substrate interface Glotzl hydraulic tangential pressure cells to monitor hoop stresses in the shotcrete lining multi-point plastic rod extensometers to measure deformations of the surrounding ground mass 0

precise levelling of the crown points and sidewall points to monitor absolute movement of the tunnel lining measuring rockbolts installed to determine the effective length of rockbolts required. These were installed in the initial sections of the tunnel only.

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Computer analyses were undertaken on parameters derived from the site investigation, and expected crown settlements, sidewall deflection, earth and shotcrete stress values were established for the three tunnels. Although absolute value trigger levels were not stated on this project, these design values were a useful tool to the NATM engineer in determining the degree of allowable deformation of the shotcrete lining before the installation of the invert section. In addition to the in-tunnel monitoring, surface and subsurface monitoring was required and consisted of the following: 0

inclinometers through the base of the slip surface to measure the direction and elevation of movement

0

piezometers to measure pore pressures during construction and for long-term monitoring

0

standpipes to observe water table variations

0

surface settlement monuments.

Although no specific design document explicitly identified contingency measures, the full-time on-site NATM supervisors and foremen from the design consultant had standard NATM contingency options available to them, including to:

ClRlA Report 185

0

stop all excavation, and to shotcrete all faces

0

incline the excavation face

0

install forepoling

0

reduce round length

0

subdivide excavation face into smaller sections

0

reduce ring closure time and distance

0

install additional rockbolts

0

repair shotcrete shell

0

install additional shotcrete and mesh layers

0

use timber propping

0

install permanent inner lining concrete

0

backfill the tunnel.

117

7.4.2

Construction control and monitoring Monitoring plan

1.

The contractor undertook and was responsible for all monitoring. The monitoring plan was implemented by the following activities and personnel to check that the behaviour of the tunnel lining and surrounding ground was according to the design, and, if not, to modify the design to make sure of construction safety and economy. A dedicated NATM geotechnical monitoring team was responsible for all in-tunnel instrumentation. Included in the team was a supervising geotechnical engineer from the NATM design consultant, who made sure of the correct installation of equipment and the timely recording of base readings.

OFFSITE DESIGNER AUSTRIA I

-

’

SENIOR NATM ENGINEER

I

-

-.

’

DESIGN MODIFICATION

L

A

I

I

TUNNEL ENGINEERS

REVIEW BY

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CONSTRUCTION AGENT

SUPERVISING ENGINEERS

I

WORKMANSHIP

MATERIAL

CONSTRUCTION PROBLEMS

SHOTCRETE PARAMETERS

MONITORING

,

GEOLOGICAL F A C T S

~

1 MODIFICATIONS IMPLEMENTED IN TUNNEL

Figure 7.7

Typical tunnel monitoring review procedure

Face logging to establish face stability, and probing ahead with three 25 m-long holes were carried out to reduce geological uncertainties. These logs were recorded by the contractor’s geologists. The materials laboratory performed shotcrete testing by coring in-situ samples. The NATM designer’s on-site representative interpreted the geotechnical results and stabilising trends against the design trigger values and was responsible for instructing support andor sequence changes when necessary. In association with the in-tunnel geotechnical monitoring, review of the face and probe logs, subsurface instrumentation and surface settlements were undertaken. .The NATM design team in Austria which was available for consultation received all geotechnical data every week for review, comment and back-analysis (see Figure 7.7).

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2.

Site monitoring

The service tunnel started with full-face excavation. Joints in the face that were initially closed, opened and became wet. Overbreak occurred regularly. It was noted from the geotechnical measurements that the horizontal deformation of the shotcrete lining was considerably higher than the crown settlement. The crown settlement of the running tunnels could not be stabilised when excavating using the top heading and bench sequence, even with enlargement of the heading sidewall footings to increase the load-bearing capacity of the top heading shotcrete arch. The design indicated grouted or resined rockbolts for the temporary support. However, trials demonstrated that a significant number of these bolts failed to achieve the required loading capacity.

Review and implement planned modification

7.4.3

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The instability of the service tunnel face was the result of relief of horizontal stresses in the overconsolidated Gault Clay. Installation of 4 m long spiles ahead of the face, appeared to have a minimal effect on face stability. Subsequently, steel lattice arches were inclined to the face with a 2 m batter, effectively preventing the blocks from loosening, and providing a safe canopy for the miners. The relatively large horizontal deformations caused by high lateral stress in the overconsolidated clay were controlled by invert closure being nearer the advancing face. Main changes in the design and monitoring results are summarised in Figure 7.8. Grey Chalk

Ground water table lauconitic Mar1

3.496%

Excavation method

-

Gault Clay

___)

1.071%

Cut and cover heading

Inclined face 23.3"

Expected

STC

STB

STE

Actual

STCl

STA4

STE

120

120

~~~

Support tyres

Shotcrete Expected thickness Actual (mm) Roof settlements fO

I -=

Expected

*20

Measured

*40

=

Expected

'

200 200

I 240

160

+80 180

-70

STB

STA4

~

STA

I STA3 I STAZ ISTAl I 2 I Ti 120

160

160

-20

-Measured -

Figure 7.8

ClRlA Report 185

-

Service tunnel actual shotcrete thickness, supporf types and deformation compared to design

119

As the service tunnel passed through the landslip zone, extensive monitoring of the ground behaviour resulted in restraints being imposed on the running tunnel construction through the slip, ie: 0

constructing the running tunnels from east to the west

0

constructing Running Tunnel South first

0

the Running Tunnel South face to be seven tunnel diameters ahead of Running Tunnel North no secondary lining to be placed until all works in the slip zone had been completed and had stabilised.

The relatively large crown settlements in the running tunnels were controlled in the same manner as for the service tunnel. The heading-and-bench method was changed to full-face excavation with inclined girders, followed by earlier invert closure, thereby stabilising the tunnel. This change occurred only 50 m from the portal. Figure 7.9 shows the modified support arrangement and sequence.

A new approach was adopted for the installation of rockbolts simply driving ribbed steel bolts into the ground with the drill rig hammer, enabling pull-out loads in excess of the design load to be achieved.

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Spiling was introduced at the start of the tunnels and continued throughout their length. The quantity and length varied to match the geology encountered and advance lengths.

e

7--------

HEADING 1. Excavation 2. Sealing shotcrete 3. Mesh and lattice girder 4. Fore poling 5. Shotcrete 6. Rock dowels

i Figure 7.9

120

Modified running tunnel temporary support design ClRlA Report 185

Excavation applications

This section discusses the use of the OM in the following applications: cuttings including slopes 0

retaining walls for cut-and-cover tunnels, roads and basements.

Cuttings and retaining walls can be supported temporarily by different systems - see Table 8.1 for examples. The application of the OM to cuttings involves the application of planned modifications to the temporary support system during construction. For retaining walls, modification can also be applied to the installation method, excavation sequence and rate of excavation - see Table 8.1. Table 8.1

Examples of excavation applications

Application

Installation method

Temporary support system

Construction sequence ~

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Cuttings

Retaining walls

not applicable

pre-cast units in situ (diaphragm wall, contiguous or secant pile walls)

counterfort trenches nails piles bolts/anchors geotextile lime stabilisation drains wall element props/slabs tiedanchors bebuttresses

not applicable

top down bottom up cantilever bays

There are two ways of applying the OM to excavations. First, the observations at one location are interpreted and the knowledge gained is applied to modify the design at other locations linearly along the route of a highway or around the boundary of a site. Symons (1992) considered that highway schemes involving long lengths of similar construction might be especially suited to the OM because of the scope they give for monitoring to be carried out and the design evaluated at an early but non-critical part of the work. Second, the knowledge gained through observations at the early stage of an excavation at a location can be used to modify the excavation sequence and temporary support design at that location during later stages. For example, the observations at the early stage of a basement excavation can be used to confirm the geology or the behaviour of the project or the adjacent structures. The information can then be used to calibrate the design parameters, allowing the excavation sequence to be modified before progressing to a deep level - see for example the work by Dawson et a1 (1996) at London Underground’s Bermondsey station. The principles of applying the OM to staged excavation works are discussed by Ikuta et a1 (1 994) and are summarised in Figure 8.1. ClRlA Report 185

121

In the view of Ikuta er a1 (1994), the OM is used to judge in advance whether the actual condition of the earth retaining structure is approaching a “danger line” (Figure 8. l), so that planned countermeasures can be taken, if necessary. In addition, if the measured data are in the vicinity of the “rationalisation line” (Figure 8. l), subsequent stages of the construction scheme could be revised to save time and money. The safety of the excavation is also re-evaluated as construction progresses.

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The danger line shown in Figure 8.1 is equivalent to the “red trigger” discussed in Section 3.9. This is normally governed by health and safety regulations, the damage criteria set by the owners of adjacent properties or the serviceability limit of the project structure at the final construction stage (see Section 3.9). The rationalisation line represents the chosen factor of safety and is likely to be governed by health and safety regulations and economic criteria, eg whether the financial benefits after design modification outweigh the cost of redesign.

Designed values Before excavation

Figure 8.1

122

First stage

Second stage

Third stage

Fourth stage

The Observational Method applied to multi-stage excavation (based on lkuta et al, 1994)

ClRlA Report 185

8.1

CUTTlNGS

8.1.I

Design and planning 1. Uncertainties Designers face both geological and parameter uncertainty when designing cuttings. Geological uncertainties can arise from complex geology, such as existing shear planes and tectonic features. As a result of these uncertainties (whether in terms of location or strength), conservative design parameters are used to cover the worst scenario. This is illustrated by the case of Bay Tree Cottage (Box 8.1). The OM based on phased site investigation and staged excavation, as described by Lord et aZ(l991) in Box 8.2, is useful in dealing with geological uncertainties. Box 8.1

The application of the Observational Method to design cuttings in soil at Bay Tree Cottage, Bath

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A 6 m-high temporary cutting was excavated in Fuller’s Earth at the Bay Tree Cottage site in Bath. Borehole records indicated that shear planes existed within the Fuller’s Earth. However, the designer could not determine with certainty the following in advance of the site work: a. b.

the continuity of the shear planes the depth and extent of the shear planes.

In the face of these uncertainties over the complete nature of the shear plane, the project team decided that the OM should be used on the temporary works. Based on “most probable” conditions, a scheme using counterfort trenches would be adequate to stabilise the slope and to allow steep temporary batter slopes to be adopted. These comprised 10 m-long by 1 m-wide trenches at 6 m centres, the base of each trench extended to 1 m below the shear plane (see Figure 8.2a). These trenches were filled with gravel to provide additional resistance across the shear plane. The following procedures were adopted: 1.

Two trial trenches were excavated in advance of the construction work for confirming the most probable conditions in the ground and for trying out the construction method.

2.

Excavationsfor the counterfort trenches were carried out in stages. The geology at each trench location was logged and the position of the shear plane exposed during construction recorded. Movement criteria were set for the adjacent areas at each stage of the construction (based on case histories). A “traffic light” control system was set up, together with an instrumentationscheme of surveying and inclinometers.

3.

In the event of excessive movement, the excavation would be backfilled with the excavated material and a soil nail scheme, based on the most unfavourable conditions (Figure 8.2b), would be implemented.

Measurementstaken during the excavation indicated that the movements were less than the predefined criteria. The contingency plan was never implemented. (Courtesy of Arup Geotechnics, Amey Construction Ltd, Highways Agency and Sir Alexander Gibb and Partners.)

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123

Box 8.2

Use of staged excavation at Izmit-Adapazari, Turkey (after Lord et at, 1991)

During the construction of the Izmit-Adapazari section of the Anatolian Motorway in Turkey, an 80 m-high cutting was excavated through the Saribayir Headland (Figure 8.3), a heavily vegetated headland formed from Pleistocene soils. Design of the cutting was based on conventional investigations, including site reconnaissance, aerial photographs, desk study and boreholes. The boreholes showed the headland to consist predominantly of sandy silts and beds of gravel, with some evidence of thin lenses or perhaps continuous seams of clay. However, there was uncertainty regarding the continuity and the dip direction of these clay seams. It was considered that there was a possible risk of failure on these clay seams when the cut was made. As the geological structure and stratigraphy could not be fully determined by borehole investigations, a staged excavation was instructed by the engineer with the following procedure: 0

0 0

excavation to proceed from the top of the cut to the temporary profile indicated in Figure 8.3 the face of the cut to be carefully inspected and logged during excavation 20 m-deep exploration shafts to be sunk from the level of the temporary profile, as indicated in Figure 8.3, in order to examine the strata in detail, in particular for the possibility of continuous clay layers, prior to excavation to the final design profile.

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Furthermore, a design capable of accommodating modification was prepared. This design (Figure 8.3) can be summarised as: 0

maximum side slopes of 1:2

0

berms, 6 m wide, at 10 m intervals vertically

0

10 m-long inclined drains drilled at 10 m intervals above each berm to maintain a lowered groundwater level seepage encountered during excavation of the cut to be intercepted by 2 m-deep counterfort drains at 10 m centres.

0

The number of drains (which are the slope stabilisation measure) could be increased or reduced in accordance with the actual geological conditions logged by the engineer as excavation proceeded. The exposed faces of the excavations were logged. Important geological features, particularly clay layers and faults, were photographed and sketch maps and elevations were drawn. Dip and direction of the beds were measured using compass-clinometers. The positions and elevations of any key features were determined by survey. All the information was plotted on a working map of the cutting to build up a detailed picture of the stratigraphy and structure of the hillside. The 1 m-diameter, 20 m-deep shafts were sunk using a bored piling rig. A purposemade slotted steel lining permitted safe inspection. Several shallow trenches and trial pits were also excavated down the slope batters to enable the clay layers to be identified, surveyed and correlated along the length of the cutting. These supplementary investigations revealed that the thin clay seams, 200-300 mm thick, were much more frequent than anticipated, typically at 1-2 m intervals. Furthermore, many of them contained shear surfaces as a result of tectonic disturbance. The deposits had been uplifted, tilted and faulted. This information enabled suitable design modifications to be made before the second stage of excavation to the final design profile. Vertical drains were installed in some areas to enhance stability. However, the inclined drilled drains and counterfort drains in the original design were no longer required in some areas because of the frequent gravel bands. The operations were successful and the motorway was opened in 1991.

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ClRlA Report 185

Counterfort trenches filled with coarse gravel, penetrating to 1 m below the observed shear plane

/

a. Design for the most probable conditions

Soil nails

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Closely spaced soil nails up to 24 rn long

-z

'Most unfavourable' shear plane

-- ----I

b. Design for the most unfavourable conditions

Figure 8.2

The design options at Bay Tree Cottage, Bath Nominal bench level

Oriqinal wound level, Saribayir headland Temporary profile,

Realigned State Road

--------

Cutting height 80m

Inspection shafts

Design profile

LEGEND h

@ )-

Berm level in meters above datum

Figure 8.3

ClRlA Report 185

Staged excavation at Izmit-Adapazari, Turkey (Lord et al, 1991)

125

In some construction operations, standard design theories may not be adequate to fully assess the effects of the operations on adjacent ground and structures, eg blast-induced vibration. The OM may then be used as a “best way out” to control the work procedures adopted in these operations. Such a case is described by Nicholls (1996) in Box 8.3. Box 8.3

The application of the OM to rock slope excavation (after Nicholls, 1996)

Nicholls (1996) described the use of the OM to prevent instability of a rock slope that could result from blast-induced vibration during rock excavation. Concern was expressed about the stability of rock blocks resting on adversely oriented sheeting joints near the top of the slope. Standard limit equilibrium models were considered to be inadequate for assessing the likely effects of blast-inducedvibration on the blocks. The OM was initiated as a “best way out” solution to conditions that were found to be less favourable than those assumed during design (the sheeting joints had not been identified as being likely to affect block stability).

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The approach relied upon the conviction that before block failure, significant lateral displacements would occur. Therefore a limiting translational displacement could be identified, and movement towards this limit monitored. A limiting displacement threshold of 5 mm was established on the basis of rock mechanics theory and published data. A number of blocks were fitted with reflector targets and monitored from a remote stable monitoring station by theodolite. The monitoring extended over several weeks to confirm that no long-term movements were occurring. If movements approaching the limiting criterion had occurred, a contingency procedure would have been invoked that prohibited further blasting until either: a.

a retaining buttress had been installed, or

b.

the relevant blocks had been stabilised.

No significant movements were identified during monitoring and the excavation was completed successfully without the need to carry out contingency measures.

2.

Limits of behaviour

EC7 (BSI, 1995) Section 9.2 states that the following limit states, have to be considered when designing slopes and cuttings: 0

loss of overall stability or bearing resistance failure due to erosion

0

failure due to surface erosion or scour failure due to hydraulic uplift deformations (including those due to creep) of the slope and their foundations that cause structural damage, loss of serviceability or failure in adjacent structures, roads or services rockfalls.

In addition, the following papers and design codes can provide a basis for establishng the range of behaviour of cuttings and slopes.

BS 603 1 :198 1 Code ofpracticefor earthworks (BSI, 198 1 b) discusses the factors governing the stability of cutting slopes in soils and rocks. It also recommends safe slope angles for various rock types for preliminary design. Chandler (1984) tabulated the slope angles and heights of short-term failures in fissured clays. The paper discusses soil brittleness and its relation to progressive failure in slopes. 126

ClRlA Report 185

The TRRL Research Report 199, A survey of slope condition on motorway earthworks in England and Wales (Perry, 1989), examined the factors that contributed to shallow failures on the side slopes of embankments and cuttings and quantified the long-term problems. It also examined the percentage of failures of existing slopes in various types of geology and recommended suitable slope angles for economic construction. 3.

Risk and responsibility

Many cuttings are designed for temporary use only. This design responsibility normally lies with the contractor. Nicholson (1992) proposed that temporary works such as battered slopes should be classified as low, intermediate or high, according to their risks (see Figure 8.4). In his view, the involvement of the engineer.in temporary slope design should vary depending on the level of risk. Where the risk is high, the engineer should: 1. Identify the design basis, taking account of site investigation data, relevant case histories, adjacent structures and possible construction sequence. 2. Consider options for designing temporary works based on the OM (Plan A based on “most probable” conditions, eg undrained conditions, Plan B based on “moderately conservative” or “most unfavourable” conditions, eg drained conditions). 3. Assess the risk and consequence of failure, and tailor specifications to suit.

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4.

Provide supervision, monitoring and remedial measures.

Identify design basis

Temporary works -options

-site investigation -adjacent structures -possible construction

- Plan A - eg Undrained (c,) - Plan B - eg Drained (c; 4 ’)

Assess risks and revie with client

Low risk

T

Performance specification

Intermediate

High risk

Engineer scheme design

Engineer detailed design

T

Performance specification with:

T

Method specification with performance limits

-settlement limits - dewatering limits -monitoring -supervision

Figure 8.4

ClRlA Report 185

The engineer’s approach to temporary battered slope design (after Nicholson, 1992)

127

The Hong Kong Geotechnical manual for slopes (GCO, 1984) also suggested use of different factors of safety for slope design in Hong Kong in accordance with categories of risk to life and economic risk (see Figure 8.5). Failure of a slope in one of Hong Kong’s country parks would have negligible risk to life or damage to property. On the other hand, a failure that could cause structural damage to an occupied building could present high risk under both categories. The recommended factors of safety are larger than 1.O and 1.4 for the design of new slopes for these situations respectively (Figure 8.5).

RISK TO

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ECONOMIC

Recommended factor of safety against loss of life for a ten-year return period rainfall

Negligible

Low

High

Votes: (1) In addition to a factor of safety of 1.4 for a ten-year return period rainfall, a slope in the high risk-to-life category should have a factor of safety of 1.1 for the predicted worst groundwater conditions. (2) The factors of safety given in this table are recommended values. Higher or lower factors of safety might be warranted in particular situations in respect of economic loss.

Figure 8.5

128

Recommended factors of safety for new slopes and rainfall with a ten-year return period in Hong Kong (GCO, 1984)

ClRlA Report 185

8.1.2

Construction control and monitoring In excavating a cut, maintaining stability of the slope above is critical. The method of checking.will depend on what is known about the slope and on how the cutting and the associated works will make for potential instability. It may not be feasible to monitor the cut face until it is fully formed, so instrumentation is likely to be put in the slope above. In which case, the instrumentation might include: 0

inclinometers, electrolevels etc for monitoring lateral ground movements

0

magnetic extensometers, electrolevels for monitoring vertical ground movements

0

survey targets embedded in the ground for monitoring surface movements.

When the cut face has been formed, it can also be monitored, for example by survey or by electro-levels.

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Pore water pressures can also be a control criterion. Construction control measures could include not deepening the cut until drainage has been installed or the pore water pressure reduces. At Folkestone Warren, in south-east England, the cyclic variation of the pore water pressure in the slope has caused periodic landslips. Observation of the pore water pressure could therefore indicate a greater or lesser likelihood of resumed movement (Muir Wood, 1990).

8.1.3

Review and implement planned modification Commonly used strategies for modifying cutting designs are as follows: 1. Addition of surface protection and surface drainage

Examples of such protection include sprayed concrete, mesh, crest and toe drains etc.

2. Regrading slope angle Benches can be formed in the slope, or the slope angle can be reduced or steepened depending on the site observations. 3.

Additional drainage Slope stability may be improved by installing drains and relief wells, as in the case described by Lord et aZ(1991); see Box 8.2.

4.

Toe fill or earth berm Placing fill or an earth berm at the toe of a slope may improve the slope stability.

5.

Retaining wall or piles An example is the use of gabion walls to stabilise slopes along motorways.

6.

ClRlA Report 185

Installation of reinforcement For slopes containing existing shear planes or where landslides have occurred, slope stability may be improved by installing reinforcement such as nails, bolts, geotextiles, anchorages, or counterfort trenches as in the Bay Tree Cottage site; see' Box 8.1.

129

8.2

RETAINING WALLS

8.2.1

Design and planning 1. Uncertainties

Uncertainty about geology leads to difficulty in estimating matters such as: values of soil strength parameters 0

rate of consolidation, ie the time required for the soil to change from undrained to drained behaviour

0

rate of softening of soil on the passive side of the wall

0

depth of soft and compressible deposits

0

continuity, extent and pressures of water-bearing layers

0

safety.

Symons (1992) also identified the following other uncertainties in predicting the behaviour of retaining walls:

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initial stress state in the ground 0

pre- and post-construction water levels and drainage boundaries

0

wall stiffness and sequence of construction existing and projected changes in superimposed loading.

These uncertainties affect the choice of support system, the installation method and the excavation sequence. 2.

Limits of behaviour

Design codes and standards should be used when setting the limits of behaviour, those used in the UK in relation to retaining wall design being: 0

BS 8004: 1986 Foundations

0

DD ENV 1997-1 : 1995 Eurocode 7: Geotechnical design

0

BS 8002: 1994 Earth retaining structures

0

CIRIA Report 104 (Padfield and Mair, 1984).

Limits of behaviour that have to be explored before intiatingthe OM include the following: Ultimate limit states 0

loss of overall stability

0

failure of a structural element or failure of a connection between elements

0

combined failure through the ground and structural elements bearing capacity, sliding and toppling failure for gravity walls

0

for embedded retaining walls, failure by rotation or translation or lack of vertical equilibrium of the wall safety to people.

130

ClRlA Report 185

serviceability limit state

movements of ground or retaining structure causing damage to adjacent buildings 0

movements of ground or retaining structure to cause loss of performance of drainage or to affect aesthetics of wall

0

unacceptable leakage through or beneath the wall

0

unacceptable transport of soil grains through or beneath the wall

0

unacceptable change to the flow of groundwater

0

structural durability.

Accident limit state 0

overload by surcharge

0

temporary works damage

0

burst water main.

The performance of a retaining wall is best expressed in terms of its movements and the movements it causes to the adjacent ground during construction. Papers detailing observed movements induced by the construction of different retaining wall types and construction sequences are listed in Table 8.2. The empirical relationships from these papers can assist the determination of trigger criteria.

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3.

Rapid deterioration

The potential hazard of rapid deterioration during retaining wall construction includes the following situations: 0

sudden increase in water pressure at the back of retaining wall, eg a burst water main

0

sudden overload of the support system during excavation of adjacent bays (progressive failure) rapid deterioration of temporary works caused by an accident, eg props knocked away by site traffic

0

failure as a result of over-stressing soils with brittle characteristics.

Although the effect of these might be localised and containable, the deterioration in one place could overload adjacent support and lead to progressive collapse and potentially very serious consequences.

ClRlA Report 185

131

\

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Table 8.2 Publications describing experience on retaining wall behaviour Choice of supporting system

Relevant content

Institution of Structural Engineers (1975) Design and construction of deep basements.

Advantages and disadvantages of various temporary and permanent support systems.

NAVFAC (1 982) Design Manual 7.2

Factors involved in the choice of a support system for a deep excavation.

GEO (1 990) Review of design methodsfor excavations

Various design methods for excavation support.

Card and Carder (1996) Movement trigger limits when applying the Observational Method to embedded retaining wall construction on highway schemes

Guidance on suitable movement trigger limits for use in developing the Observational Method for highway retaining structures.

Twine and Roscoe (1997) Prop loads: guidance on design

Guidance on design of temporary prop support to retaining walls.

In relation to movements in the adjacent ground

Relevant content

Peck (1969b) Deep excavations and tunnelling in soft ground.

Settlements adjacent to open cuts in various soils, as a function of distance from the edge of excavation.

Boscardin and Cording (1989) Building response to excavation-induced settlement.

Range of deformation in varipus soils relative to building damage potential.

Clough and O’Rourke (1990) Construction induced movements of in situ walls.

Observed movements of different wall types in different soils and observed movements caused by installation of walls

St John et a1 (1993) Prediction and performance of ground response due to construction of a deep basement at 60 Victoria Embankment.

Observed movements of walls constructed by different support systems in London Clay.

A database of ground and wall movements at sites Carder (1995) TRL Report 172: Ground movements caused by different retaining wall where diaphragm walls and bored pile walls were constructed using different construction construction techniques. techniques. Carder and Bennett (1996) The effectiveness ofInvestigation of the effectiveness of both soil berms and rakedprops as temporary support berms and raked props as temporary supports during construction of embedded retaining walls. to retaining walls

132

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8.2.2

Construction control and monitoring Retaining wall movement can be used as a criterion for construction control. In general, the movement monitoring systems for slopes described in Section 8.1.2 could also be applied to monitor retaining wall movements. When props are used for temporary support, the prop force could also be used as a supplementary criterion for construction control. Vibrating-wire strain gauges are typically used to measure strain in props, which can subsequently be converted to load for a specific steel section. Guidance is given in the CIRIA Report FR/CP/48 (Twine and Roscoe, 1997).

Review and implement planned modification

8.2.3

Commonly used strategies for modifying embedded retaining wall design include: prop installation, elimination, or replacement by earth berms controlled variation of earth berm geometry.

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1.

Props

Temporary support to an embedded retaining wall is commonly provided by steel props. Various calculation methods are in use which predict significantly different prop loads. Prop loads can be assessed by analytical methods that model the excavation sequence. Measured prop loads are frequently less than the predicted values. Prop loads can also be derived from semi-empirical methods. For example, Peck (1969a) described a semiempirical approach in which the mean value of the measured prop forces at nearby sites was used to design the props at a new site, see Figure 8.6. Recently, thermal expansion effects have been found to be important. For further guidance see CIRIA Report FR/CP/48 (Twine and Roscoe, 1997).

G I

.i i

51.5' Struts 14 BP 89

c

1

Predicted envelope of strut loads

I 110'

I11'

I'

18W 96 at 6' ctrs

1

Pressure: kip/sq.ft N.B. 1 kip/sq.ft = 47.9 kN/m2

(b) Comparison of measured and predicted apparent earth pressures

(a) Section

Figure 8.6

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0

Design value for scuts 2 3

Prop design at Harris Bank and Trust CO, Chicago (Peck, 1969a)

133

In some situations, observations may suggest that props are not required to achieve the specified performance. The so-called “soft prop” strategy was used in Limehouse Link (Glass and Powderham, 1994), as described in Box 8.4. A pre-determined gap was left between the prop and the retaining wall. The closure of the gap was monitored. If the gap did not close during the excavation, it was considered justifiable that the prop was not needed and could then be removed. In other situations, earth berms are used instead of steel props. This is illustrated by the A4lA46 case history in Section 8.3. Box 8.4

Modification strategy at Limehouse Link, London, using soft props (after Glass and Powderharn, 1994)

The original top-down design used side walls constructed by diaphragm and secant pile walling techniques. During top-down excavation, this design required temporary support to be provided at mid-height between the roof and base slabs, see Figure 8.7. One of the principal objectives of using the OM at Limehouse Link was to eliminate the mid-height propping and create unhindered working space beneath the roof slab, leading to a substantial increase in progress.

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A progressive modification approach was adopted. The implementation procedure is summarised as follows: 1.

Prior to installation of a prop, a propping trial was initiated at each construction front ahead of the advancing the excavation.

2.

The first stage involved installing a “hard prop” in accordance with the original design. These were normally prestressed to 10 per cent of the specified design load. Prop loads and temperatures were monitored as excavation progressed, the former being measured by vibrating wire strain gauges linked to automated data loggers. Actual prop loads were temperature corrected. They was found to be considerably lower than was originally assumed and typically not more than the nominal pre-load.

3.

The second stage repeated the first and the base slab blinding was increased to 300 mm to act as a strut. When it had gained sufficient strength, the props were destressed and wall closure monitored.

4.

The third stage was to install “soft props”. These left a gap of about 20 mm at one end could be monitored but would allow only limited wall movement before the prop started to act. Up to this stage, the propping system as originally designed was still being used.

5. Observed wall movements remained acceptable, and the fourth stage proceeded without props. Excavation commenced at the designated prop level and was taken down the full width of the tunnel to base slab formation. Excavation was limited to a length of 5 m; the blinding strut was cast the same day.

6.

Measurements of side wall movement were taken at designated positions and times during construction using tape extensometers and surveying techniques - the former to monitor convergence and the latter to give absolute movement. Measurements were found to be similar to those obtained by monitoring the gap in the “soft props”. To guard against large wall movements, a contingency prop and a number of reserve props were kept near to each construction front.

The OM was applied to all nine tunnel construction fronts for the top-down sections. In each case after the initial stage of hard and soft prop installation, further propping was not required.

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1-Original

designl + k i l u e engineering design

Figure 8.7 Use of soft props at Limehouse Link, London (Glass and Powderham, 1994)

2.

Earth berms

Earth berms are also frequently used to support in-situ embedded retaining walls. The advantages of using an earth berm rather than steel props are: under careful control and planning it is a cheaper method it integrates into the overall excavation sequence more readily it leaves greater headroom for heavy construction plant, and because there is no need to erect and dismantle props it provides an intrinsically safer working environment. The effectiveness of earth berms as.a.,meansof temporary support to retaining walls was discussed by Carder and Bennett (1996). Based on the results of finite element analyses, they reported that short- and long-term lateral movements of the top of a wall when using progressive construction with 6 m-high berms were similar to those to be expected from using temporary horizontal props. A 6 m-high berm would therefore be equally effective in controlling wall movements. Short-term movements with a 2 m-high berm were closer to the cantilever situation although longer-term movements were similar to those with horizontal propping. Powrie and Daly (1996) on the basis of their research studies, concluded that there was quantitative evidence of the ability of large berms to reduce wall movements. In the short-term, however, friction between the wall and the berm may be unreliable because the relative movement at this interface is small.

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Padfield and Mair (1984) warned that the effectiveness of clay berms can be impaired by changes in moisture content. If the berm dries out, it tends to shrink away from the wall, thereby permitting wall movement. If the moisture content increases, the berm softens. It is good practice, therefore, to blind or blanket the clay surface with a waterproof membrane to maintain its natural moisture content. The Minster Court case history, see Box 8.5, shows how a conservative berm geometry was progressively modified in response to the actual ground movements. The A4lA46 case history in Section 8.3 shows how earth berms were used to replace the steel props for the temporary support. Box 8.5

Modification strategy at Minster Court, London, using an earth berm (after Tse and Nicholson, 1992)

At Minster Court, a 9 m-deep basement was formed using a 0.8m-thick diaphragm wall. About 5 m from one side of the deep basement are the London Underground Limited District and Circle Line tunnels (see Figure 8.8). The basement was constructed using a “semi top-down’’ construction technique, ie the sides of the basement wall were excavated using the top-down excavation technique while the centre of the basement was excavated by bottom-up excavation technique. An earth berm was leftto support the retaining wall. Concern was raised about possible movements on the side of the basement adjacent to the District and Circle Line tunnels.

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The likely construction movements included those from (1) demolition of existing buildings (2) diaphragm wall installation and (3) berm excavation. The following procedures were adopted in the implementation of the OM: The allowable movements of the adjacent tunnels were agreed between the owner of Minster Court and London Underground Limited. The behaviour of the diaphragm wall was analysed using the computer program FREW. In conjunction with case history data for similar construction methods and ground conditions, the allowable movement in each activity was defined. They were : Demolition lateral movement less than 15 mm lateral movement less than 10 mm Installation of diaphragm wall Berm excavation lateral movement less than 30 mm Total lateral movement less than 40 mm (NB Not all maximum movements apply to the same level.) These are based on the “moderately conservative” assumptions. Two berm sizes were designed: a. a small berm for the moderately conservative condition with a 4 m-wide top, 12 m-wide base, 5.2 m high and a slope angle of 30° b. a large berm for the most unfavourable condition with a 8 m-wide top, 12 mwide base, 3 m high and a slope angle of 40° The ground movements were measured at an agreed frequency by surveying, inclinometers and extensometers. In-tunnel surveys were made and deformations of the tunnels measured by tape extensometer. The movements of the tunnel walls were measured by surveying. Construction of the basement proceeded with the excavation to the ‘large berm’ initially. The cumulative movement at the end of this stage was about 10 mm, which was significantly less than the specified criteria. This gave confidence to the contractor to trim the berm progressively to the ‘small berm’. The contingency plan was to backfill the berm to its original size in case of large movements. The measured wall movement at the end of the excavation of the ‘small berm’ was about 13 mm and the contingency measure was not required.

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1

3. Basement excavation and pro Berm size reduced if movement recorded is satisfactory pping'

'1

Stanchio:

Supporting the existing retaining wall of old building.

Capping beam

\\

\

Floor slat; 7

r---\Armco casing /

Existing District and Circle Line tunnel -]con Less than 15 mm lateral movement Less than 10 mm lateral movement Less than 30 mm lateral movement

of diaphragm wall.1

Diaphragm wall Docklands Light Eastbound

' l

'Bored

pile

Railway City extension Westbound

Note that not all maximum movements act at the same level

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N.B. Details of the superstructure not shown

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Figure 8.8

Variation of berm geometry at Minster Court, London (Tse and Nicholson, 1992)

Figure 8.9

Site location plan at Batheaston-Swainswick Bypass

137

CASE STUDY - A4/A46 BATHEASTON-SWAINSWICK BYPASS, BATH, UK The alignment of the Batheaston Swainswick Bypass required the construction of approximately 800 m of parallel diaphragm retaining walls. A plan of the area is shown in Figure 8.9. The retained height of the ground behind these walls ranged from 3.5 m to 10 m, requiring many different wall design cases. The pre-tender ground investigations at the site showed the stratigraphy to be Midford Sand overlying Lower Lias Clay. In some sections along the alignment the geology was further complicated by the possible presence of ancient landslips. The conventional division of responsibilities had been adopted on the project, the engineer being responsible for design of the permanent works while the contractor was responsible for construction including the design and construction of the temporary works.

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As part of the permanent works “predefined” design, the engineer proposed the construction sequence shown in Figure 8.10a, involving placing props between pairs of facing diaphragm wall panels followed by excavation to the formation level of the road. Prop stiffness per metre run and the load capacities per metre were defined by the engineer. The temporary prop layout was to be designed by the contractor. The implication of this construction sequence was that heavy steel sections would be required to prop between diaphragm wall panels. This would result in restricted working space and a complicated construction sequence. The contractor, after discussions with their temporary works design consultant, proposed a simpler and more economical alternative construction sequence, see Figure 8. lob, involving the use of the OM and supporting the excavation with temporary berms. The berm size and the sequence of berm excavation were assessed during the temporary works design and trigger values were defined. The wall deflection and excavation sequence were monitored against the trigger values during excavation.

8.3.1

Design and planning 1. Deskstudy Prior to detailed analysis of the excavation sequence it was necessary to ascertain the most probable ,and most unfavourable geology and soil parameters which could be expected during excavation. The contractor’s designer reviewed the existing site investigation information and assessed case histories of excavations in similar Lower Lias Clay strata. These records included details of wall movements. Based on this review some further ground investigation was carried out, giving the consultant confidence that undrained conditions would prevail during the expected excavation period. Calculations predicted that the proposed wall designs would work as cantilevers without temporary propping. A contingency plan was developed for the most unfavourable conditions, necessitating rapid excavation of the berms in bays and construction of the propping slab.

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1

-4

5 8

2

3

5 6

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Original construction sequence Excavate to wall installation platform level Construct diaphragm walls Partial excavation inside walls, local excavations outside walls Construct capping beam Place temporary prop Excavate to road formation Place permanent prop in local excavation Remove temporary prop Construct pavement

1 I

5

Alternative construction sequence Excavate to top of guide wall level Construct diaphragm walls Construct capping beam Excavation inside walls leaving berms Excavate to road formation level (within bays if necessary) Construct permanent prop Construct pavement

Figure 8.10

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(a) Original construction sequence (b) Contractor's alternative construction sequence

139

In order to help the contractor assess the net benefit of using the OM, the advice given by the contractor’s designer can be summarised as:

Advantages

Disadvantages

1 . Immediate savings (propping system)

1. Additional investigation and design work (design of temporary works and monitoring)

2. Extra working space (no props in the way)

2. Risk of failure (lives, reputation, financial)

3. Shorter work programme (simple construction method)

3. Difficulty of getting the OM procedure and design accepted by the engineer.

The contractor decided to proceed with the OM.

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2.

Site investigation

The initial (pre-tender) site investigation was designed by the engineer for the entire route of the bypass. The contractor was not involved at that stage. An additional site investigation was carried out by the contractor, acting on the advice of his temporary works designer and with the agreement with the engineer consisting of Standard Penetration Testing and undisturbed sampling, with quick undrained triaxial and index tests. Test results were interpreted and compared with case history data. They confirmed that the “most probable” mode of behaviour for the Lias Clay during excavation would be undrained rather than drained. The strength parameters obtained from the testing allowed site-specific design lines to be derived and these parameters were used to refine the temporary works design. 3.

Data interpretation

From case histories and site investigation data, the “most probable” and the “most unfavourable” design conditions were determined. In this case the “most unfavourable” design conditions were similar to those adopted by the engineer for the predefined design - see Table 8.3. Table 8.3 Design conditions adopted for retaining wall design at A4lA46

Most probable

Active 0

Q

Total stress design. Adopt 100% CII.

Water level at the top of the Lias Clay. Q

Most unfavourable (based on CIRIA Report 104)

Passive Q

o

Total stress design parameters. Adopt 100%

Active 0

0

CU. Q

0 Q

No softening. K O = 1.0 El,& = 1000.

0

Effective stress design parameters. Water level at the top ofthe Lias Clay E&,= 1000.

Passive 0

0 0

0

Total stress design parameters. Adopt 10% cfl. Top lm softening EJc,,= 1000.

K,,= 1.0 E,, / c I ,= 1000.

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In addition to undrained strengths, it was necessary to estimate realistic values for ground stiffness and the initial stress state in the ground required for an accurate prediction of wall movements during excavation. A monitored wall supporting an excavation in Lias Clay at a nearby site had been back-analysed by Ford et aZ(1991) and this case history was used, in combination with site investigation data, to optimise the ground stiffness and KOprofiles. Design was carried out for serviceability and ultimate limit states. Structural (serviceability) design was based on “most unfavourable” soil parameters and ultimate limit state was checked using both “most probable” and “most unfavourable” conditions with appropriate factors of safety on soil shear strength. 4.

Design

Design was carried out using the two sets of soil parameters described above. Feasibility of the proposed method was assessed and contingency plans were proposed by the contractor’s consultant.

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The “most probable” design parameters were used to calculate the amber trigger while the “most unfavourable” design parameters provided the red trigger. The construction programme was based on “most probable” conditions in keeping with Peck’s approach. If wall movements exceeded the amber trigger, the following would be carried out: 0

increase the frequency of monitoring readings

0

mobilise site equipment should the construction sequence have to be altered review the data with the contractor’s consultant to decide on further actions and to implement the following contingency measures where necessary: a. b. c. d.

8.3.2

excavation within bays counterfort drainage behind the wall overburden pressure reduction from the retained side of wall use of ground anchors.

Construction control and monitoring 1.

Monitoring plan

Development of the monitoring system was begun during the design stage in association with development of the contingency plans. The monitoring plan had two main roles: 1.

To lay down a clear line of communication from site personnel, through the contractor to the consultant.

2. To confirm that the assumptions made by the designers were valid during construction. The quantities to be monitored were specified: 0

wall head movement and wall inclination geology at the panels undrained strength of the Lias Clay

0

pore water pressures behind and in front of the wall.

Having planned the site monitoring procedures, the contractor and the consultant subsequently simplified and rationalised the monitoring procedures to produce a construction control manual. ClRlA Report 185

141

The manual described the relation of the OM to the excavation process, excavation sequences and the contingency measures. The manual was intended for site and office reference and was an integral part of the excavation control. The manual also defined the roles to be taken by the various parties in relation to the temporary works: 0

the contractor carried out all monitoring and construction progress recording. The review was undertaken by the temporary works construction manager

0

on behalf of the employer the engineer review monitoring records and the contractor’s decisions for the effects on the permanent works

0

the contractor’s consultant designed the temporary works, provided the trigger values, audited the site monitoring records and reviewed decisions relating to temporary works design and assumptions.

2.

Site monitoring

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Day-to-day monitoring was carried out using construction control charts. A chart was drawn for every diaphragm wall panel, because of the rapid variation in their retained heights. The charts provided a template on which all the monitoring information could be recorded and enabled site personnel to assess the behaviour of the excavation in relation to the stage of construction. The charts provided the following information: 0

anticipated geology and undrained shear strength of the Lias Clay

0

berm geometry and excavation levels at different dates

0

80 per cent of amber trigger value for the initial warning

0

amber trigger value.

The monitoring results were plotted on the charts on a regular basis during construction, their frequency of reading depending on the type of activity at a particular section. Quantities recorded on the charts were: 0

current state of excavation call head movement shear strength of Lias Clay in the berm measured using a hand vane.

Before starting excavation, the temporary works designer held a briefing session with the site staff, contractor’s site engineers and foremen to explain the principles of the OM and its application to the project.

8.3.3

Review and implement planned modification Figure 8.1 1 shows a comparison of the predicted (amber trigger) and measured movements at the end of berm excavation, with the straight line representing measured displacement equalling predicted displacement. Also shown in Figure 8.1 1 are two situations in which the monitoring data caused concern and resulted in contingency plans being invoked. The situations were for short walls, ie less than 4 m retained height and a geology different from expected. 1. Short walls The monitoring results for the short walls along the north side of the site showed movements exceeding the amber trigger. Inclinometer readings showed movements to be predominantly mass rotation rather than wall bending. Observation gave the consultant confidence that the long-term serviceability of the wall had not been compromised. Wall movements were controlled by removing soil from behind the wall thereby reducing the earth pressure acting on the retained side.

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Different geology

Short walls 0 0

0

0

Figure 8.11

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2.

10

20 30 40 50 Amber trigger value (mm)

60

70

Comparison of measured wall deflection with amber trigger value after berm excavation (stage 5 of Figure 8. lob)

Different geology

At one position along the alignment an unexpected sand-filled channel was discovered within the Lias Clay reducing stability of the wall. The provision of geology checks on the construction control charts gave site staff an immediate signal for caution. Excavation of the road was carried out in narrow bays (small lengths of berm removed at a time) and the permanent propping was rapidly placed. These two operations provided additional stability compared with the usual procedure (excavation of long lengths of berm before placing the permanent propping). By the time the excavation had been completed and the permanent road slab placed, there was a large volume of available data against which the success of the design approach could be assessed. The design approach is described in Section 8.3.1. The amber trigger was calculated using “most probable” design parameters and the red trigger was based on “most unfavourable” ones that, in this case; were the moderately conservative parameters used for the predefined design. Figure 8.12 shows the occurrence of measured readings to be greater than the amber trigger values for two stages of excavation (excavation to berm geometry and excavation to formation). It would appear that the design had not been optimised as a majority of the measured readings fell below the 100 per cent mark (ie displacement is less than predicted). Only 15 per cent of readings exceeded the amber trigger.

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50 45 40

35

2

f 25 3

U

2 20

U

15 10 5 0

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Figure 8.12

144

Occurrence of measured deflection normalised by amber trigger value (50 records)

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9

Other applications

This section covers the use of the OM in the following four applications: ground treatment including grouting, dewatering and deep compaction

9.1

0

construction of embankments and reclamation works

0

environmental geotechnics

0

structures.

GROUND TREATMENT Listed below are three types of commonly used ground treatment operations illustrating the use of the OM. 1. Grouting

compensation grouting compaction grouting 0 permeation grouting jet grouting. 0

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2.

Dewatering

3. Deep compaction

well points deep wells ejectors. vibrocompaction dynamic compaction 0 vibro-replacement 0 compaction/driven piles.

0

Other examples are shown in Table 3.3.

9.1 . I

Design and planning 1. Uncertainties Advocacy for use of the OM as a good engineering practice in ground treatment has been expressed by Greenwood (1992), Stroud (1991) and Preene and Roberts (1994). Greenwood (1992) points out that the inherent variation in the ground makes it necessary to employ a site-specific observational method for grouting applications. The more accurately the work is monitored, the better the control and corrective reaction. The quality of the work is thus enhanced. In considering the design of dewatering schemes, Stroud (1991) remarks that the process of identifying not only the most probable scenario (Plan A), but also other less probable but still credible scenarios - and being prepared with Plan B and Plan C should the need arise - is at the heart of good engineering thinking. The costs of Plans B and C etc can be estimated, discussed and agreed with the client in advance of the work. Firm prices are obtained for them in the normal way from contractors through the tendering process. With reference to dewatering, Preene and Roberts ( 1 994) note that, conventionally, a site pumping test is considered the best way to obtain design data. However, a pumping test represents a significant proportion of the total cost of dewatering works. It can be

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145

more economical for the pumping test to be a part of a continuous design process towards a more efficient system. The investigation pumping test should continue after commissioning of the construction dewatering system into the initial period of operation. Using the measured flow rates and drawdowns can be compared with those predicted by the pumping test analysis. Any major discrepancy between predictions and observations should be investigated and, if necessary, the construction dewatering system modified further. 2.

Limits of behaviour

Clauses 5.4 and 5.6 of EC7 (BSI, 1995) list factors to be considered in designs for dewatering and ground improvement (which EC7 combines with reinforcement) respectively. Some of these factors are, in effect, controls on what can be done, eg Ground improvement and reinforcement 0

the properties of the soil the magnitude of water pressures the prevention of damage to adjacent structures or services

0

the anticipated deformations

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the long-term effects with respect to deterioration of materials. Dewatering 0

in the case of excavations, the sides of the excavation are to remain stable at all times and that there should not be excessive heaving or rupture of the base

0

avoidance of excessive settlements or damage to nearby structures

0

avoidance of excessive loss of ground by seepage from the side or the base of the excavation maintenance of the water levels and pore pressures anticipated in the design without significant fluctuations

0

9.1.2

care when allowing the groundwater to recover to its original level, to prevent problems such as collapse of soils having a sensitive structure, eg loose sand.

Construction control and monitoring The monitoring of ground treatment can be carried out during the operations and after their completion. Table 3.3 in Section 3.4.1 gives examples of the types of monitoring for controlling ground treatments. For grouting, dewatering and deep compaction, additional comments about their control and monitoring are given below. Grouting With reference to permeation grouting, advice from Greenwood (1992) is that although monitoring centres can handle simultaneous injections in different parts of the site, and may even be remote, it is undesirable to rely solely on electronic output to judge the progress of current treatment. Preferably, monitoring units should be within visual distance of the job site, so that unexpected events can be observed, explanations quickly sought and records made. Further relevant discussions on grouting can be found in CIRIA Report The use of grouting techniques for ground improvement (CIRIA, in preparation).

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Dewatering Roberts and Preene (1994) discussed the observable parameters relevant to a dewatering system and how they may be measured. These are shown in Table 9.1. Further relevant discussion can be found be found in CIRIA Report 113 Control of groundwater for temporary works (Somerville, 1986). Table 9.1

Observable parameters for a dewatering system (afier Roberts and Preene, 1994)

Parameter

Measuring method

soil stratification

0 0

drawdowns

0 0

flow, system total 0 0

flow, individual well

0

water level in pumped well

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mechanical performance

0 0 0

0 0

water quality

0 0

suspend solids

0

settlement of adjacent structures 0

tidal effects

0

rainfall 0

barometric pressure

well borehole log jetting return. piezometer/standpipe unpumped well. V-notch weir flow meter volumetric control. V-notch weir flow meter volumetric control. dip pipe in well. vacuum (wellpoints) supply pressure (ejectors) discharge back pressure engine speed (diesel pumps) power supply alarms. on-site (pH, conductivity etc) off-site, laboratory testing. weir discharge tank. levelling crack monitoring. hourly monitoring of drawdown over total cycles. rain-gauge on site weather centre data. barometer weather centre data.

Deep compaction Construction control and monitoring are particularly important in deep compaction operations because they are largely based on empirical rules. Design is usually by specialist (treatment) contractors with their own particular in-house knowledge. In addition to a system of construction control and supervision, the client’s principal designer - the engineer - should decide on a strategy for trials and testing that is a continuing part of the design. This could include advising the client .on the need to vary the ground treatment as work progresses. Table 9.2 shows typical responsibilities of the engineer and specialist (treatment) contractor. Control and monitoring feature highly in the responsibilities of both parties.

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Table 9.2

Typical responsibilities of the engineer and specialist (treatment) contractor

The engineer

Specialist (treatment) contractor

obtains and interprets site and ground investigation data

0

provides materials and equipment for treating the ground

selects the treatment method

e

plans the working arrangements, sequence and operations within the specified design

decides on testing strategy

@

assesses risks of scheme in such matters as noise, vibration and impact damage and details of adjacent structures

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assesses compatibility of the design of the proposed structure with the expected behaviour of the treated ground and the programme of the project, assesses overall stability prepares specification and contract documents

e

sets out and executes the works tests and monitors the construction operations for internal quality control purposes

@

@

carries out tests and monitoring specified as contractual requirement, which may include noise and vibration measurements

completes the ground treatment works including additional or extra works instructed under the contract.

supervises the ground treatment works, including inspection by sampling and testing advises on the need to vary the works

a .

9.1.3

monitors the behaviour of the structure built on the treated ground.

Review and implement planned modification The precise response of the ground to treatment operations cannot be predicted by calculation. Therefore, a trial is normally carried out to define the amount of treatment effort required to achieve the target performance standard (normally specified in terms of the properties of the ground after the treatment). Usually, treatment operations can be modified, if required, by: changing the layout of the treatment, eg reducing or increasing the spacing of grout holes, deep wells, stone columns etc, in effect changing the volume of ground treated at one point 0

changing some elements in the scheme, eg uprating the dewatering system by increasing the number or size of the pumps in a dewatering scheme, in effect changing the input of energy at a treatment point.

In addition, there may be scope or need for applying a different technique to supplement the treatment scheme, eg combining dynamic compaction with stone columns.

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Grouting A commonly used strategy for modifying a grouting scheme layout is to vary the spacing of grout holes. As an example of this strategy, the following steps show how Millet and Engelhardt (1982) applied it to grouting solution cavities in a dolomite. 1.

A geological model of the site was established by boring and probing. This was supplemented by aerial photographs and a desk study of existing literature.

2.

Before grouting operations started, a minimum number of holes (ie a maximum grout hole spacing) was determined. These holes were considered as “primary” grout holes.

3. After all these primary holes were drilled and grouted, the intake in the grout area was evaluated from data collected during drilling and grouting. If this evaluation determined that additional grouting was needed, intermediate secondary holes could be drilled and grouted. The grout area would then be re-evaluated against predefined criteria and tertiary holes drilled and treated if necessary.

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A similar strategy was adopted by Solera and Lehane (1992) when forming a grout curtain for cutting off seepages in London Docklands. Francescon and Twine (1992), however, used a different approach to deal with geological uncertainties during the construction of a foundation in chalk with solution features, see Box 9.1. Prior to the start of site work, the range of likely geological conditions were categorised into five design cases. A corresponding range of grouting operations and foundation types were designed for each case. The actual geology revealed during excavation was compared with the design cases to select the appropriate grouting operations and foundation type. Grouting is often carried out in conjunction with other operations, such as tunnelling and excavation. The design modification may therefore be a combination of operations involving changes to the grouting scheme (layout or injection rate) as well as to the associated operation. This can be illustrated by the example in Box 9.2.

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149

Box 9.1

Treatment of solution features in Upper Chalk at Castle Mall, Norwich (after Francescon and Twine, 1992)

The Castle Mall development is in the centre of Norwich. It comprises a two-storey shopping centre with a basement up to 18 m deep alongside a five-storey underground car park. The plan area of the site is about 11 ha. The general stratigraphy across the site before construction was made ground (up to 10 m thick) overlying Norwich Crag (up to 10 m thick) and Upper Chalk. Solution features were present in the Upper Chalk. The foundations adopted were pad footings founded in Upper Chalk with ground treatment of solution features. Selection of foundation type took into account the following: 1.

Uncertainties in ground conditions. Location of solution features not known until excavation close to formation. Limited data on size and frequency of solution features, voids and loose material present within the solution features.

2.

Uncertainties in ground treatment results.

3.

Cost effectiveness. Aim to maximise benefit from good founding stratum for spread foundations.

4.

Programme time constraints. Foundation construction required to start immediately after completion of excavation.

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In the event of a solution feature being encountered three methods of treatment were envisaged: 1.

Probing and grouting (bulkfilling or compaction grouting).

2.

Excavating to backfill with concrete.

3.

Increasing original size of foundation.

The range of likely design scenarios, depending on size and location of the features was identified prior to the start of working, and is illustrated in Figure 9.1. CASE 1 - Solution features present between proposed foundation locations would be probed to locate the extent of the feature below ground and bulkfilled with a cementl pfa grout to fill any voids within the features. CASE 2 - For small solution features below a proposed foundation, the feature would be probed and bulkfilled with grout, with the pad foundation possibly being increased in size. CASE 3 - For medium-sized solution features, the feature would be probed and bulkfill grouted. A large mass concrete slab would then be constructed to spread the footing load on to good chalk around the feature. CASE 4 - For large-sized solution features, the feature would probed and compactiongrouted with a reduced bearing pressure allowed on the compacted feature. The effective footing size would be increased to allow for the reduced bearing pressure by constructing a mass concrete slab below the footing. CASE 5 - For the perimeter bored pile retaining wall, the solution feature would be compaction grouted from a higher level. The compaction grouting treatment required a minimum overburden for treatment at any one level and depth of excavation near the retaining wall was limited.

It was possible to modify the foundation design on site within the general design scenarios envisaged. In order to be able to implement the different cases on site, contractual arrangements were organised to provide a high degree of flexibility with regard to programme and occupation of areas of site.

Monitoring of the grout pressures and volumes was particularly careful for compaction grouting, a relatively untried technique in the UK. Standard penetration tests and plate load tests were carried out on treated solution features. Based on the monitoring and testing, it was then possible to modify treatment techniques as the works proceeded.

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1.habove SF

Slab formatim level (SF

CASE 1

-

Treatment

-

Feabres up to Im auoss dear of bundelions or up to 3m across at contiguous pilod well

CASE 2 Treatment

Prabe and bulkfil grout h a 1.above dab formation l e d

-

Feahres up to 2m a u o u beneath fwndalianr Prabe end bulkfd grout h a 1 . h above dab h a t i o n level

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SFL +1

CASE 3 Treatment

Feabrss krween 2m and 4m auoss beneath foundations

Prcbe md bulkfil grout han 1.5m above dab formation level Found oringinal fwlings on maae concrete

Chalk

Contiguous piled paimetsr

strfeer

ratamim wal

CASE 4 Treatment

Feobrcn larger then 4m auma bane& foundations

.above P r b d compactim grout h a led

1 . h dab fpmation Found ormginal fwoolingr on mass concrete

U

CASE 5

Feabres greabr than 3m diameter at contigruous piled wall

Treabnent *

Prcbe end compactim grout han high l e d Combine b o h g s i n b a rak bearing on beated material

Figure 9.1

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Treatment of solution features at Castle Mall, Norwich (Francescon and Twine, 1992)

151

Box 9.2

Compensation grouting at Waterloo Station, London (after Harris et al, 1994)

Harris et a/ (1994) described the use of compensation grouting to control potential ground settlement near the Victory Arch resulting from the construction of an escalator tunnel at London Waterloo Station. The OM was used to control compensation grouting works. Acceptable behaviour limits were calculated before the site work. It was planned that the performance during construction would be monitored by fitting curves to the measured settlement profiles at strategic locations and plotting the resulting width of the settlement trough against the maximum settlement. The amount and location of compensation grouting undertaken would be determined by the observed behaviour and would be directed to make sure that the limits on acceptable behaviour were not approached. If, under normal tunnelling and compensation grouting conditions, the measured movements were approaching the limiting behaviour, the planned contingency was: to slow the progress of tunnelling operations to reduce the rate at which ground volume loss occurred to undertake increased grouting to reduce the distortions of the structures 0

if the above two measures proved to be inadequate, tunnelling would be stopped and jacking of structure by grouting would be undertaken until an acceptable condition was attained.

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Dewatering Stroud (1987, 1991) noted that the OM has particular relevance to the control of groundwater. He also pointed out that a proper assessment of the risks and uncertainties ought to be carried out prior to initiation of a dewatering scheme. A “Plan B” has to be ready for action even though “Plan A” may address the “most probable” circumstances. He listed the following questions to be thought through: what could go wrong? what variation of stratigraphy could reasonably be expected along the route? what could be the consequences of undetected variations in the level of permeable or impermeable layers? 0

what would we do if this or that set of rather less likely circumstances were to occur?

Roberts and Preene (1 994) are concerned that dewatering system hardware is normally specified by a designer to include everything from the filter pack material and screen to the type of pumping plant and the discharge pipe work. A dewatering system can fail because one or more of these components is inappropriately sized. Some components, such as discharge pipe-work, can readily be uprated, whereas other items, such as the well-screen size, may not be easily changed once installed. In order to identify potential failure mechanisms for a dewatering system, knowledge is needed not only of soil properties and analytical techniques for groundwater flow, but also of the performance characteristics of the various dewatering techniques. This allows an initial dewatering system to be specified that is flexible enough to be readily modified to different conditions. The potential failure mechanisms of the dewatering system can be divided into five categories: unexpectedly high flow from the ground 0

unexpectedly low well yields (ie high well losses)

0

well/aquifer connection problems

0

long-term performance deterioration unacceptable off-site effects, eg consolidation settlement, environmental effects.

152

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To cope with these failures, the following strategies are commonly used for modifying the designs of dewatering systems (Roberts and Preene, 1994): A dewatering system can be designed such that the size or number of pumps may be increased to cope with unexpectedly high flow. If the flow proves to be much greater, a physical cut-off may be used as a contingency plan. (A switch from dewatering to physical cutoff could carry a substantial cost and programming penalty.) When the well yield is low, one solution may be to provide vacuum assistance to overcome well losses and to accelerate soil drainage. Depending on the severity, additional wells and replacement wells may be involved. A dewatering system has to connect with the more permeable zones in the substrata. Failure to connect these zones will result in poor performance of the dewatering system. Troughton (1987) described a dewatering case history in Cairo in which the discharge from shallow made ground was found to be too small. This was because the deep wellpoints were separated from the shallow made ground by a relatively impermeable layer of clay. After reconsideration of the actual site geology, additional “shallow” wells were installed at strategic locations in the made ground to improve the discharge. Additional wellpoints were also installed to provide a driving head across the clay layer.

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If the clogging potential of the dewatering system in the long term is high, it is possible to specify the system components so as to minimise the clogging impact.

Deep compaction Raison (1996) describes the use of the OM in relation to the installation of driven piles and the formation of stone columns using vibrocompaction. The strategy for modifying designs that he put forward is summarised in Table 9.3. This is largely in line with Table 9.1 and its principles can generally be applied to other deep compaction operations. Table 9.3 Application of the Observational Method to the installation of driven piles and stone columns (after Raison, 1996)

Preliminary design

0

initial trials ongoing measurements

Observations Trigger values m

Contingency measures

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based on site investigation moderately conservative conditions (governed by quality of SI, degree of risk and commercial considerations) worst-case (most unfavourable) conditions, are considered to determine suitable contingency measures

8

vibro-compaction - cone penetration tests qc value driven piles - blowcounts vibrocompaction - closer centres driven piles - longer or more piles.

153

9.2

EMBANKMENTS AND RECLAMATION WORKS This section considers the application of OM to embankments and land reclamation schemes. It concentrates on soft clay conditions, but the principles apply to other types of soils. Peck (1969a) identified embankments as an area with a long history of OM applications. He particularly referred to Terzaghi’s work between 1943 and 1948 on the Cleveland Ore Yard, a case history which considers the overall stability of an iron ore stockpile as well as the lateral loading effects on piled foundations (Terzaghi, 1948b). The OM applications that are considered below include:

9.2.1

0

settlement and consolidation, eg land reclamation

0

embankment stability, eg embankment slopes

0

long-terdstructural interaction, eg piled foundations, bridge abutments, dams.

Design and planning

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1. Uncertainties The geological uncertainties are mainly associated with assessing the fabric, layer thickness and the degree of over-consolidation of soft clay. This fabric often means the presence of thin sandy layers or partings within the clay mass will influence drainage path lengths. Probing techniques such as the piezocone can reduce these uncertainties. Parameter uncertainties are those associated with compressibility and permeability which controls the rate of consolidation, together with soil strength which controls the height and slope of the embankment construction: Cornpressibility

The compressibility of normally consolidated and over-consolidated clays is reasonably well defined by the compression index (C,) and the swelling index (Cs). Significant uncertainties arise in the assessment of the pre-consolidation pressure (pJ, because it is sensitive to sample disturbance and loading rates. Laboratory consolidation testing has to be planned carefully to establish this pressure. The effect of creep should also be considered. It is therefore difficult to predict the settlement of foundations accurately where the applied pressure is little more than the pre-consolidation pressure. Perrnea bility

The soil’s bulk permeability, ie that of the soil mass, depends on the relationship of intact clay and soil fabric. For homogeneous soft clay it is reasonably predictable (of reducing permeability with increasing effective stress). The greatest uncertainty is when the soil fabric contains discontinuous layers or partings of varying thickness and spacing. Consolidation rate

Consolidation rates are complex because they are a function of both compressibility and permeability. These uncertainties are also greatest at the pre-consolidation pressure where the soil compressibility is changing. It is important that the information from field trials at effective stresses less than the pre-consolidation pressure is not directly extrapolated to predictions above this pressure. 154

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Strength The undrained shear strength of a soft clay foundation controls the maximum embankment height. Uncertainties in the assessment of the undrained shear strength stem from sample disturbance, inherent and induced anisotropy, rate effects and the degree and nature of cementation (which also affects the sensitivity or brittleness of the clay). These factors also make the shear modulus difficult to estimate. The initial undrained shear strength is also related to the pre-consolidation pressure. The pore pressure response to loading is influenced by all these factors and is therefore difficult to predict without information from case histories and trials. As laboratory testing does not remove these geological and parameter uncertainties, trial embankment loadings are often carried out as a final stage of investigation for design confirmation. The OM provides a procedure for extending this process into construction. Modification plans are developed at the design stage to cover the range of uncertainty. 2.

Limits of behaviour

The limits of behaviour need to be established for the embankment and the adjacent structures. These are summarised below:

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Table 9.4

Limits of behaviour influencing embankment design

Embankment condition

Limit states imposed on embankments

Embankment only. (long-term or short-term problems) ref: EC7 Section 9.2

loss of overall stability

internal damage and erosion surface erosion. deformation, eg large settlement, risk of overtopping.

Influence of existing adjacent structures

embankment deformation is limited by ultimate and serviceability limit states of existing adjacent structures hazard severity is increased by the adjacent structure and the embankment factor of safety may be increased ground treatment measures installed before embankment construction commences may impose limits on embankment design.

Influence of adjacent structures constructed after embankment completion

pavement differential settlement limits, particularly at the interface with piled structures, eg roads, runways, ports, etc. shallow foundations constructed on embankments piled foundations through embankments, eg piled foundations for bridges.

0

General guidance on embankment construction on soft compressible soils is given by O’Riordan and Seaman (1994) and on the embankment interaction with pile of foundations by Seaman (1994). 3.

Consolidation settlement of soft clays

Soft clays are highly compressible, and this can lead to large consolidation and creep settlements. In addition, the drainage path lengths and permeability of soft clays are ClRlA Report 185

155

difficult to assess leading to uncertainty about the rate of consolidation. This affects the construction programme. The integrity of the fill and the soft clay might not themselves be affected by these settlements, because they occur slowly and do not result in rapid failure of the soft clay. However existing or proposed adjacent structures could be deformed by the consolidation settlement, perhaps resulting in sudden structural failure. Peck (1969a) refers to a grain elevator case history where the consolidation settlement damaged raking piles causing a sudden and catastrophic failure of the structure. Where there are adjacent structures, settlement and lateral movement problems are often overcome by arranging planned modifications such that any remaining consolidation settlement is within the acceptable structural limits. The settlement and piezometer records are reviewed against predictions. Modification plans to be triggered, if necessary, may include controlling the consolidation settlement by varying the surcharge pre-loading and the ground treatment, such as vertical drains. Examples of these techniques include Changi Airport reclamation and Tianjin Port development (Choa, 1994).

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4.

Embankment stability

Soft clays have low undrained shear modulus and strength properties. This leads to stability problems for embankment slopes more than 2 m or 3 m high. Insensitive clays behave in a ductile manner, with increasing rates of deformation prior to failure. Instrumentation can be used to detect the onset of failure. In the case of sensitive clays, however, rapid deterioration can occur with little pre-failure movement. The failure can be extensive and progressive. The OM is therefore most applicable to insensitive clays. Local case history experience or a trial embankment may provide practical information on the behaviour of a specific soft clay. The factor of safety adopted for these embankments will depend on the degree of uncertainty in the soil properties, together with the embankment behaviour and the consequences of failure. Often occasional failures of these small embankments can be tolerated because of the low cost of the embankment and the negligible risk of human injury. Examples include the Salt Lake crossing (Casagrande, 1965) and Changi East Reclamation Project (Choa, 1994). Best way out OM strategies using Peck’s approach may be more appropriate for these conditions. Where adjacent structures are present, higher factors of safety are required because of the increased hazard severity. More rigorous monitoring systems are also required to increase control over the likelihood of occurrence. Progressive modification system may be appropriate to these situations providing the risk of sudden failure can be contained. Consolidation periods, ie pauses in construction, may be required during construction to allow the clay strength to increase below the embankment. These effects influence the planning and programming of the embankment construction as discussed previously. The Tianjin East Pier stability provides a contingency plan example of vacuum dewatering to provide an additional surcharge to increase effective stresses and accelerate the construction programme (Choa, 1994). 5.

Structural interaction during and after construction

In soft clays, structures are often piled into the underlying hard strata. Deformation of the soft clay during construction may influence pile performance and stability. These issues are discussed by Seaman (1994). During embankment construction the instrumentation may be used to monitor and review the performance of the piles against predictions. The main OM contingency measures relate to increasing the degree of consolidation and reducing shear deformation, as discussed by Stewart et a1 (1994). Other 156

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contingencies could involve unloading by using lightweight fill or ground treatment. In the post-construction consolidation phase, OM contingency plans are more difficult to introduce. One could envisage jaclung a road pavement to match an abutment or re-levelling drains, but no examples of this application have been found to date. In non-cohesive or over-consolidated clay soils, embankment stability may be sufficient to enable a structure to be founded on pad foundations on top of the fill. In these cases post-construction settlements may still be large and difficult to predict to the accuracy required for the structural design. The OM has been used to monitor and review settlements against predictions. Contingency plans are made to jack up and re-level bridge bearings - see for example Nicholson and Low (1994). It is fortunate that longterm settlements take place at log time rate, such that settlements in the first five years after construction are likely to be similar to those for the next 50 years. The OM is usually used during the construction period. When it is used during the post-construction period it is essential to provide maintenance manuals setting out the monitoring procedures, trigger criteria and contingency plans.

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Embankment dams have high hazard severity and therefore require long-term monitoring programmes. Best-way-out strategies are often used to monitor and repair dam cores, leaks below foundations and clogging of filters. U1 Haq (1996) provides an example of the best way out adoption of the OM to the Tarbela Dam seepage problems. These are discussed in detail in Section 9.4.

9.2.2

Construction control and monitoring This section considers the monitoring and construction control techniques which can be used to assess the field performance against predictions. 1. Consolidation and rate of settlement Terzaghi’s consolidation theory usually forms the basis for predicting the amount and rate of primary settlement in soft clays. Monitoring is by surface settlement targets and to give depth profiles by settlement gauges and piezometers. Interpretation involves comparing the amount and rate of settlement against the predictions and checking that the degree of consolidation derived from settlement measurements corresponds to that recorded by the piezometers. Examples of this procedure is given in NA VFAC (1986) and the ICE Vertical Drains Symposium In Print (ICE, 1982). Asaoka (1978) proposed an alternative interpretation method based on a hyperbolic model. Consolidation performance (both degree and rate) is extrapolated from the existing data. The interpreted consolidation parameters are compared with the predicted values. This method does not rely on the total settlement predictions for the assessment of the degree of consolidation. Wakita and Matsuo (1994) set out a refined version of this approach for use with the OM. 2.

Embankment stability

Correlations have been developed for typical road embankments founded on soft clays between the ratio of embankment settlement (6) to the maximum lateral deflection beneath the embankment toe (y,,) and at the embankment toe (yJ. These were reviewed by Seaman (1994) and Stewart et a1 (1994): typical results are shown in Figure 9.2. This illustrates the difference in embankment performance between undrained and drained conditions and provides a way of monitoring and reviewing both shear distortion and consolidation during construction. Increased drainage reduces the lateral movements and ClRlA Report 185

157

implies greater stability. Matsuo and Kawamura (1977) related these deformation principles to embankment stability, see Figure 9.3. Wakita and Matsuo (1994) and Russell (1996) discussed the application of this technique to the OM. The deformation and stability of embankment dams during construction has been discussed by Penman (1986). The increase in embankment critical strain rates with increasing embankment height and reducing stability has also been assessed by Marsland and Powell(l977) for Thames Estuary alluvium. They also noted the increase in the undrained shearing creep with reducing factor of safety, see Figure 9.4 and proposed correlations to limit the embankment construction height. An example of the use of the OM in multi-stage embankment construction is provided by Bauduin and Moes (1987). Maximum lateral deflection below embankment toe (y,) approximately 0.3 S,, \

I

0.1

6max

J

1-

\

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lapproximately;

I

)uring embankment construction Jndrained) verage AymiA6 = 1 tandard deviation = 0.2 I

'ost-construction 2onsolidation) verage Ay,iA6 = 0.16 tandard deviation = 0.09

Legend

6

= Settlement at the centre of the base of an embankment

6,,,

= Settlement at the centre of the base of an embankment at end of consolidation

ym

= maximum lateral deflection below

the embankment toe

Figure 9.2

158

Correlation between embankment lateral deflection and centre-line settlement (afler Stewart et al, 1994 and Tavenas et al, 1979)

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3.0

2.0

6

+ y

U)

?

0-

E

f.0

1 .o

.q/q,= 1.0

0.9 0.7

0.8

A

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0

0.8

0.4

1.2

L a

tart

Y,

'6

LEGEND

6 = Settlement at the centre of the base of an embankment

ys =

Maximum lateral deflection at the embankment toe

9 = Embankment load 9, = Embankment load failure

Figure 9.3 Embankment stability control charts (Wakita and Matsuo, 1994)

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159

IC

..m B1 ,.m 82 83 84 B5

J -12

LEGEND

I

t -I-

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7

Inclinometers 18 - E Piezometers Magnet extensometers Horizontal mole tube and magnets Surface survey targets

35 24 hours after '

load increment

30

25

20

Two times 5-hour movement 15

10

5

5 hours after load increment

5 2.8

6 7 Embankment height

0

2.6

2.4

2 p 2.2

L -32 0

2.0 1.8 1.6 1.4 1.2 1.o 0.8

5

6

7

a

8.5

Embankment height (m)

Figure 9.4 160

Creep effects (Marsland and Powell, 1977) ClRlA Report 185

9.2.3

Review and implement planned modification The methods for improving the rate of consolidation in soft clays and embankment stability are reasonably well understood. This section summarises the methods that can be incorporated into the OM modification plans during the construction process. Consolidation

The following measures could be incorporated into OM modification plans to offset the effects of slow consolidation. (Some of them could be incorporated into progressive modification strategies.) modify the construction programme vary surcharge pre-load change to a lightweight fill if insufficient consolidation is achieved install additional vertical drains if insufficient consolidation treat the ground by techniques such as jet grouting if they can be accommodated during construction supplement with vacuum surcharge

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install an electro-osmosis scheme accept larger settlements in the soft clay and modify the structure, eg jack up or lower the level of bearings or use compensation grouting techniques to re-level shallow structural foundations. Embankment stability

The following modification strategies provide options for improving embankment stability and reducing lateral movements during the course of the works. The selection of the appropriate option to use will depend on the specific site constraints, such as programme time, available adjacent land, etc. Increase soil strength uses the consolidation measures listed above to achieve an increase in effective stress and hence shear strength, eg multi-stage embankment construction Reduce loading a. lightweight fill, eg PFA b. polystyrene blocks in place of conventional fill materials Modify geometry a. flatten side slopes b. addbenns Add structural support a. install stabilising piles to increase lateral stability b. install anchors or ties at the embankment base.

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161

9.3

ENVIRONMENTAL GEOTECHNICS The OM can be applied to the following two aspects of environmental geotechnics. 1. The geomechanical aspect

Structures or excavations are required to contain waste material. Some waste materials can be engineered and subsequently used as fill or constituents of earthworks, an example being mine tailings. The OM can then be applied to design these structures, excavations or earthworks. Excavations or construction of waste management structures often proceed in an incremental manner over many years (Morgenstern, 1994). As a result, there is often ample opportunity to observe and modify the design or procedures. The discussions in Section 8 are applicable to the design of these works. Morgenstern (1994) described a successful use of the OM in controlling the slope stability of a tailings dyke at a mining waste site over a period of four years. The movement mechanism ofthe dyke was analysed prior to construction on the basis of average ground conditions and movements were monitored during construction. The contingency plan was to regrade the slope to a flatter angle if the measured movement exceeded the predefined value.

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2.

The chemical aspect

There are concerns in the UK (Wood and Griffiths, 1994) and in the USA (Brown et aZ, 1990; and Morgenstern, 1994) that the practice of designing remedial measures for contaminated sites may be over-conservative. In the UK, this is because of a lack of formal standards resulting in over-reaction to risks (Wood and Griffiths, 1994), partly due to the lack of tools to define risks meaningfully (O’Riordan, 1995b) and partly because of uncertainty about remedial treatment results. The practice of designing remedial works for contaminated sites in the USA involves a study-design-build approach in which the feasibility study, design of the remediation, and execution of the remedial treatment are carried out separately by three parties (Morgenstern, 1994). This approach relies on the reduction of uncertainties early in the life of a project. However, this is not an appropriate strategy for coping with the inherent uncertainty of ground remediation. Often, difficulties in decision-making can hinder progress. Brown et aZ(1990) and Holm (1 993) advocate incorporation of the OM into the methodology of remediation schemes by requiring the whole process of feasibility study, design and construction of remedial works to be a continuous process (Figure 9.5).

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I

REMEDIAL INVESTIGATION investigations

expected conditions &

b

1

Design action & monitoring

& contaminant

PRE-RI/FS*

REMEDIAL DESIGN

REMEDIAL ACTION

Predict performance

Implement initial design

andlor pilot tests

Perform preliminary risk assessment

course of modifications

Conduct preliminary site inspection and assessment

Implement design modifications

I I

I I

Collect & analyse existing data

I I I

Formulate & plan site-specific approach

FEASIBILITY STUDY Identify & screen technologies

1

Assemble various technologies into alternatives Identify action technology-specific regulations

Analyse alternatives for expected conditions & deviations Compare & contrast alternatives

I

EVALUATE DATA & MODIFY DESIGN PER PREDETERM~NED COURSE

II '

I

I

I

))

I

7

I

OPERATION/ MONITORING

txGcn peg?&ce predictions

I

* RI = Remdial investigation

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FS = Feasibility study

Figure 9.5

9.3.1

Proposed use of the Observational Method in ground remediation (Holm, 1993)

Design and planning 1.

Uncertainties

The subsurface environment is a heterogeneous, complex environment in which small subsurface features or changes in geological conditions can have substantial impacts on water and chemical movement and performance of remedial actions (Brown et al, 1990). Morgenstern (1994) used the following example to illustrate the uncertainty involved in the widely used method for remediation of contaminated groundwater known as the pump-and-treat method. Contaminated groundwater is pumped to the surface and contaminants are removed in an appropriate treatment system so that the water can be discharged or re-injected into the ground. There are problems associated with the estimation of clean-up time. Many operations have been terminated on the basis of such a time estimation, but there are doubts about the success of the treatment process. A strategy is thus required so that the treatment process can be monitored and contingency measures can be implemented if the treatment result is not satisfactory.

2. Risk assessment Wood and Griffiths (1994) consider that before any measures are finally decided upon, those concerned should pause to make a searching appraisal of the degree of risk. This should relate to the level of hazard, taking account of the analytical results, to the sensitivity of the target, and would determine the appropriate level of protection with due regard to economy.

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163

O’Riordan (1995b) considers that often a contaminated site is over-engineered because the engineer lacks the tools to define risks in a way that is meaningful to investors, funders and end-users. Risk assessment techniques can identify areas where knowledge is truly limited and fiuther research is required; in this way policy decisions, involving the application of planning and building control guidance, can be influenced. He proposed a broad strategy for risk assessment, see Figure 9.6. There are two important components in this strategy: 0

a rigorous approach to the no-action option and option of identification of and agreement to a tolerable level of assumed risk of 10-6.This value of 10-6has been identified as a virtually negligible risk (Royal Society, 1992). Review history of site operations and available sampling data

Select indicator chemicals and

Determine exposure pathways and fate and transport of indicator chemicals

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Characterise site risk for the no-action alternative I

I

If risk is acceptable i.e. No remediation necessary

Recommend remediation of areas in which the risk is above 10-6

level of risk reduction needed

Initial screening of alternatives followed by detailed analysis of

Figure 9.6

164

Risk assessment methodology for environmental geotechnics (based on Stephanotos and Schuller, 1989)

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The principle of risk management is discussed in Section 3.2. A quantitative riskassessment (QRA) approach such as fault tree or event tree is normally used for assessing risk in relation to contaminants. Examples of the application of this type of technique can be found in CIRIA Report 152 Risk assessment for methane and other gases from the ground (O’Riordan and Milloy, 1995). O’Riordan (1995b) points out that numerical results derived from such an analysis should be treated with caution; indeed, a major part of the risk assessment process is to test the sensitivity of the result to changes in probability assumptions. The process encourages diversity in the selection of design and construction methods and enables the cost and associated benefits of particular courses of action to be rapidly established.

3. Limitations Morgenstern (1 994) warned that use of the OM in environmental geotechnics may be constrained by the following factors. 1. The regulatory environment - the environmental regulations in some countries may require the remediation works to be designed for scenarios that are not credible or call for impossibly small levels of risk.

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2.

The nature of decision-making related to environmental matters -the public might have a false impression that the OM is a “trial and error” method and consequently disapprove its use.

3. The issue of longevity - the use of the OM relies on systematic performance monitoring and effecting changes in the light of this monitoring. It is difficult to rely on the Method to control a site for over a very long period. Consequently, the effects of ageing upon the installation must be included. These limitations, together with those pitfalls described in Chapter 4, should be considered before the initiation of the Method.

9.3.2

Construction control and monitoring Published literature provides guidelines to the actions required and the methods adopted for monitoring and sampling in contaminated sites. The guidance commonly used in the UK includes: Guidance on the assessment and redevelopment of contamjnated land, published by Interdepartmental Committee on the Redevelopment of Contaminated Land (ICRCL, 1987) Specification f o r ground investigation (Site Investigation Steering Group, 1993) Risk assessment of contaminated soil: proposals f o r adjusted, toxicologically based Dutch soil clean-up criteria (Van Den Berg et ul, 1993)

CCME Guidance manual on sampling, analysis and data managementf o r contaminated sites (CCME, 1993) CIRIA Special Publications SPI 01-1 12 (CIRIA, 1995). O’Riordan (1995b) considers that the prescriptive approach described in the ICRCL guidelines places reliance on the judgement of professionals to set action levels for most of the contaminants. Explicit considerations either of hazard testing methods or of the nature of the hazard and its mobility or of the variety of receptors and their vulnerability are absent from this guidance.

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In the SpeciJcation for ground investigation (Site Investigation Steering Group, 1993) considerable reference is made to methods of examination of waters and associated materials. Many of these are developed primarily for waste water treatment systems and are often inappropriate for the testing of contaminated soils and waters. Documents published by Dutch and Canadian authorities help in the determination of appropriate testing and evaluation techniques for contaminated land. D’Appolonia (1990) proposed extending the use of the OM to the operational phase of facilities and processes through his concept of “monitored decision process”. However, as discussed in Section 9.3.1, it is unlikely that consistent monitoring can be achieved over hundreds of years and hence the use of the OM for the very long-term operations should be cautious. A consistent application of the OM over a limited period, say 50-75 years, is considered to be possible for environmental geotechnics (Morgenstern, 1994).

9.3.3

Review and implement planned modification In order to implement the OM, a range of likely scenarios and their treatment methods .should be considered. O’Riordan (1995a) considers that the design and selection of treatment method should be based on a risk assessment that considers the factors shown in Table 9.5.

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Table 9.5

0

Factors to be considered when selecting a method of treatment for contaminated sites (O’Riordan, 1995a)

applicability

0

effectiveness 0

0

information requirements planning and management needs

limitations

0

monitoring needs

cost

0

health and safety aspects

development status

potential environmental impacts

availability

post-treatment management works.

The above factors should be taken into account in examining the following aspects when evaluating and comparing the various options for a particular site: 1.

Definition of the works.

2.

Design reliability.

3. Novelty of the method. 4.

Verification and durability.

5.

Size of construction plant.

6 . Number of staff. 7.

Potential for airborne release.

8. Potential for waterborne release. 9. Haulage length to disposal site. 10. cost. 11. Access. The restoration strategy for part of a dioxin-contaminated river in northern England, described in Box 9.3, resulted from such a process. 166

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Box 9.3 An example of a restoration plan for a dioxin-contaminatedriver in northern England

In part of a river in northern England high levels of hydrocarbons, in particular dioxins, were detected. A proposal for a restoration plan was commissioned by a regulator with the objective of removing dioxin and furan pollution in a selected stretch of the river in order to develop a strategy for removal of all contaminants in the long term. A desk study examined old Ordnance Survey maps, along with the geological, hydrological and topographical profiles of the site. A limited site investigation was also performed. At this stage, the engineer considered that the following problems would dominate the restoration plan: 0

0

geological uncertainty - the river morphology varied greatly within the study area, within a short distance changing from a bare rocky bed.to a marshy area with plants. It is therefore difficult to obtain representative samples complexity of pollutants - the site has a long and complex history of pollution and intoxication, making prediction of likely pollutants difficult limited depth of sampling - previous sampling had concentrated on the upper 3 cm of sediment banks; these results may not characterise the bulk of the sediments, which are up to 1.4 m thick in places.

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Four categories of remediationwere considered. They were: 1.

Leave in situ for natural remediation of the contaminants- once the various sources of pollution are controlled, natural recovery would reduce the level of contamination. The natural scouring action of the river would disperse sediments downstream, thereby reducing the concentration of dioxins.

2.

Isolation or encapsulation-the sediment could be prevented from coming into contact with the river water by lining or capping the riverbed. Some diversion of the river flow would be required, to place the linerkapping.

3.

Removal of contaminant - sediments could be dredged from the riverbed and disposed of off site. During removal there could be a risk that contaminants would be introduced into uncontaminated areas due to re-suspensionof the particles.

4.

Treatment of contaminated material in situ or off site.

There were also concerns that, because of the uncertainties and complexity of the problem, any proposed scheme would be over-engineered.A restoration strategy, based on the OM, was subsequently proposed (Figure 9.7). This can be summarised as:

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1.

On the basis of the preliminary studies, the risk associated with each type of pollutant was assessed. It was concluded that the best course of action was to leave the sediments in place (see Option 1 above).

2.

Programmes of further investigation and long-term monitoring of the sediments by testing and logging contaminant levels were set up.

3.

An intervention value for each of the pollutants, in particular the dioxins, was set up.

4.

Remediation plans based on Options 2 to 4 above were made, for implementation if an intervention value is reached. Informationfrom the further investigation work, along with the results of biological assays, could then be used to keep the treatment method under review, for any circumstances necessitatingthe adoption of a more intensive level of remediation.

167

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168

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O'Riordan (1995b) gives an example of how risk assessment can be used in conjunction with a cost benefit analysis to determine the most cost-effective treatment solution. This example is shown in Figure 9.8. In common situations where solid contamination concentrations vary with depth, but are immobile, the relationship between the cost of clean-up and intervention level (ie the trigger level) for a particular contaminant of concern is shown in Figure 9.8. By examining systematically the consequences, the cost of cleaning up at a particular level AB (Figure 9.8) against risk to receptors of leaving untreated contamination in place can be defined. Cost-effective solutions to contamination can then be provided that are technically robust and justifiable to developers, insurers, finders and public alike.

0.5 I

Figure 9.8

9.4

'

increasing'parts per mihion (pprn)

'-

Cost of ground treatment to a selected clean-up target level for particular depth of contaminated soil (O'Riordan, 1995b)

STRUCTURES This section discusses the application of the OM to the following aspects of structures: 0

foundation design

0

operation and maintenance protection against adjacent construction operations.

9.4.1

Design and planning 1. Foundation design The choice between a deep foundation, eg pile, and a shallow foundation, eg raft, is generally governed by the foundation movement and the load capacity. Uncertainties in geology, such as the presence of solution features, and design parameter values (eg strength and compressibility) would prevent a designer from assessing these

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two criteria accurately. Consequently, a conservative solution would be adopted to cover the worst scenario. The case history of the Foyle Bridge in Northern Ireland (Nicholson and Low, 1994) illustrated how the foundation of a bridge changed from a conservative piled foundation during the tender stage to a shallow pad foundation after reassessment of the actual site information (see Figure 9.9). They also described the uncertainties in calculating the settlement of this pad foundations, which subsequently led to the use of a design based on the OM (Box 9.6). 2.

Operation and maintenance

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It is essential to monitor the performance of some structures during the operational phase because failure could result in fatalities and the cost of repairing damages would be huge. The OM therefore becomes part of the operational and maintenance strategy. The example of an embankment dam is used for illustration in the following discussions and focuses on use of the OM during the operational and maintenance phases. The postconstruction problems for embankment dams include seepages, settlement and stability. Charles (1993) describes a modified OM for monitoring the performance and maintaining the safety of embankment dams. In his Rankine Lecture, De Mello (1977) recommended the OM as one of the five design principles for embankment dams. He said that as soon as one had made the presumed average hypothesis, one ought to consider how the behaviour would change if the assumed parameters were to vary. The design should have a built-in robustness for coping with unexpected behaviour.

3. Protection of structures New construction, such as tunnelling and deep basement construction, can affect existing structures, particularly in urban areas. Interactions between the ground affected by excavation and nearby structures are complex. The OM is applied as part of the measures to protect a structure against damage induced by the adjacent work. For example, Pate1 et a1 (1982) described a strategy for protecting the foundations of buildings above a tunnelling site (see Box 9.4). The owners of historical monuments, prestigious buildings and important underground facilities will often require the contractor to put forward a plan for guaranteeing the integrity of their properties. The OM has been proved to be effective in these occasions. Harris et a1 (1994) (Box 9.2) present a strategy for protecting the Waterloo Victory Arch in London. Powderham (1994) (Box 9.5) used an OM-based strategy for protecting the Mansion House in London. Clayton et a1 (1991) (Box 9.8) and Tse and Nicholson (1992) (Box 8.5) reported the use of the OM for protecting underground railway tunnels.

170

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I

I

L Fill

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411 4I / 4 I/ 4 II \I + + \\ *\\ Y\ \

Figure 9.9

Box 9.4

The choice of foundation for Foyle Bridge: (a) tender stage, (b) reappraisal stage, (c) final stage (Nicholson and Low, 1994)

Example of using the Obsewational Method to protect structures affected by tunnelling (after Pate1et al, 1982)

An 8-foot diameter tunnel was required for installing underground sewerage facilities in a densely occupied area in Little Italy, Maryland, USA. Many historical buildings were constructed in this area and very little was known about their foundations. The tunnel was driven by TBM in conjunction with ground treatment of a 3-foot annular ring of chemical grout around the tunnel utilising the sleeve-pipe method of grouting. To monitor the effect of the tunnelling operations on the streets and adjacent buildings, an extensive instrumentation program was developed consisting of deep and shallow settlement points, inclinometers, tilt plates, tape extensometers and water observation wells. This instrumentation programme was designed and established as a contract requirement. The purpose of this programme was to serve as an early-warning system for detecting incipient damage to adjacent structures in time to take preventive action. In addition, it was intended, in the event that damage did occur, to provide the baseline data as well as the ongoing measurements needed to determine the extent and cause of the damage. The building damage criteria and trigger values were established from published architectural damage criteria and they were as follows: 1.

Settlement points - 1 in settlement at centre-line of tunnel.

2.

Inclinometer - change of slope of 1:I000 measured to point of maximum deflection.

3.

Water observation well - 1 ft drop in water level.

4.

Tape extensometer point - tensile strain of 0.001 of distance between two points.

5.

Tilt plate - rotation of 1:1000.

In the event of excessive movement, the contingency plan of compaction grouting would be implemented.

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9.4.2

Construction control and monitoring 1. Foundation design The instrumentation systems for monitoring the performance of foundations can generally be summarised in Table 9.6. Movements of structures and ground and load in foundation are parameters commonly chosen for observing. However, in the case of Castle Mall (Box 9. l), the choice was the change in ground conditions. Table 9.6 Observation

Instrumentation

Structural/ground movement

Load in foundation

electro-levels surveying markers settlement gauges extensometer inclinometer Strain gauges

Ground conditions

Visual inspection

2.

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Typical monitoring systems for foundation

Operation and maintenance - embankment dams

Charles (1993) describes instrumentation systems for monitoring the behaviour of embankment dams in the operational and maintenance phases (see Table 9.7). He explains how indicators that come to notice by visual surveillance or by instrumentation monitoring may warn of possible defects and deterioration in clay cores, including: localised settlement 0

excessive rate of general movement

0

leakage

0

high downstream pore pressures.

When interpreting field measurements of parameters such as pore pressure, settlement and leakage, emphasis should be placed on whether the measured parameters are increasing or decreasing over time. In considering how to apply the OM to examining the performance of an embankment dam, thought should be given to limitations of the instrumentation, such as cost implications, installation effects and reliability. Table 9.7

Typical monitoring systems for embankment dams (based on Charles, 1993)

Observation

Situations used

Pore water pressure

0 0

0

Seepage and leakage flows

0

0

Movements

0

evaluation of slope stability assessment of the effectiveness of the watertight element investigation of vulnerability to hydraulic fracture or piping investigation of internal erosion investigation of possible distress in the dam evaluation of field performance

Instrumentatiodtest 0

0

0

0 0 0 0

172

piezometers.

chemical analyses for dissolved minerals. a special V notch incorporated in the existing drains. theodolite optical level electro-optic distance meter photogrammetry. ClRlA Report 185

3.

Protection of structures

The monitoring systems adopted for protection of structures vary with the nature of the structure. Generally speaking, structure movements, ground movements or water pressure are chosen for observation. The monitoring systems discussed in (1) above are generally applicable to the protection of structures. An example is the protection of Mansion House - see Box 9.5. Box 9.5

Protection of the Mansion House using the Observational Method (after Powderham, 1994)

The city extension of the Docklands Light Railway from Tower Hill to Bank station passes under The Mansion House in London. The first phase of tunnelling was a small-diameter passenger link directly beneath the building. In mid-I 989 concern arose about the risk of damage to the building, as short-term settlement was exceeding long-term predictions. All outstanding tunnelling work within the zone of influence was halted pending detailed evaluation of the risks to the building.

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The "best way out" application of the OM was then introduced. It was important to re-establish the confidence of the building owners by clear control of safety. The key requirement was to avoid damage to the building. The progressive modification approach to the OM enabled risk control to be demonstrated with the ideal being the avoidance of any contingency measures. At the Mansion House the OM was applied sequentially and incorporated a phased approach to the tunnelling work. Detailed consideration was given in advance to a range of potential protection measures. These were:

a.

Shielding of the foundations of the building from the imposed tunnelling settlement trough by forming a structural curtain wall.

b.

Foundation strengthening, such as underpinning and grouting.

c.

Building strengthening, including steel ties for emergency conditions

d.

Compensation grouting to eliminate tunnel-induced settlement

For the most unfavourable case scenario leading to unacceptable settlement and building distortion, contingencies of either foundation or building strengthening were considered most appropriate. As indicated in Figure 9.1 0 the associated contingent design changes would have been implemented as necessary after the relevant phase of tunnelling. In addition to this phased approach, the ability to install emergency strengthening in the form of steel ties was also carefully assessed. To operate the sequential (progressive modification) approach, it was necessary to monitor the actual movement of the building, so instrumentation was installed to monitor displacement. Primary monitoring was performed by electro-levels (101 number) capable of measuring slope changes to an accuracy of 1/80 000, together with precise levelling and inclinometers. Secondary monitoring, used as a comparison, was carried out by water levels and spatial surveys. As noted by Powderham (1994), movements were well within the predicted values and no damage, however slight, was recorded after the implementation of the OM. No contingencies were required.

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phasel.~bnnel

Tunnelling within

zone ol influence

t

Weekly monilocing reponS to tJw city Engineer

I

Penod for full inrbal senlement

Assessment ol ground and buildtngresponseandrenev ol mntrol alert level

Rtsk level

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zone 1

No

I

Consideration of report

-

Risk lWd

zone 2 Frequent C o o d t i i surveys to check building response during remaining phase 1 tunnelling work

I Approval to

No

4

I

Risk level zone 3

Hold On nexl phase Of work

1 Report to C

Phase 2. Stewlate junction

i Engineer

+ I

Agree possible preventive works

I

Install preventive works if required

I

Repon to Gly Engineer

Figure 9.10 'Application of the OM at Mansion House, London (after Powderham, 1994)

174

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9.4.3

Review and implement planned modification Strategies for implementing planned modification vary with different situations. The following examples are chosen to illustrate the principles. 1. Foundation design There was uncertainty in predicting the settlement of the foundation at Foyle Bridge see Figure 9 . 9 ~ (Nicholson and Low, 1994). Flexibility in the connection details was therefore introduced in the design to cope with the uncertainty - see Box 9.6. In facing the geological uncertainties in Castle Mall (Box 9. I), Francescon and Twine (1994) explored the possible range of geology and in advance of.the site work arranged for there to be different types of foundation each to cope with a particular geological situation. Box 9.6

Strategy adopted for modifying the Foyle Bridge foundation design (after Nicholson and Low, 1994)

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Settlement was the main design criterion for the foundation of the Foyle Bridge. It was considered that the bridge deck could be subject to settlements of some 50 mm from the start of surfacing. Recognising the uncertainty in settlement prediction and programming, with little room for extra pre-stress in the box section, a three-point strategy was adopted. The prestressed concrete deck was designed for a long-term differential settlement of 40 mm. Any abutment settlement occurring after the deck had been connected to the bridge pier was to be removed by shimming the abutment immediately before the abutment expansion joint was placed and the abutment approach road surfacing was completed. It was also proposed to shim the abutment bearings to 15 mm above the differential settlement datum. This would give 55 mm (40 + 15) allowable settlement To allow for the uncertainty in prediction, additional steel shims 20 mm thick were to be introduced under the bearings of the bridge pier to which the abutment foundation was to be connected. These shims could be removed at any time to allow additional settlement at the abutment without a change in the state of moment in the deck. This contingency would only be invoked after most of the settlement allowance has occurred, so the instantaneous moments resulting from the removal of the shims would not exist without the Settlement moments acting in the opposite direction.

2.

Operation and maintenance - embankment dams

During the operational and maintenance stage of an embankment dam, modification of the design may be required to improve its watertightness or to repair damage caused by internal erosion. This might typically be achieved by grouting or diaphragm walling (Charles, 1993). An example is Tarbela Dam in Pakistan (U1 Haq, 1996). Paine et aZ(1982) described the use of relief wells to overcome the unexpectedly high pore water pressure in Poechos Dam in Peru (see Box 9.7). This example also illustrates the best-way-out application of the OM.

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Box 9.7

The remediation strategy at Poechos Dam, Peru (Paine et al, 1982)

The Poechos Dam in northern Peru has a total embankment length of 9 km and holds 1000 m3. Monitoring of the performance of the dam during its first filling indicated unexpected cracks forming in the embankment and in the ground downstream, accompanied by varied amounts of heave. Foundation pore pressures were also higher than expected. It became necessary to re-evaluate the design. More piezometers were installed to monitor the pore pressures at different depths of the foundation. The reassessment of the design, using higher pore pressure values, indicated that there might be unacceptably low factors of safety against sliding on a known sub-horizontal shear zone when the water reached retention level. Further investigations were carried out. More piezometers were installed. Trial pits were dug in order to locate existing shear zones and to take samples for testing. The geodetic survey was extended and the groundwater chemistry examined in detail for evidence of a significant loss of soluble material from the foundation. As an interim measure for safety, reservoir water levels were kept down, to keep pore pressures within the bounds of the original design.

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After monitoring of the pore pressures over several seasons, a staged approach based on the OM was adopted for carrying out remedial works. That maintained safety but avoided unnecessary expenditure. The stages were as follows: 1.

Eleven drainage wells were installed at the downstream toe of the embankment. These were connected to a drainage trench to allow them to discharge below ground level without pumping, but they were large enough to permit pumping should additional measures to control pore water pressures be required urgently.

2.

Stability analyses were run for different pore water pressures on different parts of the potential failure surfaces to establish trigger levels. These triggers enabled the operating staff to recognise excessive pore water pressures and assess the need to hold down the reservoir level.

3.

The dam was monitored continuously so that the measured pore water pressures could be evaluated at any time to ascertain whether additional measures were needed.

4.

Further measures - pumping, additional drainage and lowering the reservoir level -were available as contingencies.

3.

Protection of structures

Based on the case histories gathered, the strategies adopted for the protection of structures are summarised in Table 9.8. These strategies include modification of temporary works and the rate of construction. Use of grouting and surcharge have also been reported. Table 9.8

176

Examples of Observational Method-based strategies for protecting structures

Examp1e

Strategy adopted to protect structures

Minster Court

Progressively modify the temporary support (earth berms), see Box 8.5

Victory Arch

Reduce the tunnelling rate and adopt compensation grouting, see Box 9.2

Little Italy

Compaction grouting, see Box 9.4

Mansion House

Phased tunnelling, see Box 9.5

Grand Buildings

Using surcharge, see Box 9.8

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Box 9.8 ~~

The strategy adopted for protecting the London Underground Jubilee Line tunnels under the Grand Buildings in London (Clayton et al, 1991) ~

~~

~

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The redevelopment of the Grand Buildings in London required design and construction techniques that guaranteed avoiding damage to the underlying London Underground Jubilee Line tunnels (Figure 9.11). Results of detailed finite element and boundary element analyses were therefore integrated with a scheme of displacement monitoring, so as to validate and allow optimisation of a demolition and reconstruction scheme developed by the design-and-construct contractor. This scheme can be summarised as below. 1.

Assessment of the available information on the London Clay, including limited ground investigation data, observations of ground movements around constructions in similar ground conditions elsewhere, and experience from the demolition and reconstruction of an adjacent building.

2.

Design based on moderately conservative parameters and adoption of a conservative demolition and reconstruction programme, followed by boundary element analyses to predict ground movements for a new scheme of demolition and construction.

3.

Incorporation in the plans of elements of work that could be omitted or discontinued if the predictions of movements proved pessimistic.

4.

The new scheme proposals were analysed to assess their acceptability with regard to heave and predicted stress changes at the tunnel levels. These analyses provided the confidence necessary to proceed. The sequence of demolition was specifically organised to allow the maximum time for monitoring movements and modification of the demolition and reconstruction process, before the most critical area was approached. This was to give early warning of any unexpected movements.

5. Reassessment of ground stiffness parameters and redesign of the demolition and construction programme, as demolition proceeded. The demolition areas were designed to provide maximum working access while minimising changes in vertical ground stress over the tunnels. Excavation was carried out in strips across the line of the tunnels. To reduce heave in some sensitive areas, these strips were made even smaller. The plan called for the installation of temporary support panels within the basement before the start of demolition, to provide temporary support for kentledge during strip excavation (Figure 9.11). Monitoring of ground movements at the early stage of demolition indicated that the analyses were significantly overestimating the heave. Therefore the planned back-load kentledge was not used in most of the areas of the building. This enabled further economies. The filling support works and more difficult steelwork erection could be eliminated. Reinforced concrete works were made much easier. A major moling operation to remove kentledge was avoided. In addition, where movements were considerably less than could be tolerated, the specification for excavation was relaxed so that two bays could be taken out at once. This resulted in simpler excavation, easier sequences of construction and further cost savings. Overall these actions considerably shortened the construction programme and reduced the overhead costs, to the extent that an estimated f500 000 and between three and four months were saved on the substructure construction.

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IKentledge filling

-2.2 1 -3.0 1

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Escalator hall at Trafalgar Square end,

\

Access tunnel

I

1B

1A

STRIP 1 EXCAVATION

Existing wall or where required kentledge

KENTLEDGE SUPPORT

KENTLEDGE TRANSFER

Figure 9.11

178

Demolition strategy adopted at the Grand Buildings, London (Clayton et al, 1991)

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10

Conclusions and ways forward

10.1

CONCLUSIONS Professor Peck set out the procedure for carrying out the OM in his Ranlune Lecture in 1969. Now, as then, the aim is to achieve economy, enhance safety and encourage team co-operation. This report updates the OM process in response to the requirements of modem civil engineering practice, but the principles and the objectives of the OM advocated by Peck are still largely retained. The procedure for implementing the Method has been stated and illustrated by examples of different applications. The following are the main conclusions. 1. For many civil engineering projects, the ground is a major source of uncertainty, because of geological variability and difficulty in selecting design parameter values or in modelling the problem realistically. The case histories in this report show that, where there are large uncertainties, the OM is an effective way of managing risks associated with these uncertainties.

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The definition of the OM is stated in Chapter 2, and the components of the OM are shown in Figure 1.2. They are: national and corporate policy 0

corporate and project organisation design and planning construction control and monitoring review and implement planned modification (including contingency plan and emergency plan)

0

technical and procedural auditing.

However, it should be noted that overall economy can be achieved through the use of the OM only if 0

more thorough, and better quality site investigations are carried out site investigations are substantially completed and the data are interpreted before the initiation of the Method, and design cases are analysed to cover all the likely scenarios.

Historically, Peck (1969a) identified two applications for the OM. They are ab initio and best way out. Although the stage at which the OM is introduced into the project is different in these two situations, the principles and procedures for implementing the OM are identical. All the OM elements shown in Figure 1.2 should be fully implemented in both situations. 2. The definition of the OM requires that design, construction control, monitoring, reviewing process and planned modifications all have to be demonstrably robust. To achieve these requirements, a risk-based approach should be adopted in design and management of construction. Hazards and risk assessments must be carried out. All risks should be managed on the basis of their hierarchy, see Box 4.1, and should take into account the following design conditions used in relation to the OM: most probable (MP) moderately conservative (Mod C) 0

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most unfavourable (MU). 179

3.

Table 3.4 compares the design parameters/conditions adopted by different guides and codes. It can be seen that modern codes identify two of the three classes and one of these is moderately conservative. Peck (1969a) identified two soil parameter conditions for the OM, namely “most probable” and “most unfavourable”. These are also shown in Table 3.4 in relation to other codes. The moderately conservative class is not used and this makes it difficult to relate the different design approaches. For consistency in the future, it is recommended that the OM uses: “most probable” and “moderately conservative” conditions for deformation . and load predictions - serviceability limit state (SLS) designs “most unfavourable” conditions for ultimate limit state (ULS) designs and for robustness check during risk assessment.

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The codes adopt single sets of partial factors or factors of safety. The risks in terms of severity and frequency of occurrence are rarely considered. Section 8.1.1 and Figure 8.5 refer to slope stability and cuttings and the effect of risk to life and economic risk on the factor of safety to be adopted. Temporary works conditions can enable better control measures to be put in place than for permanent conditions. Further work is required to develop risk assessments and relate these to factors of safety would be helpful. The OM should have a part to play in this process. 4.

A traffic light system (green, amber, red) is often used to signal the performance recorded by the monitoring system at a particular location on site. Figure 7.3 shows an example of such a system adopted in the NATM. The monitoring system should work at primary and secondary levels. The primary system is used by the site team for routine monitoring while the secondary system is for checking the performance of the primary system and also acts as a back-up. Regular meetings should be held among key project personnel to review these instrumentation monitoring results.

5.

Peck (1969a) emphasises that field observations are useful only if they are displayed promptly in such a way as to show quickly and clearly the essential features. Timely processing and reviewing of monitoring information are therefore crucial. This will enable planned modification (including contingency plan or emergency plan if necessary) to be implemented in a timely manner such that the risks arising from unforeseen events can be prevented from reaching an unacceptable level. The concept of “discovery-recovery’’ HSE, (1996) is important in this respect (see Figure 3.10).

6. Implementation of planned modifications based on Peck (1969a) and by progressive modification originating from Powderham (1994) have both been presented in this report - see Section 3.10. A progressive modification approach is preferred. Peck’s approach is only recommended where previous case history data are available on comparable ground conditions and a multi-stage construction sequence is planned. Furthermore, the design and construction team should have a clear understanding of the objectives, key criteria and individual and collective responsibilities. It should be noted that the course of action for the designer has to be set in terms of health and safety law; and this means a risk-based approach taking account of what is reasonably practicable. If the cost difference between “most probable” and “most unfavourable” set of circumstances is small but the health and safety disadvantage is considerable, the way forward is for the designer to choose the criteria that take sufficient account of the need to avoid foreseeable risk and combat risks at source.

7.

180

The components of the OM shown in Figure 1.2 have been further broken down into project activities and they are shown in Table 4.1. Technical considerations when implementing the OM have also been discussed in Chapter 4.

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It should be noted that the OM must not be used in the following situations: 0

where there is insufficient time to implement and complete the contingency and emergency plans fully, ie rapid deterioration

0

where observations would be difficult to obtain or are unreliable.

When the Method is used to control the risk of exceeding an ultimate limit state or where there is a risk to people associated with the use of the Method, a safety case needs to be provided. The risk-control measure must be an explicit part of the safety management system required by the relevant safety regulations, eg CDM. In this context, a “safety case” is defined as a systematic and, where possible, quantified demonstration that an installation or system meets specified safety criteria. Some potential inadequacies in the use of the OM are summarised in Table 4.5 in Chapter 4. The likelihood of these inadequacies occurring should be assessed before the OM is initiated and it should not be used when such inadequacies are anticipated.

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8.

There is more interaction between designers and constructors on OM projects than in ones of predefined design. The interaction needs management and co-ordination. Project team members’ commitment and willingness to own and solve the problems are therefore of critical importance. Good management is thus of particular importance to the OM projects. The OM should not be initiated unless the technical issues have been considered thoroughly and a sound management system has been set up. Chapter 5 has presented some management considerations. The key factors for success depend largely on the values that those involved in the project set up: culture 0

strategy competence

0

systems.

9. The merits and disadvantages of different contract types and other contractual considerations, are discussed in Chapter 6 in relation to the OM. Although case histories show that many OM projects were operated under both traditional and design-and-construct contracts, the latter are intrinsically more amenable than other forms to adoption of the OM. The adoption of a partnering agreement or culture, preferably involving all parties related to or affected by the OM, can only have a positive effect. In such an environment where the parties are working together to deliver the optimised solutions (in terms of cost, risk, programme, environment etc) in which the gains and the losses are shared, the OM can deliver its maximum potential. 10. Both value management and the OM have as their primary objective the elimination of unnecessary cost while meeting other project objectives such as completion time and safety. Their complementary nature is discussed in Section 3.1 1. The value criteria of the parties involved in the project have also been discussed in Section 3.1 1. To achieve these criteria through the value-management process, a value engineering clause should be incorporated in the contract. Section 6.3 describes the key features of a value engineering clause to enable success of the OM. 1 1. The degree of success of the OM is influenced by the following: contract type procurement method payment method operating environment. ClRlA Report 185

181

Different tender mechanisms can bring into play factors such as competitiveness, potential for added value, flexibility etc, which are conducive to the adoption of the OM. These factors vary with projects. It will be useful to assess these factors in relation to each of the above four items before selecting a suitable tender strategy. An assessment matrix will be useful for ranking each item systematically.

10.2

WAYS FORWARD There is no doubt that the technical requirements and management framework described in this report will be changed with the formulation of new statutory regulations, new design codes, new contractual arrangements and new technology. The OM process will also have to change. It will do so partly because of its use in applications not covered in this report. Future development and improvement of the OM is to be expected, the more so because its purpose is to be responsive to change. The areas in which further development can be looked for are listed below. 1. Powderham and Nicholson (1996) consider that historically attention has been

concentrated on geotechnical aspects of the OM. They consider the OM development should also focus more on structural aspects of soiVstructural interaction because it is often in adjacent structures that risk concerns are concentrated.

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2.

Powderham and Nicholson (1996) also consider that there are opportunities to extend the OM into long-term situations. However, a true application of the OM will require the ongoing facility to make changes with the complementary need to assure timely collection and assessment of the critical observations. Long-term monitoring is not the same as long-term OM. The demands for clear communication and consistent understanding of the particular criteria for a given project present a significant constraint to long-term applications.

3. Possible technical developments on aspects of the design and planning include: promotion of phased SI in order to maximise the value of the SI work 0

development of national database for benchmarking

0

development of simple and practical risk assessment tools

0

further development of the concept of trend and rate of change of monitored value improvement on the usability of analytical tools such as finite element methods.

4. Potential developments are more likely on new contract types which: 0

minimise the gap between designers and constructors

0

reward each party fairly

0

allocate risk appropriately provide flexibility in design and construction changes

0

182

incorporate some of Latham’s (1994) recommendations, eg fair dealing and co-operation and provision of incentives for exceptional performance.

ClRlA Report 185

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WHITMAN, R V (1984) Evaluating calculated risk in geotechnical engineering Journal of Geotechnical Engineering, American Society of Civil Engineers, 110 (2), pp 143-1 88 WOOD, A A and GRIFFITHS, C M (1 994) Debate: contaminated sites are being over-engineered Proceedings of Institution of Civil Engineers Civil Engineering, 104, August, pp97-105

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Recommended reading

The following is a list of publications reviewed by the authors. They either contain examples of OM applications or discussions on issues related to the elements of the OM. They are recommended for further reading, to complement the information provided in this report. These have been catalogued under the following topics: history and background contractual framework 0

risk management

0

value management instrumentation health and safety tunnel applications

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excavation applications ground treatment

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embankment and reclamation works environmental geotechnics

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structures.

History and background

From theory to practice in soil mechanics - selections from the writings of Karl Terzaghi L Bjermm, A Casagrande, R B Peck and A W Skempton eds, John Wiley and Sons, NewYork The art and science of geotechnical engineering: at the dawn of the twenty-first century: a volume honouring Ralph B Peck E J Cording ed, Prentice-Hall Inc, New Jersey Judgement in geotechnical engineering - the professional legacy of Ralph B Peck J Dunnicliff and D U Deere eds, Wiley, New York FORERO-DUEfiAS, C A (1996) Discussion In: The Observational Method in Geotechnical Engineering, Institution of Civil Engineers, Thomas Telford, London, pp191-192 MUIR WOOD, A (1990) The Observational Method revisited In: Proceedings of 10th Southeast Asian Geotechnical Conference, April 1990, Taipei, 2, pp37-42 MUIR WOOD, A (1 994) Control of uncertainty in geotechnical engineering In: Geotechnical Engineering Emerging Trends in Design and Practice ( K R Saxena ed), Oxford and IBH Publishing COPvt Ltd, New Delhi, pp155-175

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NICHOLSON, D P (1 994) Preface to the Ninth Gkotechnique Symposium in Print: The Observational Method in Geotechnical Engineering Gkotechnique, 44 (4), pp6 13-6 18 BECK, R B (1969) Advantages and limitations of the Observational Method in applied soil mechanics Giotechnique, 19 (2), pp171-187 PECK RB (1977) Pitfalls of overconservatism in geotechnical engineering Civil Engineering, American Society of Civil Engineers, 47 (2), February, pp62-66 PECK R B (1980) Where has all the judgement gone? Canadian Geotechnical Journal, 17 (4), pp584-590

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PECK R B (1983) Preface to Judgement in Geotechnical Engineering - The Professional Legacy of R B Peck (J Dunnicliff and D V Deere eds), Wiley, New York PECK, R B (1985) The last sixty years In: Proceedings of the 11th International Conference on Soil Mechanics and Foundation Engineering, San Francisco, Golden Jubilee Volume, pp 123-1 33 POWDERHAM, A J and NICHOLSON, D P (1996) The way forward In: The Observational Method in Geotechnical Engineering, Institution of Civil Engineers, Thomas Telford, London, pp 195-204 POWDERHAM, A J (in preparation) The Observational Method - Implementation by progressive modification Boston Society of Civil Engineers Journal, Fall 1997 TERZAGHI K (196 1) Past and future of applied soil mechanics Journal of Boston Society of Civil Engineers, 48, ppl10-139 TERZAGHI, K and PECK, R B (1967) Soil Mechanics in Engineering Practice. 2nd Edition. John Wiley & Sons, New York, pp 293-295 TOMLINSON, M J (1995) Foundation design and construction. 6th Edition Longman, London, pp404 Contractual framework

BOARDMAN, M F (1 995) Discussion In: The Observational Method in Geotechnical Engineering, Institution of Civil Engineers, Thomas Telford, London, ppl88-189

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CHARTRES R (1 996) Discussion In: The Observational Method in Geotechnical Engineering, Institution of Civil Engineering, Thomas Telford, London, pp 189-190 INSTITUTION OF CIVIL ENGINEERS (1993) The New Engineering Contract. 1st Edition Thomas Telford, London. INSTITUTION OF CIVIL ENGINEERS (1995) NEC Engineering and Construction Contract. 2nd Edition Thomas Telford, London LATHAM, M (1994) Constructing The Team. Final report of the governmenthdustry review ofprocurement and contractual arrangements in the UK construction industry HMSO, London

Risk management

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BLOCKLEY, D (1994) Uncertain ground: on risk and reliability in geotechnical engineering Ground Engineering, March, pp29-34 CASAGRANDE, A (1965) Role of the ‘calculated risk’ in earthwork and foundation engineering Journal of the Soil Mechanics and Foundations Division, American Society of Civil Engineers, 9 1, SM4, pp 1-40 CHOWDHURY, R ( 1994) Evaluating risk Ground Engineering, March, pp 29-34 GODFREY, P S (1 996) Control of risk: a guide to the systematic management of risk from construction CIRIA Special Publication 125, CIRIA, London COLE, K W Building Over Abandoned Shallow Mines. Paper 1: Considerations of Risk and Reliability Ground Engineering, JanuaryEebruary, pp35-37 COTTAM, M P and MAGUIRE, J R (1995) Hazard and risk assessment in other industries The Structural Engineers, 73 (21), pp364-365 FOCHT, J A (1994) Lessons learned from missed predictions Journal of Geotechnical Engineering, American Society of Civil Engineers, 120 (lO), ~~1653-1683 HEALTH AND SAFETY EXECUTIVE (1989) Quantified risk assessment: its input to decision making HMSO, London ClRlA Report 185

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HEALTH AND SAFETY EXECUTIVE (199 1) Successful health and safety management HMSO, London HEALTH AND SAFETY COMMISSION (1992) Management of health and safety at work HMSO, London HEALTH AND SAFETY COMMISSION (1994) Managing construction for health and safety, Construction (Design and Management) Regulations 1994. Approved Code of Practice HSE Books, Suffolk, England HEALTH AND SAFETY EXECUTIVE (1996) Safety of New Austrian Tunnelling Method (NATM) tunnels. A review of sprayed concrete lined tunnels with particular reference to London clay HSE Books, Suffolk, England

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HM TREASURY (1993) Managing risk and contingency for works projects HM Treasury Central Unit on Procurement Guidance Note No 4 1, HM Treasury, London INSTITUTION OF CIVIL ENGINEERS (1995) Proceedings of the Conference on Risk management in civil, mechanical and structural engineering, Institution of Civil Engineers, February, Thomas Telford, London MENZIES, J B (1995) Hazards, risks and structural safety The Structural Engineer, 73 (21), pp357-363 NEALE, B S (1995) Hazard and risk assessment for constructions: a regulator’s view The Structural Engineer, 73 (22), pp388-390 O’RIORDAN, N J (1995a) Environmental geotechnics: risk management and remediation In: Proceedings of 9th Young Geotechnical Engineer’s Conference, September 1995, Gent O’RIORDAN, N J (1995b) Discussion on Debate: Contaminated sites are being over-engineered Proceedings of Institution of Civil Engineers Civil Engineering, 108, November, pp 180-1 85 O’RIORDAN, N J and MILLOY, C J (1995) Risk assessment for methane and other gases from the ground CIRIA Report 152, CIRIA, London POWDERHAM, A J and TAMARO, G J (1 995) Mansion House London: Risk assessment and protection Journal of Construction Engineering and Management, American Society of Civil Engineers, 121 (3), pp266-272 ROYAL SOCIETY (1992) Risk: analysis, perception, management Royal Society, London 198

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THOMPSON, P A and PERRY, J G (1992) Engineering construction risks: a guide to project risk analysis and risk management Thomas Telford, London THORBURN, S (1994) Uncertainty and judgement in geotechnical engineering Ground Engineering, April, ppl8-22 WHITMAN, R V (1984) Evaluating calculated risk in geotechnical engineering Journal of Geotechnical Engineering, American Society of Civil Engineers, 110 (2), pp 143-1 88 WU, T H (1974) Uncertainty, safety, and decision in soil engineering Journal of the Geotechnical Engineering Division, American Society of Civil Engineers, 100 (GT3), ~ ~ 3 2 9 - 3 4 8 Value management

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CONNAUGHTON, J N and GREEN, S D (1996) Value management in construction: a client’s guide CIRIA Special Publication 129, CIRIA, London DELL’ISOLA, A J (1982) Value engineering in the construction engineering, 3rd Edition Van Nostrand Reinhold, New York GUTHRIE, P and MALLET, H (1995) Waste minimisation and recycling in construction - a review CIRIA Special Publication 122, CIRIA, London INSTITUTION OF CIVIL ENGINEERS (1996) Creating value in engineering Thomas Telford, London POWDERHAM, A J and RUTTY, P C (1994) The Observational Method in value engineering In: Proceedings of 5th International Conference on Piling and Deep Foundations, Bruges, pp5.7.1-5.7.12 Instrumentation

AOKI, M (1995) Monitoring of field data and its application to excavations - A survey on Japanese literature In: Underground Construction in Soft Ground (K Fujita and 0 Kusakabe eds), Balkema, Rotterdam, pp295-299 AVGHERINOS, P J, JOBLING, P W, VARLEY, P (1993) The Channel Tunnel - Use of geotechnical monitoring to assist the design and construction of tunnels through an ancient landslip at Castle Hill In: Optionsfor Tunnelling (H Burger ed), Elsevier Science Publishers BV, Netherlands, ~~689-698

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BIENIAWSKI Z T (1984) Rock mechanics design in mining and tunnelling AA Balkema, Rotterdam BOARDMAN, M F (1995) Discussion In: The Observational Method in Geotechnical Engineering, Institution of Civil Engineers, Thomas Telford, London, pp 171-1 74 CHOA, V (1994) Application of the Observational Method to Hydraulic Fill Reclamation Projects Giotechnique, 44 (4), 3333735-745 COX, D W (1995) Discussion In: The Observational Method in Geotechnical Engineering, Institution of Civil Engineers, Thomas Telford, London, ppl69-170

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DIBIAGIO, E and HBEG, K (1989) Where has all the judgement come from? In: The art and science of geotechnical engineering at the dawn of hventy-first century, (E J Cording ed), Prentice-Hall Inc, New Jersey, pp248-268 FORBES, J, BASSETT, R H and LATHAM, M S (1994) Monitoring and interpretation of the Mansion House due to tunnelling Proceedings of Institution of Civil Engineers Geotechnical Engineering, 107, April, pp 89-9 8 FRISCHMANN, W W, HELLINGS, J E, GITTOES, G and SNOWDEN, C (1994) Protection of the Mansion House against damage caused by ground movements due to the Docklands Light Railway extension Proceedings of the Institution of Civil Engineers Geotechnical Engineering, 1994, 107, April, pp65-76 GREENWOOD, D A (1 995) Discussion In: The Observational Method in Geotechnical Engineering, Institution of Civil Engineers, Thomas Telford, London, pp 174-1 75 HARRIS, D I, MAIR, R J, LOVE, J P, TAYLOR, R N and HENDERSON, T 0 (1994) Observations of ground and structure movements for compensation grouting during tunnel construction at Waterloo station Giotechnique, 44 (4), pp69 1-7 13 PARKHOUSE, J G (1995) Discussion In: The Observational Method in Geotechnical Engineering, Institution of Civil Engineers, Thomas Telford, London, ppl70-17 1 PECK, R B (1943) Earth pressure measurements in open cuts, Chicago (Ill.) subway Transactions of American Society of Civil Engineers, 108, pp 1008-1036

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PECK R B (1 970) Field measurements, underground structures Publication No 123, American Society of Civil Engineers Soil Mechanics Lecture Series, Chicago, pp91-115 SYMONS, I F (1992) The role of field measurements in the design and construction of geotechnical structures In: TRL Workshop on the Observational Method for Geotechnical Structures, Transport Research Laboratory, Berkshire, England TSE, C M and NICHOLSON, D P (1992) Design construction and monitoring of the basement diaphragm wall at Minster Court, London In: Proceedings of the Conference on Retaining Structures, Institution of Civil Engineers, Cambridge, pp323-332 Health and safety

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HEALTH AND SAFETY EXECUTIVE (1989) QuantiJied risk assessment: its input to decision making HMSO, London HEALTH AND SAFETY EXECUTIVE (199 1) Successful health and safety management HMSO, London HEALTH AND SAFETY COMMISSION (1992) Management of health and safety at work HMSO, London HEALTH AND SAFETY COMMISSION (1994) Managing construction for health and safety, Construction (Design and Management) Regulations 1994. Approved Code of Practice HSE Books, Suffolk, England HEALTH AND SAFETY EXECUTIVE (1996) Safety of New Austrian Tunnelling Method (NATM) tunnels. A review of sprayed concrete lined tunnels with particular reference to London clay HSE Books, Suffolk, England NICHOLSON, D P (1994) Preface to the Ninth Geotechnique Symposium in Print: The Observational Method in Geotechnical Engineering Gkotechnique, 44 (4), pp61 3-6 18 HER MAJESTY’S STATIONERY OFFICE (1996) The Construction (Health, Safety and Welfare) Regulations 1996 Statutory Instruments 1996 No 1592, HMSO, London Tunnel applications

AVGHERINOS, P J, JOBLING, P W and VARLEY, P (1993) The Channel Tunnel - Use of geotechnical monitoring to assist the design and construction of tunnels through an ancient landslip at Castle Hill In: Optionsfor Tunnelling (H Burger ed), Elsevier Science, Netherlands, pp689-698 ClRlA Report 185

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BARTON, N and GRIMSTAD, E (1994) Rock mass conditions dictate choice between NMT and NATM Tunnels and Tunnelling, October, pp39-42 BIENIAWSKI Z T (1984) Rock mechanics design in mining and tunnelling AA Balkema, Rotterdam BOSCARDIN, M D and CORDING, E J (1989) Buildings response to excavation-induced settlement Journal of Geotechnical Engineering, American Society of Civil Engineers, 115 (l), January, pp 1-2 1 FARLEY, R and GLASS, P R (1994) The construction of the Limehouse Link Tunnel Proceedings of Institution of Civil Engineers, 105, August, ppl53-164

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FORBES, J, BASSETT, R H and LATHAM, M S (1994) Monitoring and interpretation of the Mansion House due to tunnelling Proceedings of Institution of Civil Engineers Geotechnical Engineering, 107, April, pp89-98 FUGEMAN, I C D, MYERS, A G, LAFFORD, G M and JOHN, M (1991) The Channel Tunnel: development of design and construction methods for the United Kingdom undersea crossover In: Tunnelling '91,pp427-439 FUJIMOTO, A, TAKAHASHI, Y, IWASHITA, H and YAMAGUCHI, Y (1995) Analysis on ground movements during shield tunnelling - a survey on Japanese shield tunnelling In: Underground Construction in Soft Ground (K Fujita and 0 Kusakabe eds), Balkema, Rotterdam, pp353-357 HARRIS, D I, MAIR, R J, LOVE, J P, TAYLOR, R N and HENDERSON, T 0 (1994) Observations of ground and structure movements for compensation grouting during tunnel construction at Waterloo station Gkotechnique, 44 (4), pp691-713 HEALTH AND SAFETY EXECUTIVE (1996) Safety of New Austrian Tunnelling Method (NATM) tunnels. A review of sprayed concrete lined tunnels with particular reference to London clay HSE Books, Suffolk, England INSTITUTION OF CIVIL ENGINEERS (1996) Sprayed concrete linings (NATM)for tunnels in soft ground Thomas Telford, London JOHN, M and ALLEN, R (to be published) NATM on the Channel Tunnel In: Engineering Geology and the Channel Tunnel KAVVADAS, M., HEWISON, L R, LASKARATOS, P G, SEFEROGLOU, C and MICHALIS, I (1996) Experiences from the construction of the Athens Metro In: Proceedings of the Symposium on Geotechnical Aspects of Underground Construction in Soft Ground, City University, London, 15-17 April, pp2 17-222

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MOROTO, N, OHNO, M and FUJIMOTO, A (1995) Observational control of shield tunnelling adjacent to bridge piers In: Underground Construction in Soft Ground, (K Fujita and 0 Kusakabe eds), Balkema, Rotterdam, pp24 1-244 MUIR WOOD, A (1987) To NATM or not to NATM Felsbau, 5 (l), pp26-30 MUIR WOOD, A (1 990) The Observational Method revisited In: Proceedings of 10th Southeast Asian Geotechnical Conference, April 1990, Taipei, 2, pp37-42

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MUIR WOOD, A (1995) Discussion In: The Observational Method in Geotechnical Engineering, Institution of Civil Engineers, Thomas Telford, London, p 169 NAKASHIMA, M, HIRATA, A and IWASAKI, Y (1988) In-situ measurements and construction control of an underground excavation beneath the subground structure for a subway construction in Nagoya In: Proceedings of 2nd International Symposium on Field Measurements in Geotechnics (Sakurai ed), Balkema, Rotterdam, 1, pp473-480 PATULLO, G (1995) Construction of the A20 Round Hill tunnels, Kent Proceedings of Institution of Civil Engineers Civil Engineering, 108, May, pp54-63 PECK, R B (1969) Deep excavations and tunnelling in soft ground In: Proceedings of 7th International Conference on Soil Mechanics and Foundation Engineering, Mexico, State-of-the-art Volume, pp225-290 PENNY, C (1995) Discussion In: The Observational Method in Geotechnical Engineering, Institution of Civil Engineers, Thomas Telford, London, pp190-191

PENNY, C, STEWART, J, JOBLING, P W and JOHN, M (1991) Castle Hill NATM Tunnels: Design and construction In: Tunnelling 91, Institute of Mining and Metallurgy, London, pp285-297 POWDERHAM, A J (1994) An overview of the observational method: development in cut and cover bored

tunnelling projects Gdotechnique, 44 (4), pp619-636 POWDERHAM, A J and RUTTY, P C (1994) The Observational Method in value engineering In: Proceedings of 5th International Conference on Piling and Deep Foundations, Bruges, pp5.7.1-5.7.12

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POWDERHAM, A J and TAMARO, G J (1995) Mansion House London: Risk assessment and protection Journal of Construction Engineering and Management, American Society of Civil Engineers, 121 (3), pp266-272 Excavation applications BATTEN, M and POWRIE, W (1996) Prop loads in large braced excavations Ground Engineering, October, pp29-30 BOSCARDIN, M D and CORDING, E J (1989) Buildings response to excavation-induced settlement Journal of Geotechnical Engineering, American Society of Civil Engineers, 115 (I), January, pp 1-2 1

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BRITISH STANDARDS NSTITUTION (1995) Geotechnical Design, Part 1: General Rules (together with United Kingdom National Application Document) Eurocode 7: British Standards Institution, London CARDER, D R (1995) Ground movements caused by diferent embedded retaining wall construction techniques TRL Report 172, Transport Research Laboratory, Berkshire, England CARD, G B and CARDER, D R (1996) Movement trigger limits when applying the Observational Method to embedded retaining wall construction on highway scheme TRL Report 228, Transport Research Laboratory, Berkshire, England CARDER, D R and BENNET, S N (1996) The effectiveness of berms and rakedprops as temporary support to retaining wulls TRL Report 2 13, Transport Research Laboratory, Berkshire, England CHANDLER, R J (1 984) Recent European experience of landslides in over-consolidated clays and soft rocks. In: Proceedings of 4th. International Symposium on Landslides, Toronto, 1, pp 61-8 1 CHUNG, H S, HWANG, Y C and JANG, C S (1995) In-situ monitoring and feed back analysis of braced excavation system In: Underground Construction in Soft Ground, (K Fujita and 0 Kusakabe eds), Balkema, Rotterdam, ppl75-178 CLOUGH, G W and O’ROURKE, T D (1990) Construction induced movements of in-situ walls In: Proceedings of ASCE Specialty Conference, Cornell, ASCE Geo Special Publication 25, pp439-470 DAWSON, M P, DOUGLAS, A R , LINNEY, L, FRIEDMAN, M and ABRAHAM, R (1996) Jubilee Line Extension, Bermondsey Station Box, design modifications, instrumentation and monitoring In: Proceedings of the Symposium on Geotechnical Aspects of Underground Construction in Soft Ground, City University, London, 15-1 7 April, pp49-54

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EVERTON, S J (1995) Discussion In: The Observational Method in Geotechnical Engineering, Institution of Civil Engineers, Thomas Telford, London, pp167-168 EVERTON, S J (1996) Design of low-propped embedded retaining walls in stiff clays

Ground Engineering, October, pp37-41

1

,

FARLEY, R and GLASS, P R (1994) The construction of the Limehouse Link Tunnel Proceedings of Institution of Civil Engineers, 105, August, pp 153- 164 FITZHUGH, M M, MILLER, J S and TERZAGHI, K (1 947) Shipways with cellular walls on a mar1 foundation Transactions ofAmerican Society of Civil Engineers, 1 12, pp298-324

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GEOTECHNICAL CONTROL OFFICE (1990) Review of design methods for excavating Hong Kong Government GEOTECHNICAL CONTROL OFFICE (1984) Geotechnical manual for slopes Hong Kong Government, pp 183-1 85 GEOTECHNICAL ENGINEERING OFFICE (1993) Guide to retaining wall design Hong Kong Government GLASS, P R and POWDERHAM, A J (1994) Application of the observational method at the Limehouse Link Giotechnique, 44 (4), pp665-679 GORDON, T, LORD, J A and STATHAM, I (1 99 1) Influence of Tectonic Structure on Landslipping: Thrace Motorway, Turkey In: Slope Stability Engineering (R J Chandler ed), Thomas Telford, London, pp349-354 IKUTA, Y, MARUOKA, M, AOKI, M and SATO, E (1994) Application of the Observational Method to a deep basement excavated using the topdown method Giotechnique, 44 (4), pp655-664 IWASAKI, Y, WATANABE, H, FUKUDA, M, HIRATA, A and HORI, Y (1994) Construction control for underpinning piles and their behaviour during excavation Giotechnique, 44 (4), pp68 1-689 LORD, J A, GORDON, T and BLOWER, T (1991) The design and construction of the 80 m high Saribayir Cut on the Izit-Adapuzari section of the Anatolian motorway In: Proceedings of Technical Meeting on Geotechnical Problems on Motorways and their solutions, Ankara, pp 99-123 NAKASHIMA, M, HIRATA, A and IWASAKI, Y (1988) In-situ measurements and construction control of an underground excavation beneath the subground structure for a subway construction in Nagoya ClRlA Report 185

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In: Proceedings of 2nd International Symposium on Field Measurements in Geotechnics (Sakurai ed), Balkema, Rotterdam, 1, pp473480 NATIONAL COAL BOARD (1970) Spoil heaps and lagoons National Coal Board Technical Handbook, National Coal Board, p120 NAVAL FACILITIES ENGINEERING COMMAND (NAVFAC) (1982) Design Manual 7.2: Foundations and earth structures US Department of the Navy, Washington DC, USA, p244 NICHOLLS, K H (1996) Discussion In: The Observational Method in Geotechnical Engineering, Institution of Civil Engineers, London, pp185-188 NICHOLSON, D P (1992) Contribution to ICE Ground Board meeting I8 March 1992 Ground Engineering, October, pp30-32

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PADFIELD, C J and MAIR, R J (1984) Design of retaining walls embedded in stiffclay CIRIA Report 104, CIRIA, London PECK R B (1941) Soil mechanics on the Chicago subway Ill Tech Eng, October, pp7-12 PECK, R B (1943) Earth pressure measurements in open cuts, Chicago (Ill) subway Transactions ofAmerican Society of Civil Engineers, 108, pp1008-1036. PECK, R B (1969) Deep excavations and tunnelling in soft ground In: Proceedings of 7th International Conference on Soil Mechanics and Foundation Engineering, Mexico, State-of-the-art Volume, pp225-290 PERRY, J (1989) A survey of slope condition in motorway earthworks in England and Wales TRRL Research Report 199, Transport Road Research Laboratory, Berkshire POWDERHAM, A J (1994) An overview of the observational method: development in cut and cover bored tunnelling projects Gdotechnique, 44 (4), pp619-636 POWDERHAM, A J and RUTTY, P C (1994) The Observational Method in value engineering In: Proceedings of 5th International Conference on Piling and Deep Foundations, Bruges, pp5.7.1-5.7.12 POWRIE, W and DALY, M (1996) The design of earth berms for the temporary support of embedded retaining walls In: Notes accompanying a Symposium on Earth Retaining Systems, July, Southampton, pp 15-1 6 206

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RATNAM, K (1996) Discussion In: The Observational Method in Geotechnical Engineering, Institution of Civil Engineers, Thomas Telford, London, pp168-169 ROYAL SOCIETY (1992) Risk: analysis, perception, management Royal Society, London ST. JOHN, H D, POTTS, D M, JARDINE, R J and HIGGINS, K G (1993) Prediction and performance of ground response due to construction of a deep basement at 60 Victoria Embankment In: Predictive Soil Mechanics (G T Houlsby and A N Scholfield-eds),Thomas Telford, London, pp581-608

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SYMONS, I F (1992) The role of field measurements in the design and construction of geotechmcal structures In: TRL Workshop on the Observational Method for Geotechnical Structures, Transport Research Laboratory, Berkshire, England TERZAGHI, K (1948a) Stabilization of landslides In: From Theory To Practice In Soil Mechanics, John Wiley & Sons, New York, pp409-4 15 TERZAGHI, K (1948b) Settlement and bearing capacity In: From Theory To Practice In SoilMechanics, John Wiley & Sons, New York, pp299-337 TSE, C M and NICHOLSON, D P (1992) Design construction and monitoring of the basement diaphragm wall at Minster Court, London In: Proceedings of the Conference on Retaining Structures, Institution of Civil Engineers, Cambridge, pp323-332 TWINE, D P and ROSCOE, G H (1997) Prop loads: guidance on design CIRIA Funders Report FR/CP/48, CIRIA, London WINTER, D G (1990) Pacific First Centre Performance of the tieback shoring wall In: Proceedings of ASCE Specialty Conference, Cornell, ASCE Geo Special Publication 25, pp764-777 WU, T H and BERMAN, S (1953) Earth pressure measurements in open cut: Contract D8, Chicago subway Giotechnique, 13 (3), pp248-258 YAH, K (1992) Report on the ICE Ground Board Meeting Held on 18 March on ‘The Use of Battered Excavation in Construction’ Ground Engineering, October, pp30-32

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YOUNG, D K and HO, E W L (1994) The observational approach to design of a sheet-piled retaining wall GCotechnique,44 (4), pp637-654

Ground treatment

(a)

Grouting

BRITISH STANDARDS NSTITUTION (1 995) Geotechnical Design, Part I : General Rules (together with United Kingdom National Application Document) Eurocode 7: British Standards Institution, London CIRIA (in preparation) The use of grouting techniquesfor ground improvement CIRIA C5 14, CIRIA, London

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FRANCESCON, M and TWINE, D (1992) Treatment of solution features in upper chalk by compaction grouting In: Proceedings of the Conference on Grouting in the Ground, Institution of Civil Engineers, London, pp327-348 GREENWOOD D A (1992) Report on Session 1 : Permeation grouting In: Proceedings of the Conference on Grouting in the Ground, Institution of Civil Engineers, London, pp7 1-90 MILLET, R A and ENGELHARDT, R (1982) Matrix evaluation of structural grouting of rock In: Proceedings of the Conference on Grouting in Geotechnical Engineering, New Orleans, February 1982, American Society of Civil Engineers, New York, pp753-768 PATEL, M, MURRAY MILLER, S, LI, K L, WAUGH, H and WELSH, J P (1982) Chemical grouting for construction of 8 foot diameter tunnel through “Little Italy” Baltimore, Maryland In: Proceedings of the Conference on Grouting in Geotechnical Engineering, New Orleans, February 1982, American Society of Civil Engineers, New York, pp591-605 SOLERA, S and LEHANE, B (1992) Grout curtain at the Old Billingsgate Market In: Proceedings of the Conference on Grouting in the Ground, Institution of Civil Engineers, London, pp 15-24 (b)

Dewatering

BRITISH STANDARDS INSTITUTION (1995) Geotechnical Design, Part 1: General Rules (together with United Kingdom National Application Document) Eurocode 7: British Standards Institution. London BAUDUIN, C M H L G and MOES, C J B (1989) Excess pore water pressure measurement as a method for a embankment stability control In: Proceedings of 9th European Conference on Soil Mechanics and Foundation Engineering-Groundwater effects in geotechnical engineering, Dublin, 1, pp373-376

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CREMONINI, M G, VAROSIO, G, MIRONE, M and SAVERI, E (1987) Automatic and continuous control of groundwater level effects on foundation behaviour In: Groundwater Effects in Geotechnical Engineering. Proceedings of the Ninth European Conference on Soil Mechanics and Foundation Engineering, Dublin, pp 133-137 PREENE, M and ROBERTS, T 0 L (1 994) The application of pumping tests to the design of construction dewatering system In: Groundwater Problems in Urban Areas, Thomas Telford, London, pp12 1-1 33 ROBERTS, T 0 L and PREENE M (1 994) The design of groundwater systems using the observational method Giotechnique, 44 (4), pp724-734 SOMERVILLE, S H (1986) Control of groundwater for temporary works CIRIA Report 113, CIRIA, London

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STROUD, M A (1987) General Report to Session 2 In: Proceedings of 9th European Conference on Soil Mechanics and Foundation Engineering - groundwater egects in geotechnical engineering, Dublin (3), pp983-1008 STROUD, M A (199 1) Drainage: A means of soil improvement in underground works In: Proceedings of Conference on Soil and Rock Improvement in Underground Works, Milan, March, pp261-290 TROUGHTON, V M (1987) Groundwater control by pressure relief and recharge In: Proceedings of 9th European Conference on Soil Mechanics and Foundation Engineering-Groundwater effects in geotechnical engineering, Dublin, pp2 59-264

(c)

Deep compaction

BRITISH STANDARDS INSTITUTION (1 995) Geotechnical Design, Part I : General Rules (together with United Kingdom National Application Document) Eurocode 7: British Standards Institution, London RAISON, C A (1996) Discussion In: The Observational Method in Geotechnical Engineering, Institution of Civil Engineers, Thomas Telford, London, ppl75-179 Embankments and reclamation works

ALMEIDA, M S S, BRITTO, A M and PARRY, R H G (1986) Numerical modelling of a centrifuged embankment on soft clay Canadian Geotechnical Journal, 23 (2), pp103-114 ALMEIDA, M S S, DAVIES, M C R and PARRY, R H G (1985) Centrifuge tests of embankments on strengthened and unstrengthened clay foundations Giotechnique, 35 (4), pp 425-442

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ASAOKA, A (1978) Observational procedure of settlement prediction Soils and Foundations, Japanese Society of Soil Mechanics and Foundation Engineering, 18 (4), pp 87-101 BRITISH STANDARDS INSTITUTION (1995) Geotechnical Design, Part I : General Rules (together with United Kingdom National Application Document) Eurocode 7: British Standards Institution, London BAUDUIN, C M H L G and MOES, C J B (1989) Excess pore water pressure measurement as a method for a embankment stability control In: Proceedings of 9th European Conference on Soil Mechanics and Foundation Engineering - groundwater effects in geotechnical engineering, Dublin, 1, pp373-376

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CASAGRANDE, A (1960) An unsolved problem of embankment stability on soft ground In: Proceedings of 1st Panamerican Conference on Soil Mechanics and Foundation Engineering, Mexico, 2, pp72 1-746 CASAGRANDE, A (1965) Role of the ‘calculated risk’ in earthwork and foundation engineering Journal of the Soil Mechanics and Foundations Division, American Society of Civil Engineers, 91, SM4, ppl-40 CHOA, V (1994) Application of the Observational Method to Hydraulic Fill Reclamation Projects Gdotechnique, 44 (4), pp735-745 CHOA, V (1990) Soil Improvements works at Tianjin East Pier Project In: Proceedings of 10th South-east Asian Geotechnical Conference, Taipei, 1, pp47-52 DE MELLO, V F B (1977) Reflections on design decisions of practical significance to embankment dams Gdotechnique, 27 (3), pp279-355 MATSUO, M and KAWAMURA, K (1977) Diagram for construction control of embankment on soft clay Soils and Foundations, 17 (3), pp37-52 MARSLAND, A and POWELL, J J M (1977) The behaviour of a trial bank constructed on soft alluvium of the River Thames In: Proceedings of International Symposium on Soft Clay, Bangkok, Thailand, pp505-525 NICHOLSON, D P and LOW, A (1994) Performance of Foyle Bridge east abutment Gdotechnique, 44 (4), pp757-769 O’RIORDAN, N J and SEAMAN, J W (1994) Highway embankments over soft compressible alluvial deposits - Guidelinesfor design and construction TRL Contractor Report 341, Transport Research Laboratory, Berkshire, England

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PENMAN, A D M (1986) On the embankment dam Gdotechnique, 36 (3), pp303-348 RUSSELL, D (1995) Discussion In: The Observational Method in Geotechnical Engineering, Institution of Civil Engineers, Thomas Telford, London, ppl79-185 SEAMAN, J W (1994) A guide to accommodating or avoiding soil-induced lateral loading ofpiledfoundations for highway bridges TRL Project Report 7 1, Transport Research Laboratory, Berkshre, England STEWART, D P, JEWELL, R J and RANDOLPH, M F (1994) Design of piled bridge abutments on soft clay for loading from lateral soil movements Giotechnique, 44 (2), pp 277-296

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TAVENAS, F, MIEUSSENS, C and BOURGES, F (1979) Lateral displacements in clay foundations under embankments. Canadian Geotechnical Journal, 16, pp532-550 TERZAGHI, K (1948) Final report on the performance of the ore yard of the RFC Plancor 257 during the service period 1943 to 1948 In: From Theoly to Practice in Soil Mechanics,John Wiley & Sons, New York, pp299-337 WAKITA, E and MATSUO, M (1994) Observation design method for earth structures constructed on soft ground Giotechnique, 44 (4), pp747-755 Environmental geotechnics

BROWN, S M, LINCOLN, D R and WALLACE, W A (1 990) Application of Observational Method to hazardous waste engineering Journal of Management in Engineering, American Society of Civil Engineers, 6 (4), ~~479-500 CANADIAN COUNCIL OF MINISTERS OF THE ENVIRONMENT (CCME) (1993) Guidance manual on sampling, analysis and data managementfor contaminated sites (two volumes) Report CCME EPC-NCS 62E, Canadian Council of Ministers of the Environment, Winnipeg CIRIA (1 995) Remedial treatmentfor contaminated land CIRIA Special Publications SPlOl to SP112, CIRIA, London D’APPOLONIA, E (1990) Monitored decisions Journal of GeotechnicalEngineering,American Society of Civil Engineers, 116 (l), pp4-34

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HOLM, L A (1993) Strategies for remediation In: Geotechnical Practice for Waste Disposal (D E Daniel ed), Chapman and Hall, London, pp2049-2065 MORGENSTERN, N R (1994) The Observational Method in environmental geotechnics In: Proceedings of 1st International Conference on Environmental Geotechnics, Alberta, Canada, pp965-976 O’RIORDAN, N J (1995) Environmental geotechnics: risk management and remediation In: Proceedings of 9th Young Geotechnical Engineer’s Conference, September 1995, Gent O’RIORDAN, N J (1995) Discussion on Debate: Contaminated sites are being over-engineered Proceedings of Institution of Civil Engineers Civil Engineering, 108, November, pp18&185

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O’RIORDAN, N J and MILLOY, C J (1995) Risk assessmentfor methane and other gases from the ground CIRIA Report 152, CIRIA, London VAN DEN BERG, R, DENNEMANN, C A J and ROELS, J M (1993) RisKassessment for contaminated soil: Proposals for adjusted, toxicologically based Dutch soil clean-up criteria. In: Proceedings of Contaminated Soil ’93, pp349-364 Kluwer Dordrecht, Netherlands WOOD, A A and GRIFFITHS, C M (1994) Debate: contaminated sites are being over-engineered Proceedings of Institution of Civil Engineers Civil Engineering, 104, August, pp97-105 Structures

CHARLES, J A (1993) Embankment dams and their foundations: safety evaluation for static loading In: International workshop on dam safety evaluation, Grindelwald, April 1993,4, Dam Engineering, Sutton, Surrey, pp47-75 CLAYTON, C R I, EDWARDS, A and WEBB, M J (1991) Displacements in London clay during construction In: Proceedings of 10th ECSMFE, 2, Florence, May 1991, pp791-796 COTTAM, M P and MAGUIRE, J R (1995) Hazard and risk assessment in other industries The Structural Engineers, 73 ( 2 l), pp364-365 FORBES, J, BASSETT, R H and LATHAM, M S (1994) Monitoring and interpretation of the Mansion House due to tunnelling Proceedings of Institution of Civil Engineers Geotechnical Engineering, 107, April, ~~89-98

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FRANCESCON, M and TWINE, D (1992) Treatment of solution features in upper chalk by compaction grouting In: Proceedings of the Conference on Grouting in the Ground, Institution of Civil Engineers, London, pp327-348 FRISCHMANN, W W, HELLINGS, J E, GITTOES, G and SNOWDEN, C (1994) Protection of the Mansion House against damage caused by ground movements due to the Docklands Light Railway extension Proceedings of the Institution of Civil Engineers Geotechnical Engineering, 1994, 107, April, pp65-76 GRAFTIO, H (1 936) Some features in connection with the foundation of Svir 3 hydro-electric power development In: Proceedings of 1st International Conference on Soil Mechanics, Cambridge, Massachusetts, 1, pp284-290

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HARRIS, D I, MAIR, R J, LOVE, J P, TAYLOR, R N and HENDERSON, T 0 (1994) Observations of ground and structure movements for compensation grouting during tunnel construction at Waterloo station Gkotechnique, 44 (4), pp691-7 13 INSTITUTION OF STRUCTURAL ENGINEERS (1 990) The achievement of structural adequacy in buildings The Institution of Structural Engineers, London MENZIES, J B (1995) Hazards, risks and structural safety The Structiiral Engineer, 73 (21), pp357-363 NICHOLSON, D P and LOW, A (1994) Performance of Foyle Bridge east abutment Gkotechnique, 44 (4), pp757-769 PAINE, N, ESCOBAR E, D, HALLOWES, G R, SODHA, V G and ANAGNOSTI, P (1 982) Surveillance and re-evaluation of Poechos Dam, right wing embankment, Peru In: Proceedings of Quatorzidme Congrds des Grands Barrages, Commission Internationale des Grands Barrages, Rio de Janeiro, pp333-343 PATEL, M, MURRAY MILLER, S, LI, K L, WAUGH, H and WELSH, J P (1982) Chemical grouting for construction of 8 foot diameter tunnel through “Little Italy” Baltimore, Maryland In: Proceedings of the Conference on Grouting in Geotechnical Engineering, New Orleans, February 1982, American Society of Civil Engineers, New York, pp591-605 PENMAN, A D M (1986) On the embankment dam Gkotechnique, 36 (3), pp303-348 POWDERHAM, A J (1994) An overview of the observational mei..od: development in cut and cover bored tunnelling projects Gkotechnique, 44 (4), pp619-636 ClRlA Report 185

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POWDERHAM, A J and TAMARO, G J (1995) Mansion House London: Risk assessment and protection Journal of Construction Engineering and Management, American Society of Civil Engineers, 121 (3), pp266-272 TERZAGHI, K and LACROIX, Y (1964) Mission Dam, an earth and rockfill dam on a highly compressible foundation Gkotechnique, 14, (4), pp14-50 TERZAGHI, K and LEPS, T M (1960) Design and Performance of Vermilion Dam Transactions ofAmerican Society of Civil Engineers, 125, Paper 3014, pp63-100 TSE, C M and NICHOLSON, D P (1992) Design construction and monitoring of the basement diaphragm wall at Minster Court, London In: Proceedings of the Conference on Retaining Structures, Institution of Civil Engineers, Cambridge, pp3233-32

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TWINE, D and ROSCOE, G H (1997) Prop loads: guidance on design CIRIA FR/CP/48, CIRIA, London

UL HAQ, I (1996) Tarbela Dam: Resolution of seepage Proceedings of Institution of Civil Engineers Geotechnical Engineering, 119, January, ~~49-56 ZEEVAERT, L (1957) Foundation design and behaviour of Tower Latino Americana in Mexico City Gkotechnique, 7 (3), ppll5-133

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Core Programme members

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September 1999 Alfred McAlpine Construction Ltd

IMC Consulting Engineers Ltd

AMEC Plc

Institution of Civil Engineers

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Transport and the Regions

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The Construction Directorate of the DETR supports the programme of innovation and research to improve the construction industry's performance and to promote more sustainable construction. Its main aims are to improve quality and value for money from construction, for both commercial and domestic customers, and to improve construction methods and procedures.

The Observational Method in ground engineering is a continuous, managed, integrated process of design, construction control, monitoring and review.The aim is to enable modifications to be incorporated during or after construction as appropriate. All these aspects have t o be demonstrably robust.The objective is to achieve greater overall economy without compromising safety. This book provides important guidance on the use and management of the Observational Method, covering historical background, concepts, technical and management considerations, and contractual implications.The Method's practical application is considered in more detail for tunnelling, excavation work, ground treatment, embankment and reclamation works, environmental geotechnics and structures.These sections of the book are illustrated with case studies drawn from UK experience but equally applicable worldwide. The Observational Method is likely t o be of increasing significance as a safe and efficient way of dealing with uncertainties in the ground and monitoring the performance of civil engineering works.This book provides valuable and timely guidance on the choice and application of the Method to managers, clients and supervising engineers involved in civil engineering and geotechnical works.