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Cover photograph: Propping at Canada Water station, Jubilee Line Extension, London (courtesy Ove Arup & Partners). Printed and bound in Great Britain by The Basingstoke Press (75) Ltd, Basingstoke, Hampshire.
CIRIA C517
London, 1999
Temporary propping of deep excavations guidance on design
David Twine
BSe MSe DIC CEng MICE
Howard Roscoe
BSe MSe DIC CEng MICE
............. sharing knowledge. building best practice
6 Storey's Gate, Westminster, London SW1 P 3AU TELEPHONE 0171 2228891 FAX 0171 2221708 EMAIL
[email protected] WEBSITE www.ciria.org.uk
Summary
This report provides guidance on the design of temporary propping systems for deep excavations which will lead to greater efficiency in their use. The guidance covers single and multi-propped excavations of varied geometry, flexible and stiff walls and the range of ground conditions commonly encountered in the UK. An extensive, international survey of field measurements of prop loads is presented and interpreted, based on the Peck approach, to propose a new empirical method for establishing temporary prop loads for design: the distributed prop load method. The application of this method is explained, together with practical aspects oftemporary propping. Worked examples illustrate the new approach. Guidance on temperature effects, the observational method and the instrumentation of props is given. The report also describes the factors affecting the choice of temporary propping systems and examines the contractual context for their design and specification. A summary of main recommendations from the study is drawn together in the conclusion to the report.
Temporary propping of deep excavations - guidance on design Twine, D and Roscoe, H Construction Industry Research and Information Association CIRIA Publication C517
© CIRIA 1999
ISBN 0860175170
Keywords Temporary props, design, Peck's method, case histories, structural considerations, buildability, observational method, instrumentation
Reader interest
Classification
Temporary works designers, geotechnical engineers, project managers, party wall surveyors, clients
AVAILABILITY
Unrestricted
CONTENT
Guidance based on review and current practice
STATUS
Committee-guided
USER
Civil and structural engineers
Published by CIRIA, 6 Storey's Gate, Westminster, London SWIP 3AU. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any other means, including photocopying and 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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CIRAC517
Acknowledgements
This report presents the results of a research project in CIRIA's ground engineering programme on temporary propping. The research project was carried out under contract to CIRIA by Ove Arup and Partners in collaboration with Cementation Piling & Foundations Ltd. The report was written by Mr D Twine ofOve Arup and Partners and Mr H Roscoe of Cementation Piling & Foundations Ltd. The authors would like to give a special acknowledgement to Mr R Caldwell for his work in researching, collating, interpreting and summarising the case histories presented in this report. Following CIRIA's practice, the research project was guided by a steering group, which comprised: Mr B T McGinnity (chairman) Mr D R Beadman MrRBoorman DrD RCarder Mr J D Findlay MrD P Jordan Mr W M Kilkenny Professor W Powrie MrTRoberts Mr M C Stevenson
London Underground Limited Bachy Limited Consultant Transport Research Laboratory Stent Foundations Limited Scott Wilson Kirkpatrick & Co W S Atkins Consultants Ltd University of Southampton Tarmac Construction Limited Sir Alexander Gibb & Partners Ltd
CIRIA's research managers for the project were Mr F M Jardine and Dr M R Sansom. This project was funded by CIRIA's core programme sponsors and by the Construction Sponsorship Directorate of the Department of Environment. CIRIA and the authors gratefully acknowledge the support of these funding organisations, the technical advice and help given by the members of the steering group and the following people: DrM Batten Mr ARBiddle DrDI Bush Mr E C Chaplin MrIFeltham MrRFemie MrN Finegan Dr WGK Fleming Mr SP Marchand Mr D P Nicholson Professor R B Peck MrMJPuller Mrs A Ramsay DrHD Stjohn
CIRIAC517
University of Southampton Steel Construction Institute Highways Agency Consultant Ove Arup & Partners Cementation Piling & Foundations Ltd Highways Agency Cementation Piling & Foundations Ltd Costain Construction Limited Ove Arup & Partners Consultant Consultant Ove Arup & Partners Geotechnical Consulting Group
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Contents
SUMMARy ........................................................................................................................... 2 ACKNOWLEDGEMENTS .................................................................................................. 3 FIGURES ............................................................................................................................... 8 TABLES ............................................................................................................................... 11 NOTATION ......................................................................................................................... 12 ABBREVIATIONS ............................................................................................................. 14
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INTRODUCTION ....................................................................................................... 15 1.1 Need for the report ............................................................................................. 15 1.2 Scope of report................................................................................................... 16 1.3 Structure of report .............................................................................................. 17
2
TEMPORARY PROPPING SySTEMS .................................................................... 19 2.1 Function of temporary propping systems ........................................................... 19 2.2 Props and walls .................................................................................................. 19 2.3 Ground movements and damage ........................................................................ 21 2.4 Key points .......................................................................................................... 23
3
SELECTION OF TEMPORARY PROPPING SYSTEMS ..................................... 27
4
CONTRACTUAL CONTEXT.................................................................................... 31 4.1 Temporary conditions ....................................................................................... 31 4.2 Movement criteria .............................................................................................. 32 4.3 Client requirements ............................................................................................ 33 4.4 Permanent and temporary design ....................................................................... 33 4.5 Sub-contracts ..................................................................................................... 34 4.6 Key points .......................................................................................................... 34
5
CURRENT DESIGN PRACTICE .............................................................................. 35 5.1 External pressures and temperature effects ......................................................... 35 5.2 Soil properties .................................................................................................... .35 5.3 Earth and water pressures ................................................................................... 36 5.4 Duration of loading ............................................................................................. 37 5.5 Current practice .................................................................................................. .37 5.6 Temperature effects............................................................................................ .39 5.7 Key points .......................................................................................................... 40
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CASE mSTORlES ...................................................................................................... 41 6.1 Classification ..................................................................................................... 41 6.2 Depth and width of excavation .......................................................................... .42 6.3 Ground conditions .............................................................................................. .48 6.4 Wall types .......................................................................................................... .48 Prop types .......................................................................................................... .49 6.5 6.6 Construction sequence ........................................................................................ 49 6.7 Maximum measured prop load ........................................................................... 49 6.8 Surcharges ........................................................................................................... 50 6.9 Measurement of prop load .................................................................................. 50 6.10 Duration of propping........................................................................................... 51 6.11 Variations of prop loads within an excavation.................................................... 54 6.12 Temperature measurements ................................................................................ 57 6.13 Preloading ........................................................................................................... 61 6.14 Key points .......................................................................................................... 62
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ANALYSIS USING THE DISTRIBUTED PROP LOAD METHOD .••.••..•.......••••• 63 7.1 Terminology........................................................................................................ 64 7.2 Review of Peck's recommendations ................................................................... 64 7.3 Duration of load .................................................................................................. 65 7.4 Temperature effects ............................................................................................ 67 7.5 Base stability ....................................................................................................... 68 7.6 Calculation and interpretation of distributed prop loads .................................... 71 7.7 Key points .......................................................................................................... 86
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CHARACTERISTIC DISTRIBUTED PROP LOAD DIAGRAMS .•...............••.•.. 91 8.1 Soft and firm clays (Class A soils) ..................................................................... 91 8.2 Stiff clays (Class B soils) ................................................................................... 95 8.3 Granular soils (Class C soils) ............................................................................. 98 8.4 Mixed soils (Class D soils) ................................................................................ 98 8.5 Key points ........................................................................................................ 101
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PROPPING SYSTEM DESIGN ............................................................................... 103 9.1 Shortcomings of current practice ..................................................................... 103 9.2 Design standards .............................................................................................. 103 9.3 Eurocode 7: geotechnical design...................................................................... 104 9.4 Recommended characteristic distributed prop load diagrams .......................... 105 9.5 Conditions of use for the distributed prop load method ................................... 106 9.6 Method of design ............................................................................................. 109 9.7 Key points........................................................................................................ 110
10 STRUCTURAL CONSIDERATIONS .................................................................... 111 10.1 Structural requirements .................................................................................... 111 10.2 Causes and modes offailure ............................................................................ 111 10.3 Design cases ..................................................................................................... 114 10.4 Limit state design standards ............................................................................. 114 10.5 Design loads ..................................................................................................... 114 10.6 Design of props ................................................................................................ 115 10.7 Walings ............................................................................................................ 127 10.8 Progressive collapse ......................................................................................... 128 10.9 Prop removal. ................................................................................................... 129 10.10 Key points ........................................................................................................ 130 11 BUILDABILITY ........................................................................................................ 131 11.1 General requirements....................................................................................... 131 11.2 Fabrication, delivery and on-site storage ......................................................... 131 11.3 Handling and installation of props ................................................................... 132 11.4 Constraints on method of working ................................................................... 132 11.5 Prop removal ................................................................................................... 133
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12 THE OBSERVATIONAL METHOD ...................................................................... 135 12.1 Introduction ...................................................................................................... 135 12.2 Design standards .............................................................................................. 135 12.3 Harris Trust Building ....................................................................................... 135 12.4 Additional applications ofthe observational method ....................................... 137 13 PROP INSTRUMENTATION.•...•.......................•.............••..•..•............................... 139 13.1 GeneraL ............................................................................................................. 139 13.2 Planning the load monitoring programme ......................................................... 139 13.3 Methods of prop load measurement.. ................................................................ 140 13.4 Common pitfalls ............................................................................................... 140 14 WORKED EXAMPLES ............................................................................................ 145 14.1 Design procedures ............................................................................................ 145 14.2 Example 1 ........................................................................................................ 147 14.3 Example 2 ........................................................................................................ 152 14.4 Example 3 ........................................................................................................ 154 14.5 Example 4 ........................................................................................................ 156 15 CONCLUSIONS ........................................................................................................ 159 REFERENCES .................................................................................................................. 163 APPENDICES ................................................................................................................... 169 Al SUMMARY OF CASE HISTORIES ....................................................................... 169 ALl Reference for case histories ............................................................................. 179 A2 DATA ON VARIATIONS OF PROP LOADS WITHIN AN EXCAVATION .... 193 A3 COMPARISON OF MEASURED LOADS AND PECK'S ENVELOPES .......... 199 A3.1 GeneraL ............................................................................................................ 199 A3.2 Normalised apparent pressure diagrams .......................................................... 199 A3.3 Comparison of distributed prop loads and Peck's apparent pressure envelopes ........................................................................................... 200 A3.4 Conclusions ...................................................................................................... 208 A4 BASE STABILITY OF AN EXCAVATION ........................................................... 209 A4.1 Base stability .................................................................................................... 209 A5 DISTRIBUTED PROP LOAD DATA ..................................................................... 213 A6 COMPARISON OF ULTIMATE LIMIT STATE PROP LOADS DERIVED FROM CASE B AND CASE C OF EUROCODE 7 ............................................... 217 A7 EFFECTS OF TEMPERATURE CHANGES ON PROP DESIGN ..................... 221 A7.1 General. ............................................................................................................ 221 A7.2 Scope ............................................................................................................... 221 A7.3 Prop failure mechanisms .................................................................................. 221 A7.4 Preparation of axial force/strain charts ............................................................ 222 A7.5 Use of axial force/strain charts for checking limit states ................................. 226
CIRIAC517
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Figures
1.1 2.1 2.2 2.3 2.4 2.5 2.6 2.7 3.1 3.2 3.3 5.1 6.1 6.2 6.3 6.4 6.5
6.6 6.7 6.8 6.9 6.10 6.11 6.12 6.13 7.1 7.2 7.3 7.4a 7.4b 7.4c 7.4d 7.4e
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Typical excavations with temporary propping ........................................................ 16 Limit eqUilibrium method for a propped wall ......................................................... 20 Pressure envelope method for mUlti-propped walls ................................................ 20 Surface settlements caused by retaining wall installation in stiff clays .................. 22 Surface settlements caused by retaining wall installation in stiff clays .................. 22 Summary of measured settlements resulting from excavation and propping ......... 24 Ground movements caused by loading and unloading of an excavation ................ 25 Effect of deflection of wall toe on ground movements ........................................... 26 Ground support methods not requiring temporary props ........................................ 28 Comparative wall and ground movements of cantilever and propped walls ........... 29 Flowchart for deciding the most appropriate support system for a temporary excavation .............................................................................................. 30 Relationship between lateral strain and lateral earth pressure coefficient .............. 36 Depth and width of excavations in soft and firm clays (Class A) ........................... 43 Depth and width of excavations in stiff and very stiff clays (Class B) ................... 44 Depth and width of excavations in granular soils (Class C) ................................... 45 Depth and width of excavations in mixed soils (Class D) ....................... :.............. 46 Number of propping levels against depth of excavation in each soil class ............. 47 Individual prop loads and excavation progress for three contracts on the Chicago subway (Case histories AF15, 19 and 20) ............................................... 52 Apparent earth pressure for individual props and excavation progress for H Building, Osaka (Case history AF28) ................................................................. 53 Increase in prop loads after end of excavation for Harris Trust Building (Case history AF22) ................................................................................................ 53 Variations of individual prop load within an excavation (Case histories AF12, 14, 16,22 and 26) ....................................................................................... 55 Variation of individual prop load within an excavation, for case histories additional to those given by Flaate and Peck (1973) .............................................. 56 Variations oftotal prop load within an excavation ................................................. 57 Measurements of prop temperature for Prop 10 at Canada Water station .............. 58 Loss of preload due to a reduction in prop temperature (Case history Cl) ............. 61 Method for calculating the distributed prop load .................................................... 63 Degree of restraint for steel props during temperature changes .............................. 69 Base failure of an excavation .................................................................................. 70 Flexible walls in Class A soils - normalised depth versus normalised V distributed prop load .............................................................................................. 72 Flexible walls in Class A soils - normalised depth versus normalised distributed prop load ............................................................................................... 73 Flexible walls in Class A soils - normalised depth versus normalised distributed prop load ............................................................................................... 74 Flexible walls in Class A soils - normalised depth versus normalised distributed prop load ............................................................................................... 75 Stiff walls in Class A soils - normalised depth versus normalised distributed prop load ............................................................................................... 76
ClRAC517
7.5a 7.5b 7.5c 7.5d 7.6a 7.6b 7.7a 7.7b 7.7c 7.8 7.9 7.10 7.11 7.12 8.1 8.2 8.3 8.4 8.5 8.6 9.1 10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8 10.9 10.10 10.11 10.12 12.1
CIRIAC517
Flexible walls in Class B soils - normalised depth versus normalised distributed prop load ............................................................................................... 77 Flexible walls in Class B soils - normalised depth versus normalised distributed prop load ............................................................................................... 78 Stiff walls in Class B soils - normalised depth versus normalised distributed prop load ............................................................................................... 79 Stiff walls in Class B soils - normalised depth versus normalised distributed prop load ............................................................................................... 80 Flexible walls in Class C soils - normalised depth versus normalised distributed prop load .............................................................................................. 81 Flexible walls in Class C soils - normalised depth versus normalised distributed prop load .............................................................................................. 82 Walls in Class D soils - normalised depth versus normalised distributed prop load ................................................................................................................. 83 Walls in Class D soils - normalised depth versus normalised distributed prop load ................................................................................................................ 84 Walls in Class D soils - normalised depth versus normalised distributed prop load ................................................................................................................ 85 Normalised DPL diagrams for flexible and stiff walls in Class A soils .................. 87 Normalised DPL diagrams for flexible and stiff walls in Class B soils .................. 88 Normalised DPL diagrams for mainly flexible walls in Class C (granular) soils ... 89 Normalised DPL diagrams for flexible and stiff walls in Class D (mixed) soils .... 89 Characteristic distributed prop load diagrams for Class A, Class B and Class C soils ............................................................................................................ 90 Characteristic DPL diagram for flexible walls in firm clays (Class AF) ................ 92 Characteristic DPL diagram for excavations in soft clays (Class AF) with stable bases and flexible walls ................................................................................ 93 Characteristic DPL diagram for excavations in soft clays (Class AF) with base stability enhanced by the flexible wall ............................................................ 94 Normalised DPL diagram corrected for temperature effects for stiffwalls in Class B soils ....................................................................................................... 96 Characteristic DPL diagram for dry and dewatered excavations in Class C (granular) soils ........................................................................................................ 99 Characteristic DPL diagram for excavations below groundwater level without external dewatering (submerged) in Class C (granular) soils .................... 99 Procedure for determining depth of toe embedment in relatively uniform competent soil conditions ..................................................................................... 108 Types and layouts of propped excavations ........................................................... 112 Typical end detail for horizontal prop with brace web vertical ........................... 118 Typical end detail for a horizontal tubular (CHS) prop and a diaphragm wall ..... 119 Typical end detail for a horizontal tubular (CHS) prop ........................................ 120 Typical end detail for a raking prop ...................................................................... 121 Typicaljackable end detail for a tubular (CHS) prop ........................................... 122 Typical jackable end detail for a UC prop ............................................................ 123 Typical jackable end detail for a comer prop ....................................................... 124 Axial force versus strain plots for 10 m, 15 m and 25 m long steel CHS props ..................................................................................................................... 125 Force versus strain relation for UC section props ................................................. 125 Force versus strain relation for CHS section props ............................................... 126 In-plane shear connection between waling and wall (Case history BF7) ............. 129 Harris Trust Building ............................................................................................ 136
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13.1 14.l 14.2 14.3 14.4 14.5
Steps for planning prop load monitoring .............................................................. 142 Flowchart of design procedure ............................................................................. 146 Data for Example 1 ............................................................................................... 148 Solution to Example 1 .......................................................................................... 149 ULS check for temperature effects for Example 1.. .............................................. 150 ULS check for temperature effects for Example 1 with a 50°C temperature
14.6 14.7 14.8
rise and 100 per cent restraint............................................................................... Data for Example 2 ............................................................................................... Solution to Example 2 .......................................................................................... Data for Example 3 ............................................................................................... Solution to Example 3 ..........................................................................................
14.9 14.10 14.11
ALl
A3.1 A3.2
A3.3 A3.4 A3.5 A3.6
A3.7 A3.8 A4.1 A5.1 A5.2
A5.3 A5.4 A6.1
A7.1 A7.2 A7.3 A7.4
A7.5
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151 152 153 154 156
Data for Example 4 ............................................................................................... 157 Solution to Example 4 .......................................................................................... 158 Typical departures from the general construction sequence................................. 192 Peck envelopes plotted on flexible wall data for Class A soils ............................. 201 Peck envelopes plotted on stiffwall data for Class A soils .................................. 202 Peck envelopes plotted on flexible wall data for Class B soils ............................. 203 Peck envelopes plotted on stiff wall data for Class B soils ................................... 204 Peck envelopes plotted on data for Class C soils .................................................. 205 Peck envelopes plotted on dry and dewatered case histories for Class C soils ..... 206 Peck envelopes plotted on case histories excavated below the water table for Class C soils .......................................................................................................... 207 Flexible and stiffwalls in Class D soils ................................................................ 207 Methods of extending the critical depth of excavation, Dc ................................... 212 Distributed prop loads against depth for Class A soils ......................................... 213 Distributed prop loads against depth for flexible walls in Class B soils ............... 214 Distributed prop loads against depth for stiff walls in Class B soils ..................... 214 Distributed prop loads against depth for Class C soils ......................................... 215 Results of Case B and Case C analyses ................................................................ 219 Force versus strain relation for CHS section props ............................................... 223 Force versus strain relation for DC section props ................................................. 224 Determination of stable configurations ................................................................. 225 Construction for the slope of degree of restraint lines .......................................... 228 ULS check for effects of temperature ................................................................... 228
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Tables
2.1 6.1 6.2 8.1
Base stability condition of flexible wall case histories in Class A soils (soft to ftrm clays) .................................................................................................. 91
8.2 8.3
Summary of the stiff wall case histories in Class B (stiff clay) soils ...................... 96 Maximum value ofDPLlj"Hfor stiff walls in Class B (stiff clay) soils ................. 97
9.1
Eurocode 7 partial factors for ultimate limit states in persistent and transient situations ................................................................................................ 104 Characteristic distributed prop load diagrams for Class A, Band C soils............ 106
9.2 14.1 ALl A1.2 Al.3 A2.1 A3.1 A6.1
CIRIA C517
Comparison of measured and predicted prop loads for some recent projects ......... 21 Ground conditions for the case histories ................................................................. 48 Measurements of temperature effects on prop loads (steel props) .......................... 59
Calculated SLS and ULS prop loads for Example 4 ............................................. 158 Summary of case histories .................................................................................... 171 Calculation of distributed prop load values for each case history......................... 184 Case histories which depart from the general construction sequence................... 190 Summary of measured prop loads within an excavation ...................................... 194 Summary of Class C case histories excavated below the water table ................... 205 Summary of results of Case B and Case C analyses ............................................. 218
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Notation
12
ao
initial imperfections in prop shape along its length
A
cross-sectional area of prop
b
depth of universal column section
B
width of excavation
Cu
undrained shear strength
d
diameter of steel circular hollow section (CHS) or depth of steel UC section
D
depth of the excavation
Dc
depth of excavation when base failure occurs (critical depth)
De
depth of embedment of wall below excavation
e
effective axial strain
eo
initial shortening of the prop due to selfweight and initial curvature
E
modulus of elasticity of steel
Fbase
factor of safety against base heave
h
length of hinge
H
final depth of the excavation
I
section modulus
Ka Ko
coefficient of active earth pressure coefficient of earth pressure at rest
I
horizontal spacing ofthe props
L
length of a prop or waling
LE
effective length of a prop
Lw
wall length beneath lowest, or next to lowest, level of propping depending on depth to firm stratum
m
reduction factor applied to Su for calculation of Peck's apparent earth pressure
M
moment
Msine
midspan moment at onset of plastic deformation
Mull
ultimate moment capacity
My
yield moment per metre of wall
N
base stability number, ,,(Hleu
Np
axial force on a prop
Nc
bearing capacity factor
NOR
stability factor of overall resistance to base failure
P Pa PA
prop load
Pc PE
ultimate compressive capacity of a prop
total force over the retained height from Rankine active earth pressure total active force over the embedded length ofthe wall below excavation level
compressive capacity of the prop at the elastic limit
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CIRIA C517
Pp
total passive force over the embedded length of the wall below excavation level
PI
total force over the retained height given by Terzaghi and Peck's pressure envelope method
Py
design strength of steel
R
B/
RE
total force from the distributed prop load acting over half the distance from the lowest prop level to the excavation level (O.5S)
SU
undrained shear strength
S
distance from the lowest prop level to excavation level
t
wall thickness of steel circular hollow section
T
thickness of soft clay beneath the base of the excavation
V
shear at bottom of wall
w
waling load per unit length
W
self-weight of the prop
a
coefficient of thermal expansion
1
bulk unit weight of soil
1'
effective unit weight of soil
1
average unit weight of the soil over the retained height of the excavation, H
'Yf
overall load factor
A
lateral displacement of a prop
Asine
lateral displacement at onset of plastic deformation
AH
maximum horizontal displacement of the retaining wall
AKa
component of earth pressure additional to the Rankine active earth pressure coefficient, resulting from bearing capacity failure beneath the excavation
APlemp
change in prop load due to temperature change
AV
maximum settlement of the retained ground surface
At
change in prop temperature from the installation temperature
'
effective angle of soil friction
Ea
effective axial strain
1(
prop curvature
Ksine
curvature at onset of plastic deformation
.fi or thickness of soft clay beneath the base, T, whichever is the smaller
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Abbreviations
14
AISC
American Institute of Steel Construction
CHS
steel circular hollow section
DPL
distributed prop load
DP~
characteristic distributed prop load
SLS
serviceability limit state
UB
steel universal beam section
UC
steel universal column section
ULS
ultimate limit state
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Introduction
Many retaining walls require temporary propping to ensure stability during construction and to assist in controlling ground movements. The failure of a prop could have serious consequences. The purpose of the research project was to encourage greater efficiency in prop design whilst recognising that props must not fail. The report contains a comprehensive survey of field measurements of prop loads and gives an empirical method of design, the distributed prop load method, which is based on them. The method is a direct development ofthe widely used Peck's method, which has been revised in line with current UK design practice and extended to take account of the further information now available. The distributed prop load method is quick and easy to use, and gives conservative values for the prop loads to be expected for propped temporary excavations in the range of soil conditions found in the United Kingdom. In straightforward situations these loads will be sufficient for the design of the temporary propping system. For more complex retaining structures the method will provide first estimates of the loads. Designers of large or complicated retaining structures often use numerical methods of analysis. They may calibrate the prop loads obtained from these calculations against field experience by making check calculations using the distributed prop load method or by referring to the case histories contained in the report. The report highlights the importance of decisions made at the inception of a project for the design and construction work which will follow. The early sections ofthe report set the design of props within their commercial, contractual and legal context and clarify the extent to which ground movement can be reduced by propping.
1.1
NEED FOR THE REPORT The number of deep excavations is increasing due to the rising demand for buildings with extensive basements and the construction of modem urban infrastructure (highway underpasses and rapid transit systems). Many of these developments are located in urban areas in close proximity to existing buildings. In many cases, negotiations with adjoining land owners and occupiers are protracted. In order to inform and assist these discussions there is a need for a clear and authoritative statement on the practical limit to which ground movements can be restricted. There are a number of different calculation methods in use which lead to significantly different prop loads. Safety factors are applied at different stages of the calculation. This diversity of approaches often results in lengthy negotiation and technical debate; an industry standard method would help to streamline this part of the design. Computer based numerical analyses often predict higher prop loads than those derived from long established methods used in the United Kingdom and they are frequently more conservative than methods currently in use in other countries. Measured prop loads are frequently less than their predicted values.
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Both traditional and analytical methods are best evaluated by comparison with field measurements of prop loads. A comprehensive survey ofthe available records provides a frame of reference for further developments.
1.2
SCOPE OF REPORT The report deals with both single- and multi-propped excavations of depth greater than 6 m and excavations of varied geometry, see Figure 1.1. These include long narrow propped troughs (eg highway underpasses) and basements. The report covers temporary propping for flexible (eg sheet pile) and relatively stiff (eg diaphragmlbored pile) walls embedded in soil. It does not deal with the design of anchored walls, shafts and trenches or the use ofperrnanent works for support in the temporary condition (top-down construction). Other publications, such as CIRIA Report 97 Trenching Practice, describe the design of trench supports.
a) Trough excavation
b) Basement excavation
Figure 1.1
Typical excavations with temporary propping
The range of ground conditions is that normally encountered in the UK, ie soft clay deposits, stiff overconsolidated clays and cohesionless soils. The report addresses the effect of variations in temperature on prop loads but does not include the design of the support system for seismic effects. It sets out the overall scheme requirements of a propped excavation, including structural considerations and buildability. There is detailed description on the aspects of retaining wall design which directly affect the temporary propping system, but no overall review.
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1.3
STRUCTURE OF REPORT The report first examines the context for the design of temporary propping systems. Details and interpretation of case histories follow, leading to the distributed prop load method. The next sections deal with its application to design and important practical aspects. Worked examples illustrate the method and the report concludes with a summary of the main recommendations. Section 2 sets out the requirements for a temporary propping system and briefly describes current methods of determining the prop load. It gives the main causes and typical magnitudes of ground movements caused by deep excavations. Section 3 examines the factors which affect the selection of temporary propping and Section 4 shows how contractual arrangements, performance requirements and the allocation of responsibility impact on the design process. The first part of the report concludes with a review of current design methods (Section 5). Section 6 contains a summary of 81 case histories of prop load measurements. The analyses of the measured loads, described in Section 7, leads to distributed prop load diagrams (Section 8). Prop loads may be obtained from these diagrams. Sections 9 and 10 deal with their application to the design of the propping system and with important structural considerations, including end details and temperature effects. The report contains sections on buildability, the observational method and instrumentation. It gives example calculations using the distributed prop load method (Section 14). There is a summary of the key findings at the end of each main section, which are then drawn together in Section 15.
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Temporary propping systems
2.1
FUNCTION OF TEMPORARY PROPPING SYSTEMS It is the function of the props and walings comprising a temporary support system to ensure lateral stability of the excavation and to assist in controlling wall and ground deformations. A well designed system will achieve these requirements and maintain adequate working space within the excavation.
Props are the most vulnerable elements in the retaining system and are usually structurally over-designed. Failures are extremely rare and are generally caused by poor detailing, misjudgement of ground conditions or accidents. The failure of a prop could have serious consequences and might lead to progressive collapse ofthe excavation. Buckling failures tend to be sudden, which carries further risk. The cost of the propping system is usually small in comparison with the cost of the retaining wall. While efficient design of the propping system is to be encouraged, this is not an area in which a major reduction in overall construction costs should be expected.
2.2
PROPS AND WALLS The design of a retaining wall and its temporary support system is a complex problem of soil/structure interaction. Variables include the sequence of excavation and support, wall stiffness, and stiffness of the support system. In the past, simplified design methods have been used to determine the support forces. F or the design of a singly propped wall, limit equilibrium methods are usually adequate to determine the support force reliably, as shown in Figure 2.1. This method cannot easily be applied to a multi-propped wall and empirical methods of calculation have been developed. One such method is the pressure envelope method, in which envelopes of apparent earth pressure are established from load measurements in props. The most frequently used envelopes are those ofTerzaghi and Peck (1967), subsequently modified by Peck (1 969a) as shown in Figure 2.2. With the advent of powerful desktop computers, more complex methods of analysis have been developed and are widely available to practising engineers. These methods are collectively known as "deformation methods". They may be sub-divided into the following groups:
•
beam on springs
•
beam on elastic continuum
• • •
finite difference methods boundary element methods finite element methods.
In these methods the internal forces within the retaining walls and support forces for the temporary support system are calculated. The computed support forces are often greater than those obtained from the more traditional methods described above.
CIRIAC517
19
P-
De
(a)
(b)
(c)
(d)
(e)
Cross-section
Pressure diagram
Shear
Bending moment
Deflection
Figure 2.1
Limit equilibrium method for a propped wall
PRESSURE DISTRIBUTION
TOTAL FORCE
(a) Sands K.,= tan2 (45 - ill' /2)
P, = trapezoid = 0.65 K. yH 2
= (1 - sin 4»/(1 + sin 4» Equivalent Add groundwater pressures where Rankine groundwater is above the base active of the excavation
0.25H
m = 1.0 except where cut is underlain
Equivalent Rankine active
= Rankine = 0.50 K.yH 2
P,lPa = 1.30
(b) Soft to medium clays' (N > 5-6)
K. = 1 - m(4c.jyH) = 1 - (4/N) 0.75 H
p.
by deep soft normally consolidated clay, when m =0.4
m=1.0 P, = 0.875yH2(1 - (4/N)) P a = 0.50yH2(1 - (4/N)) P/P. = 1.75
1.0K.yH
0.25H
(c) Stiff clays' For N < 4(for 4 < N < 6 use the larger of diagrams (b) and (c))
O.50H
0.25H
P, = 0.15y H2 so 0.3Oy H2 PaIN = 4, p. = 0 N
'" .2
~
e
c.. 200
.
1000
o
2000
3000
Minimum and maximum prop loads (kN)
o
Minimum
•
Maximum
- - Average
- -. Average plus 60% ----. Average minus 60%
a) Class A
3000.---------------~,-----------------------,
,,
"
Z
6
"0
2000
,, ,,
al
.2 a.
o
,,
~
Q.
~
0
• ~~
••
P D . !III • ~ ~ ~ ~. n'!:! ..... ~
Q)
Ol Q)
. . o
1000
'"EI
0
>
«
. . . ... ~
O~=---------_.------------._----------~
1000
o
2000
3000
Minimum and maximum prop loads (kN)
o Minimum
•
Maximum
- - Average
- - . Average plus 60% ----. Average minus 60%
b) Classes 8, C and 0
Figure 6.10
56
Variations of individual prop load within an excavation, for case histories additional to those given by Flaate and Peck (1973)
ClRAC517
6000r---------------~r_----------_r------------~----~
! -__________-7_______:0:....;8 •• 8
5000 __ 1-
-f~========~====~H~ar~ri.s-8 08 .. 6
08 .. 8
I-------;~--------r__l
Z
~
Harris -8
4000 i------r-----I Harris 8
(/)
"0
./
CI:!
.Q
Il--------:~-/---! 08 .. 8
Cl.
5. 3000
1--....!::::;z::::::!..S~3 .4
v,od'yS1~;I~
~ .Q (I)
/' Inland -5
0>
~
~ 2000
/
lfI-,t----,"'----:>"'--~D8.6
7
,"'0 Chicago Cuts (Case histories AF12.14.22 and 26)) Site and numbers of prop indicated
1000
O~__~~~__~~__~~____~~__~______~____~
o
1000
Figure 6.11
6.12
4000 5000 Minimum and maximum total prop loads (kN)
6000
7000
Variations of total prop load within an excavation (after Flaate and Peck, 1973)
TEMPERATURE MEASUREMENTS Temperature measurements were reported in about a third of the case histories but only 17 of 81 case histories give details of the changes in prop load caused by temperature. In five ofthese temperature effects cannot be isolated from other changes in load. The remaining twelve cases are summarised in Table 6.2. Many ofthe engineers contacted during this study, and a number of the authors who have published case histories, expressed the view that temperature changes have little effect on prop loads, at least for props supporting flexible walls. In contrast, there are a number of recent examples of props supporting stiff walls in stiff (Class Band C) soils in which temperature was shown to be an important factor. Under such conditions temperature effects are normally considered in design. The available data are summarised in this section and the implications of temperature change on prop load are discussed further in Section 7.4. It is concluded that temperature effects should be allowed for by making simple checks on the structural members selected as props, but that an increase in characteristic prop loads may not be required. The proposed method of checking is set out in Section 10.6.2.
CIRIAC517
57
60.------,-------.-------,-------.------,-------~------~------~
40~----~------~-------+----~
§: ~
::l
«i 20 1__-------1---, Cii c.
E ~
o 1__-¥'----'--'lI+
-20~----~------~------~------~------~------~--~--~------~
30-Dec 1994
18-Feb
09-Apr
29-May
18-Jul
06-Sep
26-0ct
15-Dec
Date (1995) a) Top of prop
60.------,-------.-------,-------.------,-------~------~------~
401__----~-------+-------+------~~~~HHI__----+_------~------~
-20~-----J------~------~------~------~------~------~------~
3D-Dec 1994
18-Feb
09-Apr
29-May
18-Jul
06-Sep
26-0ct
15-Dec
Date (1995) b) Sides of prop
60.------,-------,-------,-------r-------.------,-------~------_,
401__-------I------~-------+------~------~------+-------+_------_4
§: ~
::l
~ "
I
"
~ O.S
Q. Q)
Cl
O.S 1
I
III
1
I
I II
c:
.Q
0.4
'~"
0.6
~
~
O.S
£ c3
I
I
I
I
~ 0.2 Q) -c
I
I
II
0.4
I O.S
I
AF4
I I
I
i
i
I
I
J.. I
I I
I I I
o
0.25 0.5 0.75 1.0 1.25 1.5 DPLlYH
AFS
!
I
I
I i
I
I
I
I
I
1
i I
~
I
I
I
I I
0 0.25 0.5 0.75 1.0 1.25 1.5 DPLlYH
Figure 7.4a
I I
I
o
I I I I
0.25 0.5 0.75 1.0 1.25 1.5 DPLlYH
AFS
I
I I
I !
I I !
I
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I I
I II i
0.6
I
I
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j
I
i
0.8
i
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c3
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0.8
~ 0.2 Q) -c c: o 0.4
i~
I
I
,
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72
I
0.4
1
AF7
Cl
I
Q.
I
I
0
Q)
I
~ O.S
-11
0 0.25 0.5 0.75 1.0 1.25 1.5 DPLlYH
:cQ.
I
0.25 0.5 O. 5 1.0 1.25 1.5 DPLlYH
c:
1
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I
0.8 1
'10*
'""
c:
I
I
I
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0.4
~
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o
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c:
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I
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>
o
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0
'*
I
I
o
0 0.25 0.5 0.75 1.0 1.25 1.5 DPLlYH
~ 0.2 Q) -c
I
I
i
J:
I
Q.
I
AF3
.2
5.Sm dig
J:
0
c:
i I
I
o 0.4
x ~
I
I
~ 0.2 Q) -c
0 0.25 0.5 0.75 1.0 1.25 1.5 DPLlYH
~ 0.2 Q) -c
I
I
I
I I
0.4
AF2a
o
I
1
!I o
I
II' I I
I
I
0.25 0.5 0.75 1.0 1.25 1.5 DPLlYH
Flexible walls in Class A soils - normalised depth versus normalised distributed prop load
CIRAC517
AF16
o
-w
I-
orily 4nl thicknes~ of soft clay behind r-thewall N-
~
AF17
o ~ 0.2 ~
"o
0.4
1
r---
~ 0.6
..
C. ~ 0.8 1
o
1
0.25 0.5 0.75 1.0 1.25 1.5 DPLlYH
~ 0.2
0.25 0.5 0.75 1.0 1.25 1.5 DPLlYH AF18
11.5m dig
0.4
I
~
1 ~c.
o
AF2b
o
., ""o
I
~
0.6
~(
w~water pressure -
~
~ 0.8
~ 0.2 t--t---t-t----t--t----t--j ~
§ 0.4 I--+--t-t---t-+--t--i
1
I--+--tl'!---t-+--t--i
~ 0.6
:5 c.
.,
o 0.8
11 1
o
1~~--~~--~~--~
AF19
o I
~ 0.2
., ".§
.j
U
1
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1
1
II ll
I I
o
0.25 0.5 0.75 1.0 1.25 1.5 DPLlYH AF22
0
""h
., 0.6
c.
CIRIA C517
AF20
AF21
1L 1 :c
Figure 7_4b
0.25 0.5 0.75 1.0 1.25 1.5 DPLlYH
~ 0.2
0.4
0.8 1
0.6
0.25 0.5 0.75 1.0 1.25 1.5 DPLlYH
~ 0.2
8
~
~ 0.8
o
., "o"
0.4
~
I o
., ""o
)
r
~ 0.8
o ~ 0.2
In
0.4
o
0.25 0.5 0.75 1.0 1.25 1.5 DPLlYH
II o
tI
., "" ii> .,u 0
~
0.4
r""'"
x 0.6
i., 0
0.8 1~~--~~--~~-~
0.25 0.5 0.75 1.0 1.25 1.5 DPLlYH
o
0.25 0.5 0.75 1.0 1.25 1.5 DPLlYH
Flexible walls in Class A soils - normalised depth versus normalised distributed prop load
73
-
~ 0.2
-w
O~IY 4rri thiC/u,eS~
CD "C
c:
of soft clay behind -the wall -
.2 0.4
-
~ ~
AF17
AF16
o
0.6
: "C
0.4
~ x 0.6
I
0
'fil
~c.
0> a 0.8
~ 0.6
~
II 1
0 0.25 0.5 0.75 1.0 1.25 1.5
DPLlYH
o
I
I
I
Q.
~ 0.8
1
80
I
I I
I !
~
..,1
Figure 7.5d
I
c: o 0.4
'fil
()
i
I
-.-
0> "C
I
c:
8510
I
I
I
0.25 0.5 0.75 1.0 1.25 1.5
DPLlYH
Stiff walls in Class B soils - normalised depth versus normalised distributed prop load
CIRAC517
Cl
o
I
1 1
I
% 0.2 OJ
I
"C
c
.Q
~
0.4 I-- ,-Preloaded
100%
!
~ 0.6
~
15. ~ 0.8 1
o
I
I ! I
I
!
to_ r--I
"C C
o
~
l ~
I
I
I
1
I
o
I
i
I
i
1
0.25 0.5 0.75 1.0 1.25 1.5 DPLlYH C4 I
I
OJ "C C
0.4 HI-+-+--+-+--+---1
0.6 H-t--I-+-+-+---t
o
o 0.4
~
~ 0.6
~
15. ~
o.s 1
0.25 0.5 0.75 1.0 1.25 1.5 DPLlYH
C5
o
I
o
o
% 0.2
% 0.2
"C
"C
Q)
0.25 0.5 0.75 1.0 1.25 1.5 DPLlYH
C6
II
Q)
C
§ 0.4
o
~OJ~
~co
%
~ 0.8 1
0.4
~ 0.6
_ 0.6
%
!
1
0.25 0.5 0.75 1.0 1.25 1.5 DPLlYH
C7
I
% 0.2
I I I
Q)
"C
0.6
I I
~ 0.8
I I
0.4
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l
~ 0.8
I
o
o
i
I I
% 0.2
lL-~--~~~--~~
.Q
! !
o
~ O.S HI-+-+---I-+---If--;
!~
I 1
~ .
'"~
"
~ 0.6
~
Q.
Q.
r3
0.8
1
I o
I
0.25 0.5 0.75 1.0 1.25 1.5 OPLlYH
Figure 7.7a
CIRIA C517
0.25 0.5 0.75 1.0 1.25 1.5 OPLlYH
~ 0.6
iC3
I
o
r3
04
~ 0.2
1~~~--~~--~~
"'c:o"
I
Q)
"'~" c3
I
0.25 0.5 0.75 1.0 1.25 1.5 OPLlYH
o
o ~--;----;--I
CD
o
0.25 0.5 0.75 1.0 1.25 1.5 OPLlYH
0.6
I
II
Corrected for O.S f-temperature 1
1
o
1
I
0.25 0.5 0.75 1.0 1.25 1.5 OPLlYH
Walls in Class D soils - normalised depth versus normalised distributed prop load
83
n ! ':1
010
09
o
O~~~~'-'-~I--'
I I
~ 0.2
0.2
1
0.4
x
~
r
0.6
1
1
1
c:
1-1
1l 0.6 1-+---J.J-+--+-+1----l
I
i
~
0.81----+-41-'__+__+__1+ - - - I I
11....--"--"----l--"----"---I
l...-~--!...---L_L-~..J
o
1-+-+--+-+-1--1
.2 0.4
!
I
i 0.81--fl-1-.!~-=:~:_--!~---ti--l 1
Hr..,-+-+--l--+-f--I
"0
o
0.250.50.75 1.0 1.25 1.5
~
0.4
1--\-\+--+-+--1-+-1
1
~ 0.6
,
Q.
~
!
0.2
0.2
I-I--+-+--I-+-+-I---j
.2 0.4
1-'==+-+--1-+-+--1
1l 0.6
I-I+-+--+-+---jf--I
~
~ 0.8 1-4--+--+-+---i!--l 1~..J...---I_...1----l._..l....--J
1~...J....!!........!--'-~-.!..--I
o
o ~ 0.2
T
OPLlYH
013
014a
H-+-+--+-+--1---I
0.6
1--11--+--+-+--1--1
~ 0.2
1-+-+--+-+--1---1
Q)
"0
c: o
0.4
1-+++--+-+---1---1
1l 0.6
f-=1""""d::I--+--+-+---I--l
~ 0.8
1-1-+---+---+--+-1--1
~
i
1 1 lL--L.---l_....!.----L_..L---I o 0.25 0.5 0.75 1.0 1.25 1.5
lL--L.---l_...1-~_.!..--I
o
0.25 0.5 0.75 1.0 1.25 1.5
OPLlYH
o
C. ~ 0.8
o
0.25 0.5 0.75 1.0 1.25 1.5
~ 0.4 H"H--f-+--+-t--l 1 i1l 1--11--+--+-+--+--1 ~
o ~""--'--I
i
~
0.8
--,---,
012
011
!
0.25 0.5 0.75 1.0 1.25 1.5 OPLlYH
OPLlYH
0.25 0.5 0.75 1.0 1.25 1.5
OPLlYH
OPLlYH
014c
014b
Or--C~or-rect-e-d~~-r~-~-'
temperature
1--+-+--+-+---j--1
~ 0.2 1-1/:""'+-+--+_+-1--1
c: o 0.4
I--+-+J--+-+--I--I
o 0.4
~-I+-+--+-+--!--j
x 0.6
I I--+-=:!:I--+-+--I--I
1l 0.6 ~
1-.l.J+.-+--+-+---I--l
~ 0.8
I---++-+--+--+-l---j
~ 0.2 Q)
"0
1 ~c.
I·
1
'""
I
L...-'-!-"----'---'---'--'
o
0.25 0.5 0.75 1.0 1.25 1.5 OPLlYH
Figure 7.7b
84
~
I
c:
~
C. ~ 0.8
1-11+-+--+-+--1--1
1~-'---l--'--.J..-.!..--I
o
0.25 0.5 0.75 1.0 1.25 1.5 OPLlYH
Walls in Class D soils - normalised depth versus normalised distributed prop load
CIRAC517
015
o ~ 0.2
..,'"c: o
0.4
~
0.6
~
~
I
...
~
Prop
Q.
I
~ 0.8 1
Figure 7.7c
I
Anchor
o
0.25 0.5 0.75 1.0 1.25 1.5 OPlIYH
Walls in Class D soils - normalised depth versus normalised distributed prop load
The resulting normalised DPL diagrams for all the 81 case histories are summarised in Figures 7.8-7.11 according to the classification given in Section 6.1. The results are divided as follows:
• • • • • • • •
Soft clays and flexible walls with enhanced base stability
(Class AF)
Soft clays and flexible walls with stable bases
(Class AF)
Firm clays and flexible walls
(Class AF)
Soft clays and stiff walls
(Class AS)
Stiff clays and flexible walls
(Class BF)
Stiff clays and stiff walls
(Class BS)
Granular soils
(Class C)
Layered cohesive and granular soils
(Class D).
Each set of results is considered in turn in Sections 8.1-8.4. The normalised depth versus normalised distributed prop load diagrams for all the case histories in each class are superimposed in Figures 7.8-7.11. A characteristic diagram has been added for each class. The characteristic diagram is a cautious estimate of the distributed prop load and has approximately a 5 per cent chance of being exceeded (see Section 9). It is preferable to avoid the term "envelope", which implies that all possible loads lie within the limit shown. The characteristic diagrams for soil Classes A, Band C are described in the Section 8 and summarised in Figure 7.12. Their application to the design of propping systems is set out in Section 9.
CIRIAC517
85
7.7
86
KEY POINTS 1.
Peck's recommended apparent earth pressure envelopes are not directly compatible with the characteristic values required for limit state design and revised diagrams of Distributed Prop Load (DPL) are proposed in Section 8.
2.
Comparison of Peck's recommendations with the further data now available shows that in stiff clay soils the original envelope gives good agreement for flexible walls but that stiff walls attract higher loads than indicated. The original envelope for granular soils similarly agrees with records from dry sites but sites that have been dewatered should not be treated as dry. Loads found by taking the buoyant weight of the soil in combination with the original envelope and the possible water pressure are consistent with site measurements and a modified diagram is proposed which incorporates this approach.
3.
Temperature changes significantly affect prop loads. The increase in load due to increased temperature is expressed as a percentage of the increase that would be caused in a fully restrained member, ''the degree of restraint". For stiff walls the degree of restraint is generally 40-70 per cent in stiff ground and reduces to 20-40 per cent in less stiff ground. Limited data for flexible walls indicates that the degree of restraint lies between 10 and 25 per cent but can reach 40 per cent. The degree of restraint can vary significantly between props on the same site.
CIRA C517
o
L~
0.2
I
'--
I-~
0.4
~
r--
'--
.r::
0.. CD
o
V'-
r--
-
~
0.6 ,-
r-
'---
-
0.8
1
o
-
I
I
i
0.25
0.5
0.75
I
I
1.25
1.5
DPUYH a) Flexible walls
o
I
I I I
-
:/
-
1(0.65 YH)
0.2 -
:r: :;::
Characteristic dia::1 for soft clay
~-
0.4 f-
I
r---
0.. CD
o
0.6
f-
0.8
-
I
1
o
0.25
I
0.5
I I I I I I I I I I I I I
Enhanced base stability
(1.15YH)
/'
'-
I
0.75
I
1.25
1.5
DPUYH b) Stiff walls
Figure 7.8
CIRIA C517
Normalised DPL diagrams for flexible and stiff walls in Class A soils
87
O~"r-...------------------------~
0.2 -
0.4
1 1 1 1 1 1 1 1
~
0.6 -
BF1
V
[1
Characteristic diagram (0.3 YHi
1/ 1
0.8
1L-__~,~i__~____- L_ _ _ _~_ _ _ _L -__~
o
0.25
0.5
1.25
0.75
1.5
DPUYH a) Flexible walls
o
0.2
-f-
:r.:
0.4 f-
:c
c. Q)
o
0.6
0.8
I-
-
1 1 -I
Characteristic diagram (0.5 '(Hi
:/
I-
1 1
I
1
o
0.25
I
!
0.5
0.75
I
I
1.25
1.5
DPUYH b) Stiff walls
Figure 7.9
88
Normalised DPL diagrams for flexible and stiff walls in Class B soils
ClRAC517
0
\ 0.2 -
\ 1\
·II V\
0.4 '-
~
15.. Cll
\
0
0.6
Buoyant unit weight plus water pressure from \ vgrOUnd level and with '::: 35°
·
\
1-
0.8 -
C1
\
1
\
V\
· 1 0
=35°
Dry, '
\
I
I
\
0.25
0.5
I
I
0.75
1.25
1.5
DPUYH
Normalised OPL diagrams for mainly flexible walls in Class C (granular) soils
Figure 7.10
o
0.2
0.4
::t:
~I\
:c
f-
1\
15.. Cll
\
o
0.6
l-
I-
f-
0.8 -
\ I
1
o
0.25
\
I
I
0.5
0.75
I
I
1.25
1.5
DPUYH
Figure 7.11
CIRIA C517
Normalised OPL diagrams for flexible and stiff walls in Class 0 (mixed) soils
89
0.2YH
0.5YH
H
0.65YH
H
H
0.65 YH
Class AF, firm
1.15 YH
Class AF, soft with stable base
Class AF, soft clay with enhanced base stability
For Class AS, tentatively as Class AF a) Class A soils (soft to firm clays)
Class BF
Class BS
b) Class B soils (stiff to very stiff clays)
0.2YH
1--+1
-. ! i
I
H
H
I
+ I
I
--t..
--t..
l--I
~
0.2Y'H
0.2Y'H
0.2YHabove groundwater table
:.
Water pressure
0.2 Y 'H + water pressure below groundwater table
-!
Class C, submerged
Class C, dry
c) Class C soils (granular soils)
Figure 7.12
90
Characteristic distributed prop load diagrams for Class A, Class B and Class C soils
CIRA C517
8
Characteristic distributed prop load diagrams
8.1
SOFT AND FIRM CLAYS (CLASS A SOILS) The flexible wall case histories in soft to fInn clays have been subdivided on the basis of strength and base stability, as shown in Table 8.1. Table 8.1
Finn clay
Base stability condition of flexible wall case histories in Class A soils (soft to firm clays)
Base condition
Case history (AF)
Comment
Stable (adequate soil strength)
10,15,20,27
Strengths and unit weights are greater than would be expected for a normally consolidated clay Unit wei~hts are greater than 19 kN/m . All these case histories are in Chicago, USA
Soft clay
Stable (stronger stratum at or near base)
Enhanced (wall contributes to base stability)
7,18,23,24,26
There is no soft clay beneath the excavation (T=O)
2b, 5, 6, 8, 12, 14, 16,17
De/T< 1 ie wall does not extend to the competent stratum(l)
19,21,22,25,28
De/T~
la
Intermediate (5.3 m) dig stage of 11.5 m deep excavation with sheet pile walls driven to rock at 12.5 m (TIB = 0.65; D/T= 1.0 but TIH= 1.36). This shallow dig is assumed to have enhanced base stability
2a(2)
Intermediate (5.8 m) dig stage of 11.5 m excavation with 16.5 m-long sheet piles and rock at 28 m depth. (TIB = 1.5; TIH = 1.4)
1b, 3, 11, 16, 13
Reported as driven to rock for stability (TIH= 0.41-D.72 except 0.13 for AFlb; TIB = 0.13-D.5)
4,9
TIH of2.4 and 0.6 respectively TIB of 0.2 and 0.4 respectively
1 but TIH < 0.33, ie only small thickness of soft clay beneath the excavation (TIB of between 0.04 and 0.37)
(1)
Competent stratum has been taken as at least firm to stiff clay or medium dense sand, etc.
(2)
Excavation below 5.8 m was carried out under water to maintain base stability
Figure 8.1 shows that a characteristic value for the DPL diagram for fIrm clays is O.31"Hreducing to 0.151"H for depths ofDIHless than 0.2.
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91
The data for the soft clay excavation with stable bases f H show a wide scatter, but a fairly well defined upper bound at about 0.75. The scatter partly reflects the various surcharges and departures from the typical construction sequence noted in Table A1.3. For example, AFl2 was dug to the Level 2 prop before installation of the Levell and 2 props. Not surprisingly, the Levell prop carried negligible load and the prop at Level 2 gave the highest normalised DPL of all the case histories. A number of the prop load measurements show the influence of a stronger underlying stratum as discussed in Section 7.5. For example in case histories AF19 and AF23 (see Figure 7.4) a stronger stratum, at approximately the final excavation depth, has resulted. in a trapezoidal DPL diagram. Stronger strata at lower levels (AF18, AF21) show a weighting towards the base of the DPL diagram and where no stronger stratum was present (AF5, AF8) an increase in DPL with depth is noted.
o
I
I I I
0.2
'-
'--
0.4
J::
-
--I I I I I
-
V
Characteristic diagram
(O.3YHJ
a.-
?:
15. OJ
o
0.6
-
0.8 t--
.--
I I I I I I I I I I I I
1
o
0.25
I
I
0.5
0.75
I
I
1.25
1.5
DPUYH
Figure 8.1
Characteristic DPL diagram for flexible walls in firm clays (Class AF)
In many cases it has not been possible to evaluate with confidence base stability number or factor of safety against base failure. The case histories do not provide the additional information required. The characteristic DPL diagram is thus based on all the relevant records taken together as shown in Figure 8.2. The recomm-ended DPL diagram is 0.65 f H reducing to 0.5 f H above a depth below ground surface of 0.2 H. Where the level of the stronger stratum is above the lowest excavation level the loads at the top and bottom ofthe excavation are likely to be over-predicted by the characteristic DPL diagram.
92
ClRAC517
Case history data for excavations where the retaining wall enhances base stability come from Norway (6), Singapore (1) and UK (1) and are plotted in Figure 8.3. The excavations range in depth from 5.3-11.0 m and the TIE ratios are 0.1-0.5.
o
Ll ...., I
0.2
-I I
F r-
0
0.4 '----
Vr--
-
0.6 -
c-
I
o
I¥'
0.25
-
II
f-
I I I
I' 1
;-/(0.65YH)
~
-
0.8
r-
Characteristic
diagr~m
I
0.5
I
I
0.75
I
I
1.25
1.5
DPUYH
Figure 8.2
Characteristic DPL diagram for excavations in soft clays (Class AF) with stable bases and flexible walls
The DPL diagrams give much higher values ofDPLl1"H than for the stable excavations and relatively small scatter. One notable departure from the general scatter of data is for the Level D props of AFll, where the measured loads are approximately 2.5 times the load in the other four propping levels and double the other data shown in Figure 8.3. Flaate (1966) suggests that this high load was due to frost whereas Norwegian Geotechnical Institute (NGI, 1962) report suggests the load increase was due to the surcharge effect of an adjacent building. The detailed measurements show variations of up to 50 per cent between loads in adjacent props. The recommended characteristic DPL diagram for excavations which rely on the retaining wall for base stability and have TIE '
c'
Su
Unfavourable
Favourable
Unfavourable
B
1.35
1.00
1.50
1.0
1.0
1.0
C
1.00
1.00
1.30
1.25
1.6
1.4
2.
Permanent actions include actions caused by ground, groundwater and free water. Variable actions may alter with time and include surcharges and temperature effects on the prop loads.
3.
Design values of actions due to ground and groundwater may be derived using the partial factors in Table 9.1 or by other methods. The partial factors in Table 9.1 indicate the level of safety appropriate for conventional design in most circumstances and are to be used as a guide to the required level of safety when the method of partial factors is not used. Where design values for ultimate limit state calculations are assessed directly, they are selected such that a more adverse value is extremely unlikely to govern the occurrence of the limit state.
4.
104
The characteristic value of a parameter is one that is a cautious estimate ofthe value governing the occurrence of limit state. If statistical methods are used the probability of a worse value is not greater than 5 per cent.
CIRA C517
5.
Table 9.1 indicates that for the method of partial factors all permanent characteristic earth pressures on both sides ofthe wall are multiplied by 1.35 if the total resulting action is unfavourable, and by 1 if the total resulting action effect is favourable. Variable characteristic earth pressures are multiplied by 1.50. However, it also permits the partial factors to be applied to the action effects derived from the characteristic earth pressures (ie multiply prop loads from permanent actions by 1.35 and from variable action by 1.50). Clause 8.6.6.4 ofEC7 states that this latter method should be used for the design of the structural elements of a retaining wall system.
6.
For the ultimate limit state, the design water pressures should be the most unfavourable values which could occur in extreme circumstances. For the serviceability limit state the design water pressures should be the most unfavourable which could occur in normal circumstances.
7.
For the ultimate limit state, calculations the excavation depth should be increased by 10 per cent of the height beneath the lowest support, up to a maximum of 0.5 m.
Analyses of propped excavations in soft clay, stiff clay and dry sand have been undertaken to establish whether Case B is more critical for the range of case histories covered in this report. These analyses are summarised in Appendix A6 and the results indicate Case B is likely to give the higher ultimate prop load in most situations (Case C only gave significantly higher loads for three out of the twenty props in the excavations in dry sand). Case C aims to address uncertainty in the ground. Where Case C gives the higher prop load, the distributed prop load method will usually account for this because it is based on actual field data. The recommended characteristic DPL diagrams have been assessed conservatively and it is reasonable for designers in conventional situations to conclude that only Case B has to be considered. The distributed prop loads are the action effects of ground and water pressures. Following EC7 Case B philosophy, characteristic values of the DPL can be multiplied by 1 to give the serviceability limit state (SLS) design values, and by 1.35 (permanent) or 1.50 (variable)to give the ultimate limit state (ULS) design values. The position of the characteristic DPL drawing has been chosen so that the few records that fall outside it are within the ULS design DPL diagram (with the exception of prop Level D for AFll).
9.4
RECOMMENDED CHARACTERISTIC DISTRIBUTED PROP LOAD DIAGRAMS Characteristic distributed prop load diagrams for Class A (soft to firm clays), B (stiff to very stiff clays) and C (granular) soils are shown in Figure 7.12. The diagrams were derived from site measurements of prop loads, see Section 7.6, and are subdivided as shown in Table 9.2. The results for clay soils (Class A and Class B) are divided on the basis of wall stiffness and the soft clay (Class A) results are further separated according to base stability (see Section 7.5). For excavations in soil types A, Band C it is recommended that characteristic loads for the design of temporary props be determined from the diagrams shown in Figure 7.12 subject to the qualifications given in Section 9.5. The characteristic prop load, DPLko is found by multiplying the appropriate coefficient from Figure 7.12 by the average unit weight of the soil, "1 , the retained height of the excavation, H, and the area of wall supported by the prop under consideration. A step-by-step guide to the method of calculation and worked examples are given in Section 14. It should be noted that the characteristic loads do not allow for the effects of temperature. These can be significant,
CIRIAC517
105
especially for props supporting stiff walls in stiff soils. The ability of the prop to carry the loads generated by temperature changes should be checked using the method given in Section 10.6.2 of this report. For Class D soils, in which no one soil type predominates, the distributed prop load method may be extended for preliminary design purposes by treating each layer of soil discretely as set out in Section 8.4. This approach requires a good understanding of the geological history of the site and of groundwater conditions and should only be used by experienced geotechnical engineers. The distributed prop load diagrams do not resemble the real distribution of earth pressure against the retaining wall. They provide a method of calculating values of prop load which are unlikely to be exceeded by any temporary support system in a similar deep excavation.
Table 9.2
Characteristic distributed prop load diagrams for Class A, B and C soils
Soil class
AF
Oto 0.2DIH
0.2 tolH
Firm clay
0.2"j"H
O.3"j"H
Soft: clay with stable base
0.5"j"H
0.65"j"H
Soft: clay with enhanced base stability
0.65"j"H
1.15"j"H
Limitations (see also Section 9.5)
TIB < 0.5 and TIH < 0.8 Sensitivity no greater than for Oslo Clays (2 to 6)
AS
Tentatively as Tentatively AF asAF
BF
O.3"j"H
O.3"j"H
BS
0.5"j"H
0.5"j"H
Dry
0.2"j"H
0.2"j"H
Submerged
0.2 "j"1fand water pressure
0.2 "j"1fand water pressure
C
9.5
Characteristic DPL/"j"H
TIB < 0.5 and TIH < 0.8. Use with great caution, see Chang and Wong (1996)
Soils must not be prone to liquefaction. Use buoyant unit weight of soil, ' below water table
CONDITIONS OF USE FOR THE DISTRIBUTED PROP LOAD METHOD The distributed prop load method is based on empirical relationships and its selection for use on any project should be considered carefully. It is advisable to use one or more alternative methods as well and to compare the results obtained.
106
ClRAC517
When considering whether the DPL method is appropriate the engineer should consider:
9.5.1
1.
Is the specific site stratigraphy covered by the data set?
2.
If the answer to (1) is ''No'', do the site specific soils behave in a similar way to the soils in the data set, ie do the specific soils behave differently from the general Class A, B or C soils? Is it reasonable to apply DPL recommendations to the site?
3.
Is the geometry ofthe excavation and propping system within the range represented by the data set? This should be considered particularly in regard to: •
width of excavation
•
depth of excavation
•
number of props and their horizontal and vertical spacing
•
duration of propping
•
installing props before excavating below the prop level.
4.
Do the limitations stated in Section 7.6 for each soil class apply, eg TIB (') CJ1
Table A1.1
Summary of case histories (for explanation see notes at foot of the table)
...... .......
Case history
Ref no
Location
Title
Excavation details
Ground conditions
Prop and waling details (6)
Wall type
(8)
......
(l)
Surcharge
Soil profile
Groundwater/ depth/dewatering (4)
(3)
(2)
Type/preloadlng/ temperature/waling
(5)
1
Oslo, Norway
Gronland l'
SP
11.1/3
14-29
No. Battered 0-2.5m depth
mdMG(1.5)/smC(11.0)/R
Y/1.8/N
SUC/N/-20
