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CIRIA C641
London, 2008
EC7 – implications for UK practice Eurocode 7 Geotechnical design Richard Driscoll
BRE
Peter Scott
Buro Happold
John Powell
BRE
Classic House, 174–180 Old Street, London EC1V 9BP TEL: +44 (0)20 7549 3300 FAX: +44 (0)20 7253 0523 EMAIL:
[email protected] WEBSITE: www.ciria.org
Summary
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The introduction of the Eurocodes represents for most civil and structural engineers a significant challenge in adapting to a very extensive set of new design and construction requirements. This is particularly so for geotechnical engineers in that Eurocode 7 and its associated new standards present some profound departures from traditional practice. The aim of this publication is to provide geotechnical engineers with an understanding of how the new documents will affect their day-to-day activities. Much information on the detail of the new Eurocode system already exists, so this book focuses on changes to common practice and their implications. The book takes the reader through a logical sequence of activities, from site and ground investigation to geotechnical element design, to construction practices introduced by the new European Execution Standards. It then concludes with an indication of the likely timing of full implementation and a prediction of the effect that the changes will have on geotechnical practice in the UK. The book seeks to give a clear overview of the main changes that will arise, adding in appendices such detail of the Eurocode system that is necessary to understand these changes. It illustrates the changes with a set of design examples covering mainstream design challenges such as piles, retaining walls, embankments and slopes, and hydraulic failure. The book is authored by three specialists who have worked closely with the development and introduction of Eurocode 7 and its application in the design office, and the content has been carefully criticised by a panel of leading UK geotechnical practitioners.
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CIRIA C641
EC7 – implications for UK practices. Eurocode 7 Geotechnical design Driscoll, R, Scott, P, Powell, J CIRIA C641
© CIRIA 2008
RP701
ISBN: 978-0-86017-641-1
British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. Keywords
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Ground engineering, Eurocode, foundations, geotechnical design, geotechnical investigation, ground investigation and characterisation, in situ testing and instrumentation, piling, soil structure interaction Reader interest
Classification
Design of geotechnical structures, limit state design, Eurocodes replace British Codes and Standards
AVAILABILITY
Unrestricted
CONTENT
Advice/guidance
STATUS
Committee-guided
USER
Client organisation, consultants, contractors , geotechnical engineers, project managers, structural design engineers
Published by CIRIA, Classic House, 174–180 Old Street, London, EC1V 9BP This publication is designed to provide accurate and authoritative information on the subject matter covered. It is sold and/or distributed with the understanding that neither the authors nor the publisher is thereby engaged in rendering a specific legal or any other professional service. While every effort has been made to ensure the accuracy and completeness of the publication, no warranty or fitness is provided or implied, and the authors and publisher shall have neither liability nor responsibility to any person or entity with respect to any loss or damage arising from its use. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any 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. If you would like to reproduce any of the figures, text or technical information from this or any other CIRIA publication for use in other documents or publications, please contact the Publishing Department for more details on copyright terms and charges at:
[email protected] Tel: +44 (0)20 7549 3300.
CIRIA C641
iii
Foreword
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The creation of the structural Eurocodes has been in progress for many years. These new EU standards have now advanced to a stage that warrants serious preparation for their implementation and the consequences of withdrawal of corresponding national documents. For a complex engineering discipline such as geotechnics, used to the piecemeal and evolutionary introduction of national codes and testing standards, the introduction of a significantly different design philosophy for dealing with engineering uncertainty and the relatively rapid replacement of national documents represent major changes for the industry. A recent report (Institution of Structural Engineers, 2004) has highlighted the challenges facing engineers in adapting to the Eurocodes and has advocated the preparation of guidance to ease their passage into practice. This publication has been produced to assist in this process by indicating the most important differences that geotechnical engineers will encounter when implementing the new suite of geotechnical Eurocode documents. It is not intended that this publication teaches the reader how to use the Eurocode since other referenced documents are available for this. However, a certain amount of explanation for some of the features of Eurocode design has been found necessary to assist in understanding the differences to practice that the Eurocode will bring. The book lists all the documents that will eventually comprise the full suite of euronorms covering geotechnical engineering. Many of these documents are still in preparation in several CENa committees and working groups. However the main design code, EC7-1, and several “execution”b standards have now been published by BSI. This mixture of published and unfinished documents leads to a rather confusing reference numbering system, with published BSI documents designated by “BS EN…”, published CEN documents by “EN…” and documents in preparation by “prEN….”. For clarity and brevity, the terms EC7-1 and EC7-2 have been used in this document for the two parts of Eurocode 7. EC7-1 concerns geotechnical design and EC7-2 refers to ground investigation and testing. EC7-1 cannot be used without EC7-2. This book begins with a short introduction to explain its purpose, content and style, and to identify the main changes that EC7-1 will bring. In Chapter 2, it discusses changes that may occur in site investigation practice before concentrating on how the Eurocode may affect general geotechnical design philosophy in the UK, with likely consequences, in Chapter 3. Chapter 4 focuses on changes that are specific to the main geotechnical elements that require designing, such as piles, retaining walls and slopes, with several worked examples demonstrating how the EC7-1 design methodology might differ from conventional practice. Chapter 5 briefly discusses differences in geotechnical construction practice that the new execution standards may introduce. Precisely how the new Eurocode suite of documents will be implemented in the UK is still a matter for debate. The intention is for packages of Eurocodes including, for example, loading, geotechnical, concrete, masonry and timber all necessary to design a complete building structure, to be available for full implementation and consequent withdrawal of national documents. It may be obvious that the timing for this
iv
a
Comité Européen de Normalisation.
b
“Execution” is defined as “all activities carried out for the physical completion of the work including procurement, the inspection and documentation thereof ”.
CIRIA C641
implementation is rather uncertain, though a prediction has been made in Chapter 6, which also briefly discusses the regulatory framework and how the new codes and standards will apply within it.
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Finally, Chapter 7 comprises a short piece on the likely overall effect of the Eurocode on geotechnical investigation, design and construction practice in the UK. The appendices provide more detail and further information. The intention is to keep this book as simple and succinct as possible in discussing what is a complex system of linked documents and which introduces a partial factor design philosophy to geotechnics. This has been carried out in several ways:
CIRIA C641
1
Endnotes for each chapter are included at the end of the book.
2
Text that quotes directly from the Eurocode has been highlighted in bold, while clause references are indicated in bold italics.
3
Key conclusions from each chapter are summarised in a table at the beginning of the chapter.
4
The examples have been formatted so that appropriate code clauses are apparent.
v
Acknowledgements
Research contractor This publication is the main output from CIRIA research project 701. It was prepared by BRE in association with Buro Happold.
Authors
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Richard Driscoll BSc MSc CEng FICE Richard Driscoll is an associate of BRE and was the lead author for this book. Richard worked at BRE for 27 years before retiring as the head of ground engineering. He spent many years as a BSI representative developing EC7 and has co-authored a book on the subject. Peter Scott BSc MSc CEng FICE MASCE FGS Peter Scott is the technical head of the geotechnical group at Buro Happold Consulting Engineers. Peter has extensive experience in geotechnical design for major projects in the UK and abroad and was responsible for providing the worked examples in the book. John Powell BSc MSc DIC DSc(Eng) CEng MICE John Powell is an associate director in the Geotechnics section of Building Technology at BRE. He chairs the BSI committee for BS 5930 and 1377 that is the mirror committee for EC7 Part 2. He represents BSI on the committee responsible for the drafting of EC7 Part 2 and is the national technical contact for associated technical specifications. David Poh of Buro Happold Consulting Engineers assisted in the preparation of the worked examples. Following CIRIA’s usual practice, the research project was guided by a steering group, which comprised:
Steering group
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Dr A Bond
Geocentrix
Mr S P Corbet
FaberMaunsell
Mr E S R Evans
Network Rail
Mr J D Findlay
Stent Foundations
Mr T Hayward
Stent Foundations
Mr A Jukes
Highways Agency
Mr A Kidd
Highways Agency
Dr P Morrison
Arup Geotechnics
Mr R Newman
Tony Gee & Partners
Mr A S O’Brien (chair)
Mott MacDonald
CIRIA C641
Dr M Pedley
Cementation Foundations Skanska
Mr S G Smith
Bechtel
Dr J Wilson
Atkins
CIRIA managers CIRIA’s research managers were Mr Chris Chiverrell and Dr Andrew Pitchford.
Project funders This project was funded by: The DTI’s Partners in Innovation scheme The Highways Agency Network Rail
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CIRIA’s Core Programme Sponsors Technical organisations CIRIA and the authors gratefully acknowledge the support of those funding organisations, the technical help and advice provided by the members of the steering group, and colleagues and specialists for reviewing the document and for assisting the authors in co-ordinating and collating all the technical contributions. Contributions do not imply that individual funders necessarily endorse all views expressed in published outputs.
Front cover photo: The piled wall for the new Wembley Stadium (courtesy Stent Foundations Ltd, a Balfour Beatty company). See Case study in Appendix A5
CIRIA C641
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Contents
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ii Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .iv Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .vi List of figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .x List of tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .x Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .xi Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .xii
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1
2
3
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 1.1
Purpose of this book . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
1.2
The status of Eurocode documents . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
1.3
Important features of EC7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
1.4
The content of this book . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
1.5
The style of this book . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
1.6
Consultation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
Site characterisation and determination of ground property design values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10 2.1
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
2.2
Ground investigation and testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
2.3
Ground identification and classification . . . . . . . . . . . . . . . . . . . . . . . .13
2.4
Determining the design values of geotechnical parameters . . . . . . . .13
The new principles of geotechnical design in Eurocode 7 . . . . . . . . . . . . . . . . . .17 3.1
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17
3.2
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17
3.3
Design by prescriptive measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18
3.4
Design using load tests and tests on experimental models . . . . . . . . .19
3.5
Design using the Observational Method . . . . . . . . . . . . . . . . . . . . . . . .19
3.6
Eurocode 7 – general design principles . . . . . . . . . . . . . . . . . . . . . . . .19 3.6.1 Limit state design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19 3.6.2 Design requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21 3.6.3 Design situations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21 3.6.4 Durability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21
3.7
Design by calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21 3.7.1 The application of safety in limit state design calculations . . . .21 3.7.2 ULS design calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22 3.7.3 Actions and their effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22 3.7.4 Geotechnical resistances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23 3.7.5 The GEO and STR ULS calculations . . . . . . . . . . . . . . . . . . . .23 3.7.6 Serviceability limit state design . . . . . . . . . . . . . . . . . . . . . . . . .24 3.7.7 The EQU limit state . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25 3.7.8 The UPL limit state . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25
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3.7.9 The HYD limit state . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26 3.8 4
The difference between DA-1 and traditional design calculations . . .26
Specific changes in design principles with examples . . . . . . . . . . . . . . . . . . . . .28 4.1
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28
4.2
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28
4.3
Spread foundations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28
4.4
Piles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29 4.4.1 Specific changes/issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29
4.5
Retaining walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41 4.5.1 Specific changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41
4.6
Embankments and slopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .72 4.6.1 Specific changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .72
4.7
Hydraulic failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .77
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4.7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .77 4.7.2 UPL design (see Clause 2.4.7.4) . . . . . . . . . . . . . . . . . . . . . . . .77 4.7.3 HYD ULS design (see Clause 2.4.7.5) . . . . . . . . . . . . . . . . . . . .80 4.7.4 Failure by internal erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . .80 4.7.5 Failure by piping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .80 5
Carrying out the construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .81 5.1
6
7
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .81
5.2
Construction requirements in EC7-1 . . . . . . . . . . . . . . . . . . . . . . . . . .81
5.3
BS EN “execution” standards discussed and compared with relevant BSs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .81
Implementing the new codes and standards in the UK . . . . . . . . . . . . . . . . . . . .83 6.1
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .83
6.2
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .83
6.3
“National choice” and the National Annexes . . . . . . . . . . . . . . . . . . . .83
6.4
The retention of valuable national code and standards material . . . .84
6.5
Time-scale and processes for change . . . . . . . . . . . . . . . . . . . . . . . . . .84
6.6
Guidance material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .85
The impact of the geotechnical Eurocode system on UK practice . . . . . . . . . . .86 7.1
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .86
7.2
The impact of EC7-1 on design practice . . . . . . . . . . . . . . . . . . . . . . .86
7.3
The impact of EC7-2 and associated documents on site investigation practice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .87
7.4
The impact on geotechnical construction practice . . . . . . . . . . . . . . . .87
7.5
Overall impact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .87
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .88
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A1
Examples of the selection of characteristic ground property values using all available site information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .93
A2
Statistical methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .96
A3
Design Approach 1 for GEO and STR limit state calculations . . . . . . . . . . . . . . .97 A3.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .97
A3.2
Design Approach 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .97
ix
A4
Conflicts of construction practice and requisite amendments . . . . . . . . . . . . .101
A5
Case studies using EC7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .104
A6
The provenance of BS EN standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .119
Endnotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .120
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List of figures Figure 1.1
Diagrammatic representation of the suite of EU geotechnical and structural codes and standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
Figure 2.1
Processing test measurements into design values of ground parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
Figure 2.2
General procedure for determining characteristic values from measured values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
Figure 4.1
Alternative procedures for pile design using profiles of ground properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31
Figure 6.1
Possible implementation timetable . . . . . . . . . . . . . . . . . . . . . . . . . . . .85
Figure A1.1
UU txl. strengths (U100) for a site with 3 b/hs . . . . . . . . . . . . . . . . . . .94
Figure A1.2
Corrected SPT “N” values for the site . . . . . . . . . . . . . . . . . . . . . . . . .94
Figure A1.3
SPT inferred strengths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .95
Figure A1.4
Assessed “characteristic” strength profile . . . . . . . . . . . . . . . . . . . . . . .95
Figure A1.5
Small building on estuarine beds near slope . . . . . . . . . . . . . . . . . . . .95
Figure A5.1
Wembley Stadium site geology and topography . . . . . . . . . . . . . . . .109
Figure A5.2
Undrained shear strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .110
Figure A5.3
CPT cone resistance profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .111
Figure A5.4
Preliminary pile load tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .112
Figure A5.5
Pile tests, observed versus predicted failure loads . . . . . . . . . . . . . . .113
Figure A5.6
Pile load settlement behaviour (observed versus predicted) . . . . . . .114
Figure A5.7
1.5 m diameter pile predicted load settlement (from load tests on 0.45 m to 0.75 m diameter piles) . . . . . . . . . . . . . . . . . . . . . . . . . . . .115
Figure A5.8
Wembley pile load test data compared with previous published results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .116
Figure A5.9
Predicted pile load settlement characteristics . . . . . . . . . . . . . . . . . . .117
Figure A5.10
Test pile 7 measured, characteristic and factored load settlement curves, compared with predicted behaviour . . . . . . . . . . . . . . . . . . . . . . . . . .118
List of tables
x
Table 1.1
The content of BS codes and their correspondence with the European documents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
Table 1.2
The content of BS codes and testing standards and their correspondence with the European documents . . . . . . . . . . . . . . . . . . .7
Table 2.1
Some of the changes introduced by EC7-2 . . . . . . . . . . . . . . . . . . . . . .11
Table 2.2
Some terminological changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Table 5.1
Correspondence between BS codes and standards and European codes and standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .82
Table 7.1
Impact of EC7-1 on design practice . . . . . . . . . . . . . . . . . . . . . . . . . . .86
Table A3.1
Values of partial factors recommended in EC7-1 Annex A . . . . . . . .100
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Table A4.1
Conflicts between BS codes and those BS EN execution standards available in January 2005 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .102
Table A5.1
Summary of vertical pile tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .106
Table A5.2
Fleming’s analyses (CEMSET), input parameters . . . . . . . . . . . . . . .107
Table A5.3
Factors that may affect choice of factor of safety . . . . . . . . . . . . . . . .108
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Examples
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Example 4.1
Design of a vertical, pre-cast concrete pile driven into sand and gravel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32
Example 4.2
Pile design incorporating negative skin friction (downdrag) . . . . . . . .37
Example 4.3
The design of a cantilever retaining wall without groundwater pressures acting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43
Example 4.4
The design of a cantilever retaining wall with groundwater pressures acting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .48
Example 4.5
The design of an embedded retaining wall with groundwater pressures acting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .55
Example 4.6
The design of a cantilever retaining wall with elevated groundwater pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .65
Example 4.7
The design of a stable slope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .73
Example 4.8
An excavation below the water table, showing design against uplift . .78
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Glossary
EC7 introduces terms and uses expressions that may require some explanation. The following table indicates what meaning these are intended to convey to the reader. The interpretations of the terminology are largely those of the authors, often using text in BS EN 1990: 2002 unless they include direct quotations from EC7.
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Action
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1
Set of forces (loads) applied to a structure (direct action).
2
Set of imposed deformations or accelerations caused, for example, by temperature changes, moisture variation, uneven settlement or earthquake (indirect action).
Characteristic value
Clause 2.4.5.2(2)P states that: The characteristic value of a geotechnical parameter shall be selected as a cautious estimate of the value affecting the occurrence of the limit state. A fuller discussion may be found in Section 2.4.
Code
Published guidance from a national standards body on how activities should be undertaken to achieve a required result using recommended best practice.
Comparable experience
Documented or other clearly established information related to the ground being considered in design, involving the same types of soil and rock and for which similar geotechnical behaviour is expected, and involving similar structures. Information gained locally is considered to be particularly relevant.
Derived value
Value of a geotechnical parameter obtained by theory, correlation or empiricism from test results. A fuller discussion is found in Section 2.2.
Design situation
Set of physical conditions representing the real conditions occurring during a certain time interval for which the design will demonstrate that relevant limit states are not exceeded.
Design value
Value of a variable used in the calculation of the dimensions of or forces on or in, the structure to be built.
Effect of action
Effect of actions on structural members (eg internal force, bending moment, stress and strain) or on the whole structure (eg deflection, rotation).
Execution
All activities carried out for the physical completion of the work including procurement, the inspection and documentation thereof.
Geotechnical action
Action transmitted to the structure by the ground, fill, standing water or groundwater (definition adapted from Clause 1.5.3.7 of BS EN 1990).
Limit states
States beyond which the structure no longer fulfils the relevant design criteria.
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Nominal value
Value fixed on non-statistical basis, for instance on acquired experience or on physical conditions.
Partial factor
A factor to either increase or decrease a variable used in part of the determination of the dimensions of or forces on or in the structure to be built.
Representative value of an action
Value used for the verification of a limit state. A representative value may be the characteristic value.
Resistance
Capacity of a member or component, or cross-section of a member or component of a structure, to withstand actions or their effects without mechanical failure, eg bending resistance, buckling resistance, tension resistance.
Serviceability limit states
States that correspond to conditions beyond which specified service requirements for a structure or structural member are no longer met.
Standard
Published instructions from a national standards body on how activities must be undertaken to achieve a required result.
Technical specification
Published instructions from a standards body on how activities should be undertaken to achieve a required result.
Ultimate limit states
States associated with collapse or with other similar forms of structural failure.
Verification
Design and checking.
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1
Introduction
1.1
Purpose of this book A new European suite of geotechnical design, testing and construction documents will in due course largely replace British codes and standards. This book has been written to identify and explain to the general geotechnical practitioner in the UK the key differences between the incoming and outgoing system and to indicate what other commonly used design documents1 will be retained. The book does not provide a clause-by-clause commentary on the main design Eurocode, EC7 Part 1 (this may be found elsewhere2), nor is it intended to be a manual of good practice in geotechnical design. Rather, it highlights the important features of the new Eurocode system and seeks to show how they may affect practice. With accompanying illustrations in worked examples, some guidance is given on how to apply the system’s Principles to ensure that designs will conform to the new requirements, and will be built and maintained as the Eurocodes intend. The main changes to geotechnical practice introduced in the Eurocodes are concentrated in Eurocode 7 Geotechnical design – Part 1: General rules, which this book concentrates on3, and Eurocode 7 Geotechnical design – Part 2: Ground investigation and testing. It is important to appreciate that the new European suite of geotechnical documents is a comprehensive, linked system of codes, standards and technical specifications. These indicate how information on the ground is to be acquired, how it is to be interpreted and transformed into design parameters and the geometry of geotechnical structures, and how these structures are to be built and maintained, with suitable monitoring and quality assurance. There is a confusing plethora of alphanumeric references within many of the new European documents. For the purposes of simplicity, this book refers to the two parts of Eurocode 7 as EC7-1 and EC7-2. It should be understood that all “Euronorms” published by CEN have the prefix “EN”, those produced by ISO4 and adopted by CEN have the prefix “EN-ISO” and all these documents, when published by BSI as UK versions will be prefixed by “BS EN” etc. Further complication is introduced by the use of “pr EN…” to signify documents that are in preparation. Figure 1.1 illustrates the system of new European documents while Tables 1.1 and 1.2 show the current BS codes and standards and their approximate relationships with those European documents that exist or are anticipated. There is direct correspondence for some documents (for example, some parts of BS 1377 are being and will continue to be replaced by an equivalent standard from CEN Technical Committee 341, see Powell and Norbury, 2007 for examples) while in most other cases there is limited overlap between the material (for example, BS 8004 covers aspects of the construction (“execution”) of pile foundations found in BS EN 1536:1999). EC7 introduces a number of important changes in the codification of design practices. In particular it:
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presents, for the first time, a unified set of Principles for all geotechnical design
bridges the philosophical divide between geotechnical design and superstructure design that has existed since BS 8110, explicitly employing limit state design and partial factors, was introduced in the UK
1
makes a clear distinction between the avoidance of an ultimate limit state (failure of the ground and collapse of all or part of a ground-supported structure) and of a serviceability limit state (undue movement and its consequences). Much “routine” geotechnical design has historically blurred these two requirements. The Eurocode should prompt greater thought about designing to prevent unacceptable movement, which should be beneficial
requires more systematic thought about the degree of uncertainty in the values of geotechnical material parameters for use in design calculations5
introduces a degree of compulsion by indicating that certain (Principle) activities “shall” be undertaken in both design and ground investigation6.
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EC7-1 is not only about carrying out design but is also about checking7 that a design will not reach a limiting condition in prescribed design situations. The code does not tell the reader how to design, rather it lays down a set of guiding design Principles, lists the many physical conditions that the ground and the structure it supports may exhibit, and states how the constructed outcome must behave. In common with the other structural Eurocodes, the foreword to EC7-1 indicates that it serves as:
a means to prove compliance with the essential requirement of “mechanical resistance and stability”
a basis for specifying contracts for construction works.
Unusual forms of construction or design conditions are not covered and additional expert consideration will be required by the designer in such cases. It is explicitly stated that appropriately qualified personnel are to provide the input data for geotechnical designs and that the design and ground investigations are to be performed by appropriately qualified and experienced personnel. In addition to the above, this book has several further aims:
to give readers a clear and simple understanding of the main issues that they will need to address when checking that their geotechnical design conforms with the Eurocode
to describe briefly the range of information presented in the Eurocode suite, to clarify the meanings of some new terms, to describe briefly the new design methods and to present easy-to-understand explanations of how the new methods work using design examples and a case study
to indicate the likely effect on geotechnical practice in the UK of the move to the Eurocode suite of documents, including how use of the Eurocode will comply with the requirements of the Building Regulations and any other local regulations, such as the London District Surveyors’ rules.
The book has been written primarily for three groups of readers:
2
1
The general geotechnical engineer who may often not have routine recourse to codes but who will, nevertheless, need to be assured that a design complies with the code requirements.
2
The non-geotechnically qualified engineer who carries out simple design for small projects for which the ground conditions are not regarded as problematical, whereby a geotechnical specialist may not be required. Such projects often comprise
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small housing developments where the foundations may be prescribed and where other geotechnical structures require recourse to relatively straight-forward design (such as small retaining walls currently designed using BS 8002:1999). 3
The general engineer and building and construction professional who may need to understand what the geotechnical engineer is doing.
This book is intended to be a companion to the suite of European geotechnical documents and is not a substitute for them, in any way.
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1.2
The status of Eurocode documents Once implemented in the UK, the Eurocode documents will have the status of current BS codes and standards. It is expected that all references to BS documents in the Building Regulations and other regulatory documents such as those of the Highways Agency and Network Rail will be replaced by references to the new BS ENs. The Eurocodes contain “Principles” that are “mandatory” ie they contain the word “shall”, as highlighted later in this book. This means that if and when the new BS ENs are used to design or to check a design, these mandatory requirements must be satisfied.
1.3
Important features of EC7 Scope It is important to appreciate that EC7-1 applies to the design of both new projects and the repair and stabilisation of existing geotechnical structures. It does not, however, specifically deal with the re-use of existing foundations nor does it apply to the assessment of existing structures. EC7-1 and EC7-2 also apply primarily to greenfield sites, and while “clean” fill is covered, contaminated land is not. Limit state design Two different types of limit state are identified, each having its own design requirements:
ultimate limit states (ULS), defined as states associated with collapse or with other similar forms of structural failure (eg exceeding the bearing resistance of the foundation). For geotechnical design, it is particularly important to note that ultimate limit states include failure by excessive deformation, leading to ... loss of stability of the structure or any part of it
serviceability limit states (SLS), defined as states that correspond to conditions beyond which specified service requirements for a structure or structural member are no longer met (eg excessive settlement leading to cracking in the structure).
Limit states are generally avoided by considering design situations in which adverse conditions apply (see Section 3.6.3). The need to identify these design situations should help to develop the routine use of risk assessment in geotechnics. Uncertainty in ground parameter values and resistance EC7-1 introduces the clear separation of actions and reactions and the application of partial factors to “characteristic” values of actions, ground parameters and resistances in place of global factors for dealing with all uncertainty and safety.
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3
Movement Because of the explicit requirement to check serviceability conditions, greater attention will need to be paid to settlements and other movement. However, note that the code does not provide explicit guidance on how to calculate movement. As will be discussed later, the separation of bearing capacity (a ULS) from settlement (an SLS) means that partial factors applied in a ULS calculation may not guarantee that settlements are sufficiently small, particularly on soft ground. Clients should be confident that appropriately qualified and experienced personnel have been involved in any EC7-1 design calculations. Compulsory reporting of information The production and communication of the Geotechnical design report and Ground investigation report are requirements of EC7. Minimum contents for these reports are specified and these comply fully with obligations under CDM regulations.
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Geotechnical models EC7-1 deals with the design of different types of foundation, retaining wall and other geotechnical structures but the code does not specify which soil mechanics theories or soil behaviour models to use, although it does suggest, in informative annexes8, means to determine, for example, the earth pressure acting on a retaining structure or the stability of a slope. A unifying set of design Principles EC7-1 presents a unified set of Principles for design (see Appendix A3). In contrast, BS codes have emerged over many years in a rather piecemeal fashion, with a collection of different design philosophies. Terminology EC7-1 introduces terms that are not widely used or defined in the UK, at least by the geotechnical engineering community. These terms are briefly explained in the glossary, with some being more fully covered in later chapters of this book.
1.4
The content of this book Chapter 2 deals with important differences in obtaining design parameters for use with EC7-1. For ground investigation, including laboratory and field testing, EC7-2 deals with basic ground data and its interpretation with the resulting “derived values” being passed to EC7-1 for conversion into a characteristic and hence design value. The differences from current practice in these processes are briefly outlined. Chapter 3 deals with the key differences in the general Principles of design between EC7-1 and the BS codes of practice. The alternative methods of design permitted in the code are briefly described after which “design by calculation” is discussed in some detail since it is here where the greatest changes from current practice will be found.
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Of course, design calculations rely on the provision of appropriate and suitably accurate input parameters. The chapter also highlights important new concepts for arriving at suitably conservative values of input parameters so that the design will avoid the occurrence of a limit state. The concept of characteristic value of a parameter and how it is acquired, starting with the elements of a site investigation, is discussed, after which the obtaining of a design parameter value is considered. Finally, the adoption in the UK of Design Approach 1 is outlined (three alternative design approaches are permitted in the Eurocode). Chapter 4 briefly describes specific differences for common design problems and illustrates them in typical worked examples and a case history.
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Chapter 5 describes the key differences involved in moving from BS codes to the BS EN standards for “execution” (construction). The resolution of any conflicts identified between the documents is outlined. Chapter 6 deals with the manner in which the Eurocodes will be implemented in the UK. It briefly discusses how national preferences for safety are incorporated into the National Annexes for EC7-1 and EC7-2 and explains how and when the Eurocodes are likely to replace the BS codes as references in Building Regulations and other regulatory and widely-adopted design documents9. Chapter 7 discusses the manner in which the move to the Eurocodes might affect geotechnical practice in the UK, from changes in site and ground investigation, through design calculations to construction activities on site. Brief mention is made of any consequences for the economics of geotechnical works and any effect on construction programmes. There are a number of appendices that contain specific details that have been separated from the main body of text to ease reading and understanding.
1.5
The style of this book Since the European geotechnical codes and standards have been developed in a somewhat disconnected manner by several different CEN committees, the emerging suite of documents does not always appear to conform to a logical pattern. Furthermore, EC7-1 itself does not always follow the sequences of events that constitute design as normally practiced in the UK. So this book does not follow the order of presentation of material in the Eurocodes. Throughout, an attempt has been made to keep the narrative simple and focused on how the Eurocode may introduce changes to practice.
1.6
Consultation During the writing of this book, consultation has taken place with a group of geotechnical design, construction and site investigation specialists. While several in the group are familiar with EC7-1, a concerted attempt has been made to address this document to people who have little or no knowledge of EC7.
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5
6
Site investigation
9
Retaining structures
Anchorages
12 Embankments
11 Overall stability
Earthworks
BS 8002:1994
8
Pile foundations
BS 6031:1981
Earth retaining structures
BS 8081:1989
7
Spread foundations
10 Hydraulic failure
Ground anchorages
BS 8008:1996
6
Some
Foundations
Safety precautions and procedures for the construction and descent of machine-bored shafts for piling and other purposes
BS 8004:1986
Foundations
BS 8004:1986
nailing etc).
Strengthened/reinforced soils (N Note: EC7-1 does not cover the design of and other fills reinforced soils or ground strengthened by
Fill, dewatering, ground improvement and reinforcement.
5
BS 8006:1995
Supervision of construction, monitoring and maintenance
4
Earthworks
Geotechnical data
Basis of geotechnical design
2
3
General
1
Section – Title
EC7-1
BS 6031:1981
Some of those below
BS 5930:1999
BS code
Design of specific elements
Design aspects of construction activities
Ground investigation
Overall approach
General issues covered
EC7-2
General Planning of ground investigations Soil and rock sampling and groundwater measurement Field tests in soils and rocks Laboratory tests on soils and rocks Ground investigation report Planning strategies for geotechnical investigations
1 2 3 4 5 6 Annex B
Section – Title
New European documents
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BS EN 14731:2005 BS EN 15237:2007 BS EN 14475:2006
BS EN 14679:2005
BS EN 12063:1999
pr EN 14490
BS EN 1538:2000
BS EN 1537:2000
BS EN 12716:2001
BS EN 14199:2005
BS EN 12715:2000
BS EN 12699:2001
BS EN 1536:2000
BS EN 12063:1999
pr EN 14490
BS EN 14475:2006
Reinforced fill
Vertical drainage
Ground treatment by deep vibration
Deep mixing
Soil nailing
Jet grouting
Grouting
Sheet pile walls
Diaphragm walls
Ground anchors
Micropiles
Displacement piles
Bored piles
Sheet pile walls
Soil nailing
Reinforced fill
Standards for the execution of special geotechnical works (CEN TC288)
Table 1.1 The content of BS codes and their correspondence with the European documents
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General requirements and sample preparation
Classification tests
Chemical and electro-chemical tests
Compaction-related tests
Compressibility, permeability and durability tests
Part 2
Part 3
Part 4
Part 5
Methods of test for soils for civil engineering purposes
BS 1377:1990
Part 1
Site investigation
BS 5930:1999
BS code
Ground investigation report Planning of geotechnical investigations
5 Annex B
Detailed information on compaction testing of soils
Annex Q Detailed information on compressibility testing of soils
Annex R
Annex N Detailed information on chemical testing of soils
Annex M Detailed information on tests for classification, identification and description of soils
Detailed information on preparation of soil specimens for testing
Laboratory tests on soil and rock
4
Annex L
Field tests in soil and rock
3
List of test results of geotechnical test standards
Soil and rock sampling and groundwater measurements
2
Annex A
General planning of ground investigations
1
EC7-2
Incremental loading oedometer test
Atterberg limits
Particle size distribution
Density of solid particles
CEN ISO/TS 17892-5*
Note: There appear to be BS ENs in existence that have been drafted by committees concerned with aggregates. These will need to be reviewed to assess their applicability to soils.
CEN ISO/TS 17892-12*
CEN ISO/TS 17892-4*
CEN ISO/TS 17892-3*
CEN ISO/TS 17892-2*
CEN ISO/TS 17892-1*
Water content Density of fine grained soils
DD EN-ISO/TS 22475-3:2007
Sampling – conformity assessment
DD EN-ISO/TS 22475-2:2006
pr EN ISO 22282-6
BS EN-ISO 22475-1:2006
pr EN ISO 22282-5
Water permeability tests with packer and pulse-like stimulation
Sampling – qualification criteria
pr EN ISO 22282-4
Infiltrometer test
Sampling – principles
pr EN ISO 22282-3
Pumping tests
pr EN ISO 22282-2
Water pressure test in rock
Water permeability tests in a borehole without packer
General Rules
pr EN ISO 22282-1
pr EN ISO 14689-2
Part 2: Electronic data exchange – rock Geohydraulic testing
BS EN ISO 14689-1:2003
Part 1: Identification and description
Rocks
pr EN ISO 14688-3
BS EN ISO 14688-2:2004
Part 2: Classification principles Part 3: Electronic data exchange - soil
BS EN ISO 14688-1:2002
Part 1: Identification and description
Geotechnical investigation and testing – Identification and classification of soil:
CEN ISO standards Laboratory and field testing standards and technical specifications (CEN TC341). (see Appendix A6 for an explanation of the Note: “Technical Specifications” are identified by “TS” in the reference number. provenance of the different standards)
New European documents
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Table 1.2 The content of BS codes and testing standards and their correspondence with the European documents
7
Shear strength tests (effective stress)
In situ tests
Part 8
Part 9
Note
* will not be published in the UK
BS 8004:1986
Foundations
Shear strength tests (total stress)
Part 7
None
Consolidation and permeability tests in hydraulic cells and with pore pressure measurement
Part 6 Detailed information on permeability testing of soils
Example of ground water pressure derivations based on a model and long term measurements.
Annex C
Standard penetration test
Annex F
Flat dilatometer test Plate Loading Test
Preparation of specimens for testing of rock material Classification testing of rock material Swelling testing of rock material
Annex J Annex K
Annex T Annex U Annex V Annex W Strength testing of rock material
Field vane test
Annex I
Annex H Weight sounding test
Annex G Dynamic probing
Pressuremeter test
Annex E
Annex D Cone and piezocone penetration tests
Detailed information on strength testing of soils
Annex P
Annex O Detailed information on strength index testing of soils
Annex S
pr EN-ISO 22282-1 pr EN-ISO 22282-2 pr EN-ISO 22282-3 pr EN-ISO 22282-4 pr EN-ISO 22282-5 pr EN-ISO 22282-6
Permeability tests in a borehole Water pressure tests Pumping test Infiltrometer tests Closed systems packer tests
pr EN ISO 22477-5 pr EN ISO 22477-6 pr EN ISO 22477-7
Testing of anchorages Testing of nailing Testing of reinforced fill
pr EN ISO 22477-X
pr EN-ISO 22476-13
General rules (permeability)
pr EN ISO 22477-4
pr EN-ISO 22476-12
Plate Loading Test
Pile Load tesr – rapid axial loaded compression test
DD CEN-ISO/TS 22476-11:2005
Mechanical cone penetration test
pr EN ISO 22477-3
CEN-ISO/TS 22476-10*
Flat dilatometer test
Pile load test – dynamic axially loaded compression test
pr EN-ISO 22476-9
Weight sounding test
pr EN ISO 22477-2
pr EN-ISO 22476-8
Field vane test
Pile load test – static transversely loaded tension test
pr EN-ISO 22476-6
Full-displacement pressuremeter
pr EN ISO 22477-1
pr EN-ISO 22476-5
Self-boring pressuremeter test
Pile load test – static axially loaded tension test
pr EN-ISO 22476-4
Flexible dilatometer test
Pile load test – static axially loaded compression test
BS EN-ISO 22476-3:2005
BS EN-ISO 22476-2:2005
Dynamic Probing Menard pressuremeter test
pr EN-ISO 22476-1
Electric cone penetration test Standard Penetration Test
CEN ISO/TS 17892-9
Consolidated triaxial test
DD CEN ISO/TS 17892-6:2009*
CEN ISO/TS 17892-8*
Unconsolidated triaxial test Fall cone test
CEN ISO/TS 17892-10*
CEN ISO/TS 17892-7*
CEN ISO/TS 17892-11
Direct shear test
Unconfined compression test on fine grained soils
Permeability test
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Geotechnical design Eurocodes: BS EN 1997-1:2004 BS EN 1997-2:2007 Eurocodes: BS EN 1990:2002 Basis of structural design BS EN 1991-1-1:2002 Actions on structures
Test standard for technical specifications for ground properties
Geotechnical projects
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European standards for the Execution of special geotechnical works
ISO/CEN Standards for identification and classification Other structural Eurocodes eg BS EN 1993-5:2007
Note
Figure 1.1
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The different sources of these documents are explained in Appendix A6.
Diagrammatic representation of the suite of EU geotechnical and structural codes and standards
9
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2
Site characterisation and determination of ground property design values
2.1
Summary
2.2
1
EC7-2 makes compulsory the provision of a ground investigation report to all relevant parties.
2
EC7-2 is more prescriptive than BS 5930 in its planning and execution requirements for ground investigation.
3
The emphasis in EC7-1 on better prediction of settlement and deformation raises the importance of ground deformation properties. While the option is available in EC7-1 to use a reduced strength value, akin to the strength mobilisation factor used in BS 8002, the need for better knowledge of the deformation properties of the ground from additional and specific testing should presage a profound change in UK geotechnical practice.
4
Some parts of BS 1377 and BS 5930 have been and will continue to be replaced by new BS EN documentation.
5
Some departures from BS 5930 terminology apply for soil and rock descriptions.
6
Design values of ground properties may be assessed directly as an alternative to applying partial factors to characteristic values.
7
Procurement processes may need to be clearer about who does what, quality assurance, professional indemnity implications and communication between interested parties.
Ground investigation and testing The processes for obtaining ground parameters to use in design with EC7-1 are specified in several parts of the suite of European codes and standards, as indicated in Tables 1.1 and 1.2. It is evident that change over to the BS EN suite will entail the acquisition of many more standards and a period of adjustment to those documents that will replace BS 5930 and parts of BS 1377. Table 1.1 lists the standards and technical specifications (TS) that are being produced by CEN and ISO, which will replace UK codes, standards and practice, covering the same subject matter. Most of the standards and TS documents are to be finalised or, in some cases, drafted, so it is not yet possible to fully identify conflicts with UK procedures10. The main areas in which EC7-2 differs from current BS codes and standards requirements are listed in Table 2.1, from which it can be seen that a fundamental change involves the introduction of some compulsory activities (the word “shall” is used). This change has consequences for the procurement of site investigation, for the clear specification of who does what and for how information is disseminated to all appropriate parties. In fact, there are only two major changes to existing British site investigation practice and it can be expected that much if not all of the good practice guidance contained in, for example, BS 5930 will continue to apply in the future (see Section 6.4). The first major change concerns the provision of a geotechnical investigation report discussed later. The second concerns the effect of the requirement in EC7-1 for much greater consideration of settlement and deformation. This will entail much more attention being paid to the determination of the deformational properties of the ground, a difficult subject.
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Table 2.1
Some of the changes introduced by EC7-2
EC7-2 novel feature
Impact on practice as embodied in BS codes and standards
Use of “shall” in Principle clauses, rather than “should”11 (examples from Section 2 Planning of ground investigations)
if the main ground investigations do not supply the necessary information, complementary investigations shall be undertaken. Clients may come to appreciate that, if they fund more comprehensive, initial investigations, they can avoid the expense of further investigation at a later stage
it is stated that investigations shall be planned and data shall be adequate to manage risks12
the document states that a visual inspection shall be undertaken before planning the investigation programme and used in conjunction with a desk study
it also says that quality assurance systems shall be in place for all aspects of the work13
the necessary number of specimens to be tested shall be determined. Recommended numbers are contained in informative annexes but the status/validity of these will need to be discussed in the NA for EC7-2. It is unclear what the implications might be if the recommendations are ignored and things go wrong
a table of applicability of various field tests is also presented.
Section 3 on soil and rock sampling introduces categories of sampling method based on BS EN ISO 22475114.
this implies that only certain sampling methods can be used to obtain samples of a certain quality class
the quality class relates to use in specific laboratory tests in order to give the test results required for the selection of characteristic values15.
Section 4 Field tests in soils and rocks specifies that CEN standards shall be used when specifying tests. Conversion of test results into “derived” values is introduced
existing BS 1377 and BS 5930 sections specifying test methods will become redundant where a corresponding standard exists. If a technical specification is listed for a particular test then either this or the BS can be used. See Table 1.2 for the tests affected.
Section 5 Laboratory tests on soils and rocks
general statements occur with “shall” throughout the section. Much is simply good practice but if things go wrong then decisions taken relating to clauses saying “shall” will need to be justified
checks shall be made that the laboratory equipment used is adequate, fit for purpose, is calibrated and within the calibration requirements
there is a requirement that all test methods and procedures shall be reported
a quality assurance system shall be in place in the laboratory16
all descriptions shall be to BS EN ISO 14688-1:2002 and BS EN ISO 14688-2:2004
for the laboratory tests listed in Table 1.2 there will be no withdrawal of corresponding BS documents as the CEN documents are all only technical specifications (TS) and EC7-2 allows the NA to adopt National Standards in preference. This will be the case in the UK (only the TS for the fall cone will be adopted) (see NA to EC7-2 in 2009).
the report shall form part of the geotechnical design report
Section 6 Ground investigation report deals with what shall be in the report
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it shall state known limitations of the results
it shall include a presentation of all available information and geological features and a geotechnical evaluation of the information
all methods shall be documented in accordance with the relevant standards
it shall include all relevant information on how the derived values were arrived at.
11
EC7-2 identifies an explicit hierarchy of investigations that is also found in BS 5930:
geotechnical investigations17, which comprise the gathering of all relevant information about the site18 and a ground investigation
ground investigations, which comprise field investigations, laboratory testing and desk studies of geotechnical and geological information
field investigations, which comprise direct investigations (drilling, sampling and trial pits) and indirect investigations (in situ tests, such as the CPT).
The code further distinguishes between investigations for the purposes of design and for control.
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In a clear departure from most current practice, EC7-2 makes compulsory the provision of some form of ground investigation report (GIR) as part of the geotechnical design report. The code specifies that the GIR should include:
a presentation of all available geotechnical information including geological features and relevant data
a factual account of all field and laboratory investigations
a geotechnical evaluation of the information, stating the assumptions made in the interpretation of the test results
a statement of methods adopted (citing the relevant standards)
all relevant information on how a direct assessment of design values or “derived values” (see below) were determined, including any correlations used
any known limitations in the results.
The size of the GIR will depend on the complexity and value of the project, varying from a single page for a simple footing to volumes of pages for a major infrastructure project. While listing the general information required to reach a decision on the values of geotechnical parameters for a suitable design, it could be argued that EC7 places too much emphasis on the manipulation of test results and not enough on desk studies and other means to determine information on such matters as:
site geology, geomorphology and overall stability
Man’s influence on the site and the sensitivity of existing structures
local experience and relevant published knowledge.
In addition to gathering all pertinent facts already known about a site, determining the ground properties for design using the Eurocode suite could be seen as a logical sequence of:
carrying out tests and interpreting the test results
determining derived values
collating all geotechnical and other relevant information about the site
selecting characteristic values for factoring into design values, taking account of the requirements of the project19.
The testing and interpretation elements of this sequence are illustrated in Figure 2.1, which has been taken from EC7-2 and is further explained in Figure 2.2. In both, the term derived value is used – EC7-2 defines this as the value of a geotechnical parameter obtained from test results by theory, correlation or empiricism.
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The processes for obtaining derived values are essentially the same as current good practice – EC7 may be seen simply as attempting to codify these processes. It is this derived value that is used in EC7-1 for selecting a characteristic value from which to determine a design value. The selection is made in EC7-1 because, in the final analysis, it should be the designer’s responsibility. This may generate difficulty for the ground investigation contractor in some instances. For example, if the contractor is asked to provide characteristic rather than derived values of ground parameters, it may become difficult or risky unless there is sufficient information about the design situations and the limit states pertaining to the project.
2.3
Ground identification and classification
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For soil and rock descriptions (ISO-EN 14688-1, 14688-2 and 14689-1) some differences from BS 5930 terminology are apparent. Examples that have required a revision of our current terminology are shown in Table 2.2. Table 2.2
Some terminological changes Old terminology
New terminology
Slightly organic
Low-organic
Organic
Medium-organic
Highly organic
High-organic
Very soft, soft, firm, stiff and very stiff (terms used in BS 5930 to describe shear strength)
Now used to describe the consistency of silts and clays. Shear strength descriptors become: very low, low, medium, high, very high and extremely high, (though maintaining the same strength ranges used in BS 5930)
There are changes in the boundaries between the classes
(Fuller details can be found in Powell and Norbury, 2007 and Baldwin, Gosling and Brownlie, 2007). Some of the new field testing standards are likely to cover not only equipment specification, sizing and operation but also to introduce requirements for what might be termed “fitness for purpose”. In these, the test specification is related to its application and the required accuracy for the ground conditions, and for intended use of the results (this will become clearer as the documents are completed). As the various documents become available, detailed comparisons with BS documents will have to be undertaken to identify any potential conflicts.
2.4
Determining the design values of geotechnical parameters One of the biggest changes for UK practice is the formalised process in the Eurocode for determining the design values of ground properties using partial factors and characteristic values. EC7-1 states that design values of geotechnical parameters (Xd) shall either be derived20 from characteristic values using the following equation: Xd = Xk / γM
(Equation 2.2, BS EN 1997-1)
or shall be assessed directly (Clause 2.4.6.2(1)P). where Xk is the characteristic value and γM is the partial material factor.
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Although the Eurocode clearly permits the direct assessment of a design value, it places priority on the use of factored characteristic values. There are situations in which it is more appropriate to assess, for example, a strength where the critical state strength value will be used in the design. This may also apply to the design values of deformation properties since these are rarely measured and are commonly deduced from correlations with strength. In selecting the characteristic value, account should be taken of a number of matters that are listed in EC7-1, which then defines “characteristic value” as the characteristic value of a soil or rock parameter shall be selected as a cautious estimate of the value affecting the occurrence of the limit state.
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Design values of parameters may be required for both ultimate and serviceability limit state considerations. It is important to appreciate that, while the partial factor used to obtain the design value will have different values for ULS and SLS, so also may the characteristic value itself differ in calculations for these limit states. The meaning and selection of a characteristic value have been debated for many years21. It is important to realise that, despite the formalisation in the Eurocode, the selected value(s) is for the judgement of the designer, having considered all relevant information, including prior knowledge of the particular site and all ground testing and assessment data. As the values of the partial material factor γM in Equation 2.2 are fixed in the National Annex (see Section 6.3), the designer has control of the design value through the selection of the characteristic value22. Much has also been said23 about the merits or otherwise of using statistics in the determination of characteristic values and it is important to appreciate that the Eurocode does not require their use.
Note:
Figure 2.1
14
It is very important that the chosen correlation is appropriate for the prevailing geological condition.
Processing test measurements into design values of ground parameters
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Figure 2.2
General procedure for determining characteristic values from measured values Selecting characteristic values A list of the issues to be considered in determining characteristic values is shown in Figure 2.2. An example of how a characteristic profile of ground strength values might be obtained for a particular site is given in Appendix A1. It is important to appreciate that the selections made in Appendix A1 are quite subjective, so a risk-averse or risktaking designer might make a rather different selection which could be seen as a retention of the status quo in UK practice. Characteristic values of ground stiffness and weight density24 The basis of structural design Eurocode, BS EN 1990, states that “The structural stiffness parameters (eg moduli of elasticity … ) … should be represented by a mean value”25.
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In problems involving ground structure interaction the stiffness of the ground is often a very important parameter26. In these cases, the use of a mean value for ground stiffness is questionable27. As EC7-1 does not define a characteristic value of ground stiffness, it is suggested that its definition should follow that for strength ie a “cautious estimate” and not a mean value. EC7-1’s definition of characteristic strength value might be assumed also to apply to the weight density of soil and rock. However, the uncertainty about weight density is usually sufficiently low that there is no need to make a distinction between mean and cautious values.
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Other attempts to deal with uncertainty in ground parameters It is useful to compare the acquisition of conservative ground parameters in EC7-1 with approaches in other geotechnical design codes and guidance for ensuring the necessary caution in values for use in design. Historically, CIRIA R104 (Padfield and Mair, 1984) suggested that design may be based on moderately conservative values of parameters. Moderately conservative is defined as “a cautious estimate of the value relevant to the occurrence of the limit state” which compares closely with the EC7-1 definition. CIRIA C580 (Gaba et al, 2003) discusses these definitions.
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3
The new principles of geotechnical design in Eurocode 7
3.1
Summary
3.2
1
Clear separation of ultimate (failure) condition from serviceability (settlement and comfort) condition.
2
Need to be aware of the important distinction between “permanent” and “variable” actions, since different values of partial factors apply to each. Similarly for “favourable” and “unfavourable” actions.
3
Use of “characteristic” to define values of ground properties for use with partial factors to form design values.
4
Application of separate partial factors to several aspects of uncertainty, rather than a single lumped factor of safety applied to cover all uncertainty.
5
Partial factor values have been largely selected to avoid failure and are not necessarily sufficient to ensure acceptable movement. A check on movements will often be required.
6
Good communication between geotechnical and structural engineers is required for clarity on permanent and variable structural loads, and on tolerable movements of foundations. Communication between concerned parties will need to improve.
7
A simple alternative to performing settlement calculations for an SLS check is permitted in specific circumstances. This involves adopting a reduced value of strength in the ULS calculation in order to limit mobilised strains.
Introduction There are several key features of EC7-1 that make it different from the current BS geotechnical codes, these are:
unlike current codes, EC7 states that Principles shall be honoured. BS codes state only that things should be done28
EC7-1 embodies a design calculation methodology that makes sub-structure design fully compatible with superstructure design using the other structural Eurocodes29
EC7-1 explicitly identifies design limit states30
EC7-1, in ultimate limit state design calculations, makes use of partial factors applied to characteristic values of parameters, to account more directly for uncertainty in the values of parameters used in the calculations31 and to achieve compatibility with structural codes that also conform to the basis of structural design laid down in BS EN 199032
the formal adoption of four alternative methods for achieving a geotechnical design (see page 18)
EC7-1 makes compulsory the provision to the client of a geotechnical design report, while EC7-2 requires the provision of a ground investigation report, to form part of the geotechnical design report.
Together, these provide a single set of guiding principles for all geotechnical designs that is absent in the current, diverse set of BS design codes.
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EC7-1 explicitly identifies one or a combination of the following four design methods to be used to ensure that the performance of a geotechnical structure will avoid exceeding the specified limit states:
using prescriptive measures33 (see Section 3.3)
using tests on models or full scale tests34 (see Section 3.4)
using the Observational Method35 (see Section 3.5)
using calculations (see Section 3.7).
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Although they are found in our current national geotechnical codes, these alternatives are not as clearly recognisable as in EC7-1. Formal use of the Observational Method post-dates BS codes, although many of its features, such as monitoring of performance in the field, have been used for many decades. To help establish design requirements, EC7-1 recommends the classification of designs into Geotechnical categories (GC): GCs 1, 2 or 3 are proposed according to the complexity of the structure, of the ground conditions and of the loading and according to the level of risk that is acceptable for the purposes of the structure36. GCs may also be used to help establish the extent of site investigation required and the amount of effort to be put into checking that a design is satisfactory. No such classification system exists in BS geotechnical codes, although a similar system has been introduced for classifying retaining wall problems in CIRIA C580 (Gaba et al, 2003). As GCs are “recommended” their use is not compulsory in EC7-1. EC7-1 is as much about the “checking”37 of a design as about carrying out design. It does not provide detail of how design calculations are to be performed but rather presents a framework for ensuring that the resistance offered by a design to the destabilising actions on it is adequate to prevent a limiting condition being exceeded. The same could be said for some of the current BS codes, for example, BS 8004 does not describe in detail how to design footings or piles against bearing capacity failure or settlement. Conversely, BS 8002 and BS 8081 provide far greater detail than is found in EC7-1. In this chapter, the first three of the general design methods are briefly discussed before concentrating on design by calculation (Section 3.7), since it is in this regard that the principles of EC7-1 are most different from BS codes.
3.3
Design by prescriptive measures Prescriptive measures involve conventional and generally conservative rules in the design, and attention to specification and control of materials, workmanship, protection and maintenance procedures. Partial factors are not intended to be used with prescriptive measures38. Prescriptive measures usually involve the application of charts and tables that have been established from comparable experience39 and they implicitly contain their own safety factor. Very often, the concept of “allowable stress on the soil” is used in these charts or tables40. Prescriptive measures can be applied in cases where calculation models are not available or not appropriate41. Examples of the application of prescriptive measures concern durability, such as sacrificial thickness to accommodate corrosion loss, and local rules of good practice, such as prescribed depth of footing to avoid seasonal volume change in clay soil42.
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3.4
Design using load tests and tests on experimental models EC7-1 permits design based on the results of load tests. An example would be the design of a bored pile in clay using the total stress α method, or the design of anchorages, where in situ load testing is extensively used or relied upon.
3.5
Design using the Observational Method
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EC7-1 introduces design using the Observational Method, in which the design is reviewed in a planned manner during the course of the construction and in response to the observed performance of the structure. The essence of the safe use of the method is (a) anticipation of the modes of behaviour expected of the structure, and their consequences for the work, and (b) a precise plan of monitoring, with contingency planning for the actions to be taken when observations show departure from these modes. The minimum requirements to be met before and during construction are indicated in EC7-1. The advantage of this method is that it facilitates design where a prediction of the geotechnical behaviour is difficult, eg where the ground conditions are complex or not sufficiently well known, but where ground conditions will provide ductile, controllable behaviour. The Observational Method allows optimistic or pessimistic assumptions to be checked by monitoring the actual behaviour. The Observational Method often permits a less conservative design to be adopted because close monitoring and observation take place during construction, with contingency plans in case of adverse behaviour. Although not mentioned in EC7-1, reduced values of partial factors or a less cautious selection of the characteristic values of the soil properties might be used in the design in the knowledge that, if unsatisfactory performance is observed during construction, a contingency plan is available. The adoption of the Observational Method in UK design practice post-dates current BS codes and its incorporation into EC7-1 is an important development. The Observational Method is clearly explained in Advantages and limitations of the Observational Method in applied soil mechanics (Peck, 1969) and is comprehensively covered in CIRIA R185 (Nicholson et al, 1999).
3.6
Eurocode 7 – general design principles Before focusing on design by calculation, some of the basic and novel concepts of limit state design in the Eurocode require brief explanation.
3.6.1
Limit state design EC7-1 is a “limit state” design code which focuses on the avoidance of limiting conditions. In effect, limit states should not be exceeded. Limit states are defined as states beyond which the structure no longer fulfils the relevant design criteria. For example, a limit state could be:
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an unsafe situation
damage to the structure
economic loss.
19
In EC7-1, the avoidance of such limit states is achieved by requiring the design to deliver a particular result, for example that:
a foundation does not fail, causing the structure to collapse
a foundation does not settle to such a degree that the structure distorts and/or cracks.
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Ultimate limit states (ULS) of full collapse in or failure of geotechnical structures are fortunately quite rare. However, an ultimate state may develop in the supported structure because of large displacement of a foundation, which has itself not failed. This means, for example, that a foundation may be stationary, after initially settling, but part of the supported structure may have failed (for example, a beam has lost its bearing and collapsed owing to substantial deformation in the structure). EC7-1 requires that both possible states are avoided. In marked contrast to current British codes, five distinct ultimate limit states are identified in EC7-1. For simplicity, they are given the acronyms shown in parenthesis and bold italics have been used by the authors for emphasis:
loss of equilibrium of the structure or the ground, considered as a rigid body, in which the strengths of structural materials and the ground are insignificant in providing resistance [EQU] (an example would be the tilting of a rigid retaining structure about the edge of its foundation which bears on rock)
internal failure or excessive deformation of the structure or structural elements, including footings, piles, basement walls, etc, in which the strength of structural materials is significant in providing resistance [STR] (an example would be the resistance to cracking of a pile in tension)
failure or excessive deformation of the ground, in which the strength of soil or rock is significant in providing resistance [GEO] (examples would be overall stability of a slope and bearing resistance of spread or pile foundations)
loss of equilibrium of the structure or the ground due to uplift by water pressure (buoyancy) or other vertical actions [UPL]
hydraulic heave, internal erosion and piping in the ground caused by hydraulic gradients [HYD].
Of these five ultimate limit states, STR and GEO represent the critical issues normally faced by designers of simple geotechnical structures such as shallow spread footings, conventional piled foundations and low-height retaining walls, while UPL and HYD will be important for deep excavations and cuttings below the water table43. EQU will apply only on infrequent occasions, such as when checking the rotational stability of a rigid structure resting upon a rigid foundation44. Accordingly, the more detailed explanation of the design calculation framework given in Section 3.7.5 concentrates on calculations to check that STR and GEO limit states are not exceeded, with a brief discussion of the other limit states. A particularly important and novel feature of EC7-1 is the clear separation it makes between the ultimate limit state and the serviceability limit state. While making many statements about the need to ensure that an SLS is not exceeded, the code says very little about how to calculate settlements and deformations. This is important because the code proposes a combination of values of geotechnical parameters (such as shear strength) and partial factors in a design calculation that is specifically intended to avoid failure, rather than to limit deformations. This is in marked contrast to much “routine” geotechnical design practice in the UK where design values of parameters in effect
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arise from the use of global factors of, say, 2-3 on bearing capacity so as to prevent excessive settlement, a condition that is usually far removed from “collapse”. This distinction becomes especially important when avoiding large settlements (but not necessarily a ULS condition in the ground) that might cause the collapse of a structural element in the supported superstructure (for example, the bearing of a beam is lost by deformation of the structure when a large foundation settlement occurs).
3.6.2
Design requirements
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Limit states are generally checked by considering design situations in which adverse conditions apply. Design values, which are deliberately pessimistic, are used for the destabilising actions (eg loads), for the material strengths that provide the reactions and for the resistances of geotechnical elements. Design values are used in calculations for both ULS and SLS45. Since the aim of limit state design is (a) generally to avoid limit states and (b) specifically to make any possibility of exceeding a ULS very remote, EC7-1 may require the adoption in calculations of design values for parameters that appear to be remote from reality.
3.6.3
Design situations EC7-1 presents a list of items for consideration when identifying design situations. The geotechnical design must be checked for all relevant design situations. These should be selected to encompass all conditions which are reasonably foreseeable as likely to occur during the construction and use of the structure. EC7-1 deals with persistent, transient and accidental situations, for ultimate and serviceability limit states46. These situations are largely distinguished from each other by time47. The safety requirements may be different in specific design situations. For example, in an accidental situation, a structure may be required merely not to collapse, with the serviceability condition being of limited concern. Seismic design situations are rarely of general concern in the UK. Though not for this reason, since seismicity is a problem for several EU member states, EC7-1 does not deal with seismic design and refers the reader to EN 1998-5.
3.6.4
Durability Durability is the ability of a structure to remain fit for use during its design life, taking into account any required maintenance. For geotechnical structures, maintenance is often difficult or impossible. The design should take into account any potential degradation of materials with time due to any aggressiveness of the environment (ground, groundwater chemistry) by providing adequately resistant materials or protection for them48.
3.7
Design by calculation
3.7.1
The application of safety in limit state design calculations Current British codes contain a range of different ways of introducing safety into a geotechnical design, from the “working state” method using a global safety factor, generally adopted in BS 8004, to the partial factor method adopted in BS 8081. BS 8002 adopts yet another approach, that of the “(strength) mobilisation factor” whereby movement is limited by reducing in the calculation the value of strength (and thus strain) mobilised in the ground.
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Most users of BS 8004 will have performed a design calculation that, by virtue of the (large) global factor of safety employed will have ensured not only that a ULS is avoided but also that settlements are acceptably small. In contrast, EC7-1, in ULS calculations, uses partial factor values that are intended to ensure that geotechnical failure is avoided, and may not be large enough to keep settlements to an acceptable level, particularly for softer, more compressible ground such as normally-consolidated or lightly over-consolidated clay or granular soil49.
3.7.2
ULS design calculations
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The Eurocode design philosophy clearly separates ULS from SLS, and deals rather more explicitly with the identification and treatment of the many uncertainties inherent in a design problem. Since most of EC7-1 is concerned with checking the avoidance of a ULS, this rigour is applied to the uncertainties in the calculation of, for example, bearing capacity from ground strength rather than to settlement calculations for checking the avoidance of an SLS. ULS design calculations in the Eurocodes involve the following systematic activities:
establishing actions50
establishing material (ground) properties
establishing (ground) resistances
setting up calculation models51 for the relevant ultimate and serviceability limit states52
showing in a calculation that limit states will not be exceeded.
Particular care is required when applying partial factors in numerical modelling using computer software.
3.7.3
Actions and their effects The values of the actions from the superstructure supported by the geotechnical substructure are obtained from BS EN 1991-1-153. These values will be characteristic ones (Fk) and will, in the geotechnical design, be factored by an appropriate partial action factor (γF), given in BS EN 1990 and repeated in EC7-1. EC7-1 identifies the geotechnical actions (eg active earth pressures on a retaining wall, and downdrag on piles) and deals with the effects of actions, given the symbol E54. EC7-1, in Clause 2.4.2(2)P states that the values of geotechnical actions to be used shall be selected, since they are known before a calculation is performed – they may change during that calculation. A definition of “geotechnical action” has been given in the Glossary, and is based on one provided in BS EN 1990. Generally, actions may be permanent (eg self-weight of structures or soil), variable (eg imposed loads on the surface of retained ground) or accidental (eg impact loads). It can be argued that EC7-1 promotes better communication between structural and geotechnical engineers by requiring greater awareness of what constitute permanent and variable structural actions. Given the greater emphasis on avoiding unwanted deformations, geotechnical engineers will need to be confident that characteristic values of actions from the structure are sufficiently well reported to them by structural engineers. It is important to distinguish between actions imposed by the structure on the ground and the effects of geotechnical actions imposed by the ground on the structure. It is
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also important to decide if actions are “favourable” (ie act to stabilise the structure) or “unfavourable” (ie act to destabilise the structure) since EC7-1 applies factors of different value depending on the stabilising (or otherwise) effect of the action, see Table A3.1. An unfavourable geotechnical action may have a favourable effect. For example, in the case of an embedded retaining wall, the unfavourable active earth pressure will generate a favourable passive earth pressure, that is a geotechnical resistance55. Care should be taken when using numerical modelling in a ULS design. The application of ULS γ values of > 1 to actions such as water pressures can lead to physically impossible stress states. In such circumstances it is appropriate to apply partial factors to the effects of actions (moments, shear forces etc). For embedded retaining walls recommended practice is to factor the effects of actions for both serviceability and ultimate states, as the serviceability condition can often be more onerous. This has become accepted practice in the UK although the factors given in Table A.3, for instance, are strictly applicable to the ULS condition.
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3.7.4
Geotechnical resistances Once the characteristic value of the ground strength parameter has been obtained, the appropriate partial factor is applied to arrive at a design value for use in the calculation model that delivers the required geotechnical resistance (Rd)56. Alternatively, the design value of the strength may be assessed directly, without recourse to factoring characteristic values.
3.7.5
The GEO and STR ULS calculations All ULS calculations in EC7-1 start from the basic inequality57: Ed ≤ R d where: Ed
is the design value of the effects of all the actions
Rd
is the design value of the corresponding resistance of the ground and/or structure.
Design values of the effects of the actions The effects of an action (such as the bending moments and shear forces in foundations) are functions of the load (action) from the structure, of any geotechnical action that depends on ground properties, and of the geometry of the geotechnical structure: Ed = E {γF Frep ; Xk/γM58 ; ad}
(this is the so-called “Design Approach 1”– see below)
The meanings of the symbols are:
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Frep
the representative59 (or characteristic Fk) value of an action (or of the effect of an action)
γF
the partial factor for an action (or for the effect of an action, γE60)
Xk
the characteristic value of a material (ground) property
γM
the partial factor for the material property
ad
the design value of a geometrical property (eg the depth to a water table)
23
Design values of resistance In general terms, the design value of the resistance, Rd, of the ground is a function of the design value of the applied load (γF Frep), the ground strength, Xk/γM, and the design value of any relevant geometrical quantity, ad. To obtain Rd the partial factors may be applied either to ground properties (Xk) or to resistance (R), as follows: Rd = R {γF Frep ; Xk/γM ; ad} (this is the so-called “Design Approach 1” that applies generally for geotechnical resistances) or Rd = R {γF Frep; Xk ; ad}/γR (in “Design Approach 1”, this expression only applies to the design resistance of piles and anchorages)
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where γR is a partial factor for the resistance of the ground. In the first expression, the design value of the resistance is obtained by applying the partial factor γM> 1 to the characteristic values of the ground strength parameters ck′ and ϕk′ or cu;k etc. If actions play a role in the resistance, design values of actions (γF Frep) are introduced into the calculation of Rd61. In the second expression, the design value of the resistance is obtained by applying the partial factor γR> 1 to the resistance obtained using characteristic values of the ground strength parameters62. If actions play a role in the resistance, design values of actions (γF Frep) are introduced in the calculation of Rd but with γG = 1 for permanent actions (and γQ = 1.3 for variable actions), so that the expression becomes: Rd = R {Frep ; Xk ; ad}/γR During the development of EC7-1 there was much debate about the different forms of these expressions. In order to reach consensus among the EU member states, EC7-1 settled on three alternative design approaches (DAs), each using different forms of the expressions and, sometimes, different recommended partial factor values. In the National Annex to BS EN 1997-1 only Design Approach 1 (DA-1) is permitted. Appendix A3 explains how DA-1 is applied and provides a reference to explanations of the other two design approaches.
3.7.6
Serviceability limit state design EC7-1, while requiring a more explicit consideration of settlement and serviceability limits than some BS codes, does not prescribe how settlements are to be calculated, although a simple elasticity method is outlined in Annex F63 of the Eurocode64. It should be remembered that settlements are notoriously difficult to calculate with any accuracy and that use of relatively simple elasticity methods is best left to a suitably experienced geotechnical engineer. Limit state design requires that the occurrence of a serviceability limit state is sufficiently improbable. Serviceability limit states may be checked in two ways:
by calculating the design values of the effects of the actions Ed (eg deformations, differential settlements, vibrations etc) and comparing them with limiting values, Cd
by a simplified method, based on comparable experience.
For checking an SLS design, design values of actions and of material properties will
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normally be equal to their characteristic values, that is the γF and γM values will be equal to 1. In cases where differential settlements are calculated, a combination of upper and lower characteristic values of deformation moduli should be considered, to account for any local variations in the ground properties. It should be appreciated that it may not be appropriate, notwithstanding that different γ values will be applied to arrive at a design values, to adopt the same characteristic value of strength in both ULS and SLS calculations. To do so may result in unreasonable estimates of forces and moments in the SLS.
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In a perfect world, limiting values of deformations, Cd, might be specified as design requirements for each supported structure. EC7-1 lists a series of items to take into account when establishing limiting values of movement65, 66 but does not recommend any specific values of limiting deformations. However this is not remarkable since nor do current BS codes67. As a simple alternative to performing serviceability checks using calculations, EC7-1 permits the designer to show that a sufficiently low fraction of the ground strength is mobilised to keep deformations within the required serviceability limit. This simplified method requires the existence of comparable experience with similar soil and structure. This clearly restricts the circumstances in which the simplified method may be applied to conventional structures and foundations in familiar ground conditions. The simplified method is applied in EC7-1 to spread foundations, to pile foundations and to retaining structures. EC7-1 gives no indication of what is a “sufficiently low fraction of ground strength”68. However, for spread foundations the code states: For conventional structures founded on clays, the ratio of the bearing capacity of the ground, at its initial undrained shear strength, to the applied serviceability loading should be calculated (see 2.4.8(4)). If this ratio is less than 3, calculations of settlements should always be undertaken. If the ratio is less than 2, the calculations should take account of non-linear stiffness effects in the ground. Clause 6.6.1(16). It should be noted that even when the ratio exceeds 2, non-linear and scale effects can play an important role in determining settlement.
3.7.7
The EQU limit state EC7-1 stipulates that the following inequality must be satisfied: Edst;d ≤ Estb;d + Td This means that the design value of the destabilising action Edst;d (eg the overturning moment from earth or water pressures) should be less than the design value of the stabilising action Estb;d (eg the restoring moment due to the weight of the structure) plus any contribution from the design value of shearing resistance, Td, on the sides of structures in the ground (the contribution from Td should be of minor significance).
3.7.8
The UPL limit state The UPL limit state applies in circumstances such as where a new building basement will be excavated below the water-table so that uplift pressures may be resisted by piling the basement slab. To check that failure will not occur, EC7-1 requires the following inequality to be satisfied:
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25
Vdst;d ≤ Gstb;d + Rd where Vdst;d is the sum of Gdst;d and Qdst;d, the design values, respectively, of the permanent and variable destabilising actions, such as water pressures under the structure and any other upward or pull-out force. Gstb;d and Rd are the design values of the stabilising permanent actions, such as the weight of the structure and/or of the ground, and the resistance of any additional structures such as holding-down piles or anchorages. Example 4.8 illustrates the design of a structure subjected to uplift water pressures.
3.7.9
The HYD limit state
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The resistance to failure by heave due to seepage of water in the ground is checked using either stresses or forces as the variables. As it does not depart from current best practices in the UK, design to avoid this limit state is not discussed further in this book. Further guidance may be found in Frank et al (2004).
3.8
The difference between DA-1 and traditional design calculations In traditional design calculations such as those largely employed in the BS geotechnical codes, a limit state is avoided by making sure that stresses in materials, eg in the soil beneath a foundation, are kept to working levels. This is ensured by applying a global safety factor in the calculation for a working state design. For example, in: E ≤ R/FS E
is the disturbance or force acting
R
is the resistance offered by the structure (eg the bearing pressure beneath the footing)
FS
is a factor to make sure that E is “sufficiently” less than R.
The precise nature and magnitudes of E and R are not often clearly specified in current national geotechnical codes. Traditionally, FS has been a fairly large number, for example 3 for a simple strip footing and 2-3 for a pile69, 70, 71, 72. Through experience, these factor values were found by testing and from the back-analysis of observations, firstly to prevent failure and secondly to ensure that settlements under working loads remained acceptably small. In the global safety factor method, checking that both ULS and SLS requirements were met was performed in the one calculation. This traditional, “lumped” factor approach merged all the uncertainty into one factor value, as well as providing an element for limiting the strength mobilised and the settlements. In contrast, the EC7-1 limit state design philosophy73 clearly separates ULS from SLS. It also deals rather more rigorously with the identification and treatment of the many uncertainties inherent in a design problem by:
26
making a clear distinction between (destabilising) actions from the superstructure and from the ground
CIRIA C641
by separating the uncertainties in these actions, with the partial factors for structural actions coming from BS EN 1990 and, for geotechnical actions, from EC7-1
by separating the uncertainty in the (stabilising) reactions from the ground from that of the structural loads.
At first sight the STR and GEO γM and γR factor values required in EC7-1 (and reproduced in Table A3.1) might seem quite small when compared with traditional “lumped” factor values. It is important to appreciate that these values have been developed in combination with the γF and γE values that are prescribed in BS EN 1990 for the avoidance of a ULS, following calibration studies in which designs using global factors were reproduced. Given the emphasis in EC7-1 on avoiding a ULS, the combination of these γ values will not necessarily be sufficient to avoid an SLS and separate checks will usually be required.
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EC7-1 permits the application of additional “model” factors in some calculations. One such is discussed in Section 4.4.1 and illustrated in Examples 4.1 and 4.2.
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27
4
Specific changes in design principles with examples
4.1
Summary EC7-1 requires the serviceability limits for the structure to be clearly identified so that the geotechnical designer is able to demonstrate that settlement of foundations is acceptable.
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1
4.2
This is likely to lead to the more frequent need for structural engineers to indicate the tolerable deformations of the super-structure, which has not been common practice.
2
A significant change for pile design involves the introduction of “correlation factors” that are intended to encourage more pile testing and ground investigation.
3
For retaining structures and in contrast to BS 8002, EC7-1 has no specific requirement for a minimum surcharge loading on the retained ground surface.
4
EC7-1 requires that the GDR be updated as observations yield further information about the geotechnical project.
Introduction Chapter 3 presented the general principles of geotechnical design that are different from traditional British practice. In this chapter, we examine some differences that are specific to particular geotechnical elements and structures and give worked examples showing how to apply EC7-1, using DA-1, and how it compares with current practice. The worked examples have been selected to illustrate as clearly as possible the way in which the principles of EC7-1 are applied.
4.3
Spread foundations The code offers one of three design methods to be adopted:
a direct method which in principle involves two separate calculations:
a ULS calculation using ground properties
a settlement calculation to check the SLS requirements.
an indirect method in which a single calculation (for all limit states) is based on comparable experience (an essential prerequisite), and which uses the results of field or laboratory measurements or other observations and SLS loads
a prescriptive method, which is usually based on comparable experience of the observation of serviceability performance (see Section 3.3).
Direct calculation method for GEO and STR ULS design In the fundamental ULS requirement, represented by the familiar inequality: E d ≤ Rd Rd may be calculated using analytical or semi-empirical formulae. Annex D of EC7-1 provides widely-recognised formulae for bearing resistance74 that apply for homogeneous ground, a condition that is only rarely encountered in the UK.
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CIRIA C641
Indirect method for combined ULS and SLS design A typical indirect method would be based on the results of a field test. While EC7-1 includes an approach using a (pressuremeter) test not commonly used in the UK75, other, more common indirect methods could be based on the results of the standard penetration test or cone penetration test76. Prescriptive method for combined ULS and SLS design Allowable bearing pressures are prescribed, for simple strip footings, for example in tables in the Building Regulations and the allowable bearing pressures in BS 8004.
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While a specific example showing the design of a spread foundation has not been included here, aspects such as checking sliding resistance are included in Examples 4.3 to 4.5 on the design of cantilever retaining walls. Example calculations for design against bearing failure may be found elsewhere (Frank et al, 2004 and Driscoll et al, 2005) and are not repeated in this publication.
4.4
Piles
4.4.1
Specific changes/issues (a) Serviceability: In EC7-1, pile design by calculation concentrates on ULS avoidance. This means that the values of partial factors for pile resistance (reproduced in condensed form in Table A3.1) have been determined, in combination with characteristic values and the partial factors for actions, to prevent failure, where this is defined as being a settlement of 10 per cent of the pile diameter77,78. Most pile design in the UK is based purely on the ULS capacity, estimated from site test data and empirical factors with sufficiently large values to control settlements. In this approach pile capacity essentially corresponds to the asymptote of a load-displacement graph. As there is no direct assessment of pile settlement, the definition of ultimate capacity is inconsistent with the 10 per cent diameter definition of EC7-1 (and incidentally BS 8004). To control serviceability the ratio of shaft friction to specified pile working load is often set at or above unity to ensure settlements are low, but serviceability is not explicitly checked. However, EC7-1 requires a check with structural engineers on the serviceability limits for the structure, which leads to the need to demonstrate the acceptable settlement of piles. As it is not usual to assess pile settlement, this constitutes a major change from routine British practice in pile design. However, EC7-1 requires a check with structural engineers on the serviceability limits for the structure, which leads to the need to demonstrate the acceptable settlement of piles. As it is not usual to assess pile settlement, this constitutes a major change from routine British practice in pile design. In the UK National Annex to EC7-1, BSI has permitted the use of different partial resistance factor (γR, set R4) values depending on the verification of SLS, with a lesser magnitude value being available for explicit verification, for example, see Table A.NA.6 for a driven pile. In this case, the lower γR values in R4 may be adopted (a) if the serviceability limit is verified by load tests (preliminary and/or working) carried out on more than one per cent of the constructed piles to loads not less than 1.5 times the representative load for which they are designed, or (b) if settlement is explicitly predicted by a means no less reliable than in (a) (authors italics), or (c) if settlement at the serviceability limit state is of no concern. Note that in particular circumstances EC7-1 also permits settlement calculations to be replaced by
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29
bearing capacity calculations with a higher value of factor so that a sufficiently low fraction of the ground strength is mobilised (Clause 2.4.8(4)). Users of EC7-1 who adopt the definition of failure as being a settlement of 10 per cent of the pile diameter may face a dilemma when estimating pile settlement using, for example, the Fleming method79 (Fleming, 1992). What value of shaft and base resistance should be used and, if available capacity beyond a settlement of 10 per cent of pile diameter is ignored, how meaningful will the settlement analysis be? A designer may need to make additional and specific allowance to further limit pile movements80. The above remarks apply to single piles and not to piles in groups. EC7-1 has very little to say about pile groups, which are generally designed for serviceability not ULS.
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(b) ULS The other significant change involves the introduction of the use of “correlation factors” ξ81. These are intended to provide increasing design benefit, in the form of reductions in the value of the correlation factor, progressively with either increasing numbers of pile load tests or profiles82 of ground parameter data. The correlation factors are used in combination with mean and minimum values of pile test result or ground profile value so as to include some allowance for the variability encountered. In the UK it is likely, given the variability commonly encountered, that ξ will only be used with pile load test results and then only where the test results show reasonable repeatability. In rare circumstances where the local geology is constant, such as an estuarine clay deposit, a consistent set of ground property profiles, obtained using the CPT for example, might permit correlation factors to be used on them. For those most common occasions when design is based on ground test data, EC7-1 provides two calculation methods:
a procedure in which the bearing resistance is calculated using results from one or more profiles of ground test results83
an alternative procedure, where the ground test results for all test locations are first combined in order to derive the characteristic values of base resistance and shaft friction in the various strata84, as illustrated in Figure 4.1.
In another departure from common practice, account should be taken when applying a correlation factor of the ability of the structure connecting the piles to transfer loads from weaker to stronger piles. If structural engineers agree that the structure is able to do so, the value of ξ may be divided by 1.1 provided the resulting value does not fall below 1. EC7-1 differs slightly from BS 8004 in stating that a check of buckling of slender piles is not required if the undrained shear strength, cu , of the soil exceeds 10 kPa, whereas BS 8004 implies that a check is required for cu less that 20 kPa. Here are two examples illustrating features of EC7-1 for the design of piles. The first demonstrates that, at least until the user has become familiar with the code, a settlement calculation will frequently be necessary to ensure that the design satisfies the SLS. However, since the authors contend that currently most designers do not have sufficient confidence that settlement can explicitly be predicted by a means no less reliable than a pile load test, they cannot justify the adoption of the lower partial factor values given in Table A.NA.6. Over time, however, as settlement calculations are
30
CIRIA C641
verified/modified to reflect actual test behaviour (for given soil types) the designer may acquire sufficient experience to use them confidently. For this reason, Example 4.1 shows a serviceability calculation, as required by BSEN1997-1, but adopts the higher partial factors given in the NA. Example 4.2 illustrates the way in which favourable resistance and unfavourable (downdrag) action are separately factored, with different factor values.
Assemble available ground test data
Note that this option is not likely unless the ground is locally uniform
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Yes
More than one profile of data?
No
Select suitable profiles of ground test data. Number of profiles = n
Find values of ξ3 and ξ4 from National Annex, Table A.NA.10, depending on value of n
Calculate characteristic pile resistances for the different profiles of ground properties, divide mean and minimum values for the resistances by their respective ξ values and take the minimum of these two results as the characteristic resistance to be factored by γ into the design resistance. Note: this process attempts in a simple way (a) to provide benefit from a greater quantity of data and (b) to reflect the variability between the ground properties at the locations of the profiles.
Figure 4.1
CIRIA C641
This alternative is likely to be most commonly used in UK practice
Calculate characteristic pile resistances from this single profile of ground properties and apply partial factors for base (γb) and shaft (γs) to determine design resistances. Note: an additional “model” factor will be required, since the ξ factor is not applied in this alternative. The value of this factor is indicated in the National Annex to EC7-1 (see Endnote 84).
Alternative procedures for pile design using profiles of ground properties
31
Example 4.1
Design of a vertical, pre-cast concrete pile driven into sand and gravel
Project
Subject
Example 4.1
Pre-cast Concrete Driven Pile
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Pile D esign Drive pile in sand and gravel
Note: This example demonstrates how a pile design should be approached using EC7. The calculation to determine the length of pile required is shown below. Section 7 of the code should be consulted when undertaking a complete design to determine which other limit states should be considered. Purpose of calculation: To determine length of pile required.
Clauses 2.4.7.3.4.2 (2) Table A.NA.3. NA.A1.2(B) (Permanent Unfavourable) [1]
7.6.2.1 (2)
Design Approach 1. (Axia lly loaded piles) Combination 1: A1 “+” M1 “+” R1
Design Action (Load) (A1) Partial Factor, ȖG Fc; d1
=
1.35
=
2000 × 1.35
Fc; d1 =2700kN
Note: For transparency in the calculation any difference in the weight of the pile and the displaced overburden load is not included.
Basic Pile Resistance Factors Material Factors (M1) Table A.NA.4
All partial factors = 1.0 Note: No modification to adopted soil parameters is required for the design of axially loaded piles.
32
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Resistance (R1) Base resistance formula: Nq. Vv ’. Ab Nq – bearing capacity factor Vv ’ – vertical effective stress at the pile toe Ab – area of the base of pile Bearing capacity factor:
Berezantsev et al (1961) [2]
I’ = 35q Nq = 55 (D/B = 20) Area of base: Ab = [ʌ x (0.6)²] /4 = 0.28m²
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Shaft resistance formula: Ks. Vv ’. tanG. As Ks – lateral load factor Vv ’ – average effective stress on the pile shaft tanG - mobilised friction at the pile-soil interface As – area of the pile shaft Kulhawy (1985)
Ks = 1 (Driven pile, large displacement) tanG = tan (0.8I’) (Precast concrete)
[3] 7.6.2.3(4) Table A.NA.6 NA (2007) A.3.3.2
tanG = 0.53 Design Resistance (R1) Partial factors for driven piles in compression, Ȗb & Ȗs = 1.0 Model Factor, ȖR;d = 1.4 Design Base Resistance: Rb;d1 = (55 x Vv ’b x 0.28)/(Ȗb x ȖR;d )
[4]
Rb;d1 = 11Vv ’b Design Shaft Resistance: Rs;d1 = (1 x Vv ’s x 0.53 x (0.6 x ʌ x L))/(Ȗs x ȖR;d )
[5]
Rs;d1 = 0.71Vv ’s L Design Compressive Resistance Rc;d = Rb;d + Rs;d Determination of length of pile to carry prescribed load Try 15m long pile Rb;d1 = 11 x (2 x 20 + 13 x 10) = 1870kN
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33
= 0.71 x {[(0+40)/2 x 2] + [(40 + 170)/2 x 13]} = 998kN
Rs;d1
[6]
Rc;d1 = 1870 + 998 = 2868kN (> Fc;d1 (2700kN))
Rc;d1 = 2868kN
Conclusion: A pile 15m long, 600mm diameter can carry the load of 2000kN under Combination 1.
Combination 2: A2 “+” M1 “+” R4 Design Action (A2)
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Table A.NA.3 NA.A1.2(C) (Permanent Unfavourable)
Table A.NA.4
7.6.2.3(4) NA(2007) A.3.3.2 Table A.NA.6
Partial Factor, ȖG = 1.0 Fc;d2 = 2000 ×1.0 Fc;d2 = 2000
Material Factors (M1) for Combination 1 and Combination 2 are the same, ie M1 Design Resistance (R4) Model Factor, ȖR;d = 1.4 Partial factors for driven piles in compression without explicit verification of SLS. Ȗb = 1.7 and Ȗs = 1.5 Therefore Rc;d2 = Rb;d1 /1.7 + Rs;d1 /1.5 For 17m long pile Rb;d1 = 11 x (2 x 20 + 15 x 10) = 2090kN Rs;d1 = 0.71x{[(0+40)/2x2]+[(40+190)/2x15]}=1253kN Rc;d2 =
2090 1253 + = 2064kN (>Fc;d2 (2000kN)) 1.7 1.5
Rc;d2 = 2064kN
Conclusion: A pile 17m long, 600mm diameter can carry the load of 2000kN under Combination 2 7.6.4.1(1)
An example of the vertical settlem ent of the pile is det ermined using procedures developed by Poulos & Davis (1980): Pile foundation analysis and design (Wiley).
For Floating Pile. Poulos & Davis (P&D). Section 5.3.2
34
ȕ= ȕo C k C ȣ
CIRIA C641
ȕ =P b/p = proportion of applied load transferred to pile tip ȕo = tip load proportion for uncompressible pile C k = correction factor for pile compressibility
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C ȣ = correction factor for poissons ratio of soil.
P&D Fig 5.11
L/d = 17/0.6 = 28; db/d = 1 ȕo = 0.55 db – diameter of bas e of pile d – diameter of shaft
P&D Fig 5.12
Take k=1000 (Ratio stiffness of pile to stiffness of ground) C k = 0.9
P&D Fig 5.13
Poisson’s ratio, ȣ = 0.3 C ȣ = 0.79 So, ȕ = 0.055 x 0.9 x 0.79 = 0.04
Overall load vs settlement Ultimate shaft resistance (unfactored values) c.f. Equation [5] P su = (0.6 x x 1 x 0.53 x {[(0 + 40)/2 x 2] + [(40 + 190)/2 x 15]} = 1763kN P&D. Eqn 5.53
Full shaft yield P y1 =
P su = 1–ȕ
1763 = 1836kN 1 – 0.04
Settlement at full shaft yield U y1 = P&D Eqn 5.54
I Esd
P y1
I = IO Rk RhRy Rk – correction factor for pile compressibility Rh – correction factor for rigid layer (take as 1.0) R y – correction for soil poissons ratio IO – settlement influence factor for incompressible pile IO = 0.07
P&D Fig 5.18
L/d = 28, d0/d = 1
P&D Fig 5.19
K – stiffness ratio soil/pile – take as 1000 Rk = 1.15
P&D Fig 5.21
Qs = 0.3 R y = 0.93 So, based on average soil stiffness of 30MPa I = 0.07 x 1.15 x 1.0 x 0.93 = 0.075
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35
ȡy1 = ȡy1
0.075 x 1836 (30 x 10 3) x 0.6 = 8mm
Settlement at 2000kN = 8 x 2000 1836
Settlement = 9mm
Overall Conclusion: 1 2
A pile 17m long, 600mm diameter can carry the load of 2000kN under Combination 2. R R Combination 2 is critical as c;d2 < c;d1 F F c;d2
3
c;d1
Settlement analysis has shown the pile will settle less than 10mm under the working load of 2000kN
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NOTE: 1
Traditional pile analysis has been based broadly on obtaining an appropriate site investigation and then varying the overall factor of safety dependent on pile testing proposed. Eg FofS = 3 – No pile t ests FofS = 2.5 – Test 1% of working piles FofS = 2 – Undertake a preliminary pile test Using this approach it has been expected that settlement criteria would be met, without the need for explicit analyses.
2
The equivalent factors of safety for the above analyses are: Combination 1 ȖG × Model Factor × Ȗ(b&s) = 1.35 × 1.4 × 1.0
FofS = 1.89
Combination 2 ȖG × Model Factor × Ȗ(b&s) = 1.0 × 1.4 × (1.5 to 1.7) 3
FofS = 2.1 to 2.38
The NA states that lower partial factors can be adopted if, “settlement is explicitly predicted by a method no less relia ble than a pile load test.”
The authors’ contention is that currently most designers would not have this degree of confidence. Over tim e, however, as settlement calculations are verified/modified to reflect actual test behaviour (for given soil types) the d esigner may acquire sufficient experience to justify the adoption of the lower partial factors. For the above reasons this example shows a serviceability calculation, as required by BSEN1997-1, but adopts the higher partial factors given in the NA. 36
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Example 4.2
Pile design incorporating negative skin friction (downdrag)
Project
Subject
Example 4.2
Pile design incorporating negative skin friction (downdrag)
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Pile D esign Bored pile in clay with downdrag
Clauses
2.4.7.3.4.2 (2)
Purpose of calculation: To determine the length of pile required.
Design Approach 1. (Axia lly loaded piles) Combination 1: A1 “+” M1 “+” R1 Characteristics values of Actions Applied force, Fk = 1000KN
7.3.2.2
Negative s kin friction, S D = D C u = Cu
(take adhesion factor, D = 1)
Downdrag force, Fnsf;k = x 0.6 x 4 x 20 = 150.8KN Design Actions (A1) Table A.NA.3 NA.A1.2(B) (Permanent Unfavourable)
Partial factor, ȖG = 1.35 (for both applied and downdrag force) Applied force, Fd1 = 1000 × 1.35 = 1350kN Downdrag force, Fnsf;d1 = 150.8 x 1.35 = 203.6kN
[1]
Total downward force, Fc;d1 = 1350 + 203.6
Fc;d1=1554kN
Note: For transparency in the calculation any difference in the weight of the pile and the displaced overburden load is not included.
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37
Skempton (1951) B/L = 1, D/B>5
Basic Pile Resistance Factors Pile Base: 9 C u Pile Shaft: Į C u For this case, adopt Į = 0.5 Pile Shaft: 0.5 C u
Table A.NA.4 Undrained Shear Strength
Design Soil Parameters (M1) Partial factor
Ȗcu
= 1.0
C u;d = C u;k /1.0 C u;d = C u;k
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Characteristic Resistance (R1) [2]
Base Resistance: Rb;k = 9.C u;d .Ab
[3]
Shaft resistance: Rs;k = 0.5.C u;d .As Note: C u is the average undrained shear strength of the clay along the pile shaft. Design Resistance (R1)
7.6.2.3 Table A.NA.7
Note: Data obtained from ground test results Partial factors for bored piles Ȗb = 1.0 Ȗs = 1.0
7.6.2.3 (8) NA (2007) A.3.3.2 [4]
Note: When d eriving characteristic values for pile design from ground parameters partial factors have to be corrected by a Model Factor. Model Factor, ȖR;d = 1.4 Partial factors for pile resistance for bored piles:
[5]
Ȗb × ȖR;d = 1.0 × 1.4 = 1.4
[6]
Ȗs × ȖR;d = 1.0 × 1.4 = 1.4
[7]
Design Base Resistance:
Rb;d = (9/1.4) C u;d .Ab = 6.4 C u;d .Ab
Design Shaft Resistance:
Rs;d = (0.5/1.4) C u;d .As = 0.36 C u;d .As
Design Compressive Resistance: Rc;d = Rb;d + Rs;d Determination of length of pile to carry prescribed load Try LR = 17m Rc;d1
38
=
[6.4 × (60 + 17 × 8) × ( × (0.6)2)/4] + [0.36 × (60 + (60 +17× 8))/2 × (17 × × 0.6)]
CIRIA C641
[8]
Rc;d1
=
355kN + 1477Kn = 1831kN (>Fc;d1 (1554kN))
Rc;d1= 1831kN
Conclusion A pile 21m long, 0.6m diameter can carry the load of 1000kN with downdrag force of 151kN under Combination 1.
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Combination 2: A2 “+” (M1 or M2) “+” R4 2.4.7.3.4.2 (2) Note 2
M1 – For calculating pile resistance M2 – For calculating unfavourable actions ie downdrag
Table A.NA.4 (Undrained Shear Stength) [9]
Design Soil Parameters (M2) Partial factor, Ȗcu = 1.4 C u;d = C u;k x 1.4 = 28kPa Note 1: The use of factors for set M2 in Table A.NA.4 is explicit when negative skin friction is derived directly from shear strength, ie C u, but in this case as it is unfavourable the factor is used as a multiplier rather than the more usual adoption as a divisor. Note 2: Where negative skin friction is derived using for instance, factored overburden pressures eg S D = Kp o’ (Bjerrum, 1973) It is suggested that the M2 factor of 1.25 is adopted (equating to an effective cohesion and angle of shearing resistance) Design Actions (A2)
Table A.NA.3 NA.A1.2(C) (Permanent Unfavourable) [10]
7.6.2.3 (4) Table A.NA.7
Partial factor, ȖG = 1.0 (for both applied and dragdown forces) Applied force, Fd2 = 1000 × 1.0 = 1000kN Downdrag force, Fnsf;d2 = x 0.6 x 4 x 28 x 1.0 = 211kN Total downward force, Fc;d2 = 1000 + 211
Fc;d2 = 1211kN
Design Resistance (R4) Partial factor for bored piles Ȗb = 2.0 Ȗs = 1.6
NA (2007) A.3.3.2
Model Factor, ȖR;d = 1.4 Partial factors for pile resistance for bored piles:
[11]
Ȗb × ȖR;d = 2.0 × 1.4 = 2.8
[12]
Ȗs × ȖR;d = 1.6 × 1.4 = 2.24
Design Base Resistance: Rb;d = (9/2.8) C u;d .Ab = 3.2 C u;d .Ab Design Shaft Resistance: Rs;d = (0.5/2.24) C u;d .As = 0.22 C u;d .As CIRIA C641
39
Design Compressive Resistance, R c;d = Rb;d + Rs;d Determination of length of pile to carry prescribed load Try LR = 19m
[13]
Rc;d2
=
[3.2 × (60 + 19 × 8) × ( × (0.6)2)/4] + [0.22 × (60 + (60 +19× 8))/2 × (19 × × 0.6)]
Rc;d2
=
192kN + 1071kN = 1263kN (>Fc;d2 (1211kN))
Rc;d2= 1263kN
Conclusion A pile 23m long, 0.6 diameter can carry the load of 1000kN with downdrag of 211kN under Combination 2.
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Overall Conclusion: 1
A pile 23m long, 0.6m diameter can carry the load of 1000kN with downdrag.
2
Combination 2 is critical as
R F
c;d2
c;d2
40
<
R
c;d 1
F
c;d1
CIRIA C641
4.5
Retaining walls
4.5.1
Specific changes Much of UK design practice uses CIRIA R104 or more recently CIRIA C580, while BS 8002 is often used for “small” walls85. In contrast with these documents, which contain quite detailed guidance on retaining structure design, Section 9 of EC7-1 gives only the basic requirements for design without specifying or describing particular calculation methods, though factored strength is the de facto method. It does not impose any fundamental changes in design philosophy for retaining structures although there are some changes of detail that may have an impact on the finished design. These are: Combination of calculation method and factor
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EC7-1 differs significantly from much UK practice in that, for the latter, factors and their values depend on the calculation method86 whereas the former does not define the calculation method but does specify factors on strength and gives values for them. Design values of geotechnical actions EC7-1 requires the factoring of characteristic values while BS 8002 states ... Design values of loads, derived by factoring or otherwise, are intended, here, to be the most pessimistic or unfavourable loads which should be used in the calculations for the structure. Note that EC7-1 (Clause 9.3.1.2) requires that the geotechnical design report shall specify the checks, to be made during backfill placement, that in situ values of fill weight density are not less than those assumed in the design. Design values of ground strength parameters While EC7-1, again, requires the factoring of a characteristic value, BS 8002 states ... Single values of soil strength should be obtained from a consideration of the representative values of peak and ultimate strength. The value so selected will satisfy, simultaneously, the considerations of ultimate and serviceability limit states. Surcharge In contrast to BS 8002, EC7-1 has no specific requirement for a minimum surcharge loading on the retained ground surface87. Depth of unplanned excavation EC7-1 adopts the same requirements as those in BS 8002. Limiting values of earth pressures (on vertical walls) In an informative Annex C, EC7-1 recommends88 charts and equations for calculating coefficients of limiting earth pressure. These charts are taken from BS 8002 and are based on the work of Kérisel and Absi (1990), they are essentially the same as those in CIRIA C580 for comparable circumstances.
CIRIA C641
41
Compaction pressures EC7-1 states that compaction pressures should not be included in GEO ULS design calculations since they are normally relieved by relatively small horizontal movement of the wall. They should, however, be included in STR calculations to check the structural wall section strength and in SLS calculations. BS 8002 makes no such distinction between ULS and SLS conditions. SLS design
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Both EC7-1 and BS 8002 indicate that the SLS generally provides the governing criteria for design. For the SLS, the former advises that appropriate earth pressures are usually larger than the limiting active earth pressures and lower than the limiting passive earth pressures, while the latter states that ... Serviceability can be sufficiently assured by limiting the proportion of available strength actually mobilised in service. For SLS calculations, EC7-1 states that the values of earth pressures should normally be obtained using design values of all ground parameters equal to their characteristic values (ie with γM = 1)89. BS 8002, however, adopts the use of M, a mobilisation factor applied to the peak strength90, or else the use, in certain circumstances, of a design value equal to ϕ′cv. EC7-1 advocates consideration of the calculation of displacements when walls retain more than 6 m of cohesive soil of low plasticity91 or 3 m of high plasticity soils, or when the wall is supported by soft clay within its height or beneath its base. There now follow four examples of wall design using EC7-1. Example 4.3 illustrates a relatively simple design check on the overturning and sliding stability of a cantilever wall where the water table is at depth.
42
CIRIA C641
Example 4.3
The design of a cantilever retaining wall without groundwater pressures acting
Project
Subject
Example 4.3
Cantilever Wall
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Retaining Walls Cantilever Wall
Diagram Notes: The actions are assessed by assuming that the retaining wall and the soil above the h eel (Soil A) behave as a unit. The point O is at the toe of the bas e. Moments will be taken about this point to check for overturning. This example demonstrates how a retaining wall design should be approached using EC7. Stability and sliding calculations are included. However for a complete design, it will be n ecessary to consider other limit states discussed in the cod e such as bearing resistance and wall movement (Clause 9.2). 1 st trial, Base width = 3.0m Clauses 2.4.2(4) & 9.3.1
[1]
CIRIA C641
Characteristic Vertical Load Soil A Wt
=
5.5 × 1.5 × 20 = 165kN/m run Point of Action from O = 1.5/2 + 1.5 = 2.25m
Wall Stem Wt
=
5.5 × 0.5 × 24 = 66kN/m run PoA from O = 0.5/2 + 1 = 1.25m
Wall Base Wt
=
0.5 × 3 × 24 = 36kN/m run PoA from O = 3/2 = 1.5m
Vk = 165 + 66 + 36
Vk =267kN
43
Stability 2.4.7.3.4.2 (1)
Combination 1: A1 “+” M1 “+” R1 Design Loads (A1)
Table A.NA.3 NA.A1.2(B) (Permanent Favourable)
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[2]
Table A.NA.3 NA.A1.2(B) (Variable Unfavourable) [3]
Design Vertical Load (Wall + Soil A) The weight of the wall and Soil A are favourable actions as increasing these loads increases the overall stability. ȖG = 1.0 Vd1 = JG x Vk Vd1 = 1.0 x 267
Design Surcharge Load For lateral load calculations, surcharge is an unfavourable action as it increases the tendency of the wall to overturn. ȖQ = 1.5 Vd1s = JQ x Vks Vd1s = 1.5 x 10
Vd1=267 kN/m run
Vd1s =15kPa
Design Soil Parameters (M1) Table A.4
Table A.NA.4 [4]
9.5.1(5) [5]
Annex C.1(3) Fig C.1.1
Weight density ȖJ= 1.0 Design Jd = 20kN/m³ Angle of shearing resistance ȖI’ = 1.0 Design tanI’d1 = (tan32q)/1 = 0.62q Soil A/soil interface, G Gd1 = 0
(Note: Rankine conditions adopted for the “virtual back of wall”)
Coefficient of horizontal active earth pressure Ka = 0.32 (for G / I’ = 0) Active lateral earth pressure at position Y-Y (“virtual back of wall”)(A1)
Annex C
Va(z) = Ka (J.z + q) – 2cKa
(in this case c’ = 0)
Lateral Design Earth Pressures (A1) Eqn C.1
Va1;d (z) = Ka (Jd x z x ȖG +Vd1s) A lateral earth load is an unfavourable action as it causes the wall to have a greater tendency to overturn.
Table A.NA.3 NA.A1.2(B) (Permanent Unfavourable)
ȖG = 1.35 Vd1s = 15kPa (Eqn 3) Va1;d (0) = 0.32 x (20 x 0 x 1.35 + 15) = 4.8kPa Va1;d (6) = 0.32 x (20 x 6 x 1.35 + 15) = 56.64kPa Lateral Design Earth Load (A1)
[6]
44
H d1 = (4.8 x 6) + ((56.64 – 4.8) x 6 x 0.5) = 28.8 + 155.5
H d1=184.3 kN/m run CIRIA C641
Passive Pressures The depth of excavation to allow for in front of the wall ('a) should strictly be limited to 10% of 6.0m ie 0.6m with a maximum of 0.5m (Clause 9.3.2.2(2)). In this example the maximum excavation of 0.5m is taken. Therefore no passive pressures are considered . Check for overturning of the wall Taking moments about Point O: M about O = [Soil A] + [Wall] – [Lateral earth load] = [2.25 x 165] + [(1.25 x 66) + (1.5 x 36)] – [(3 x 28.8) + (6/3 x 155.5)] = [371.25] + [136.5] + [0] – [86.4] – [311] = 507.75 – 397.4 = 110.35kNm
[7]
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Stabilising Moment (507.75kNm) Conclusion:
>
Overturning Moment (397.4kNm)
The stabilising moment is greater than the overturning moment at Point O therefore overall stability is satisfied. Wall of base width 3m is acceptable for Combination 1
Combination 2: A2 “+” M2 “+” R1 Table A.NA.3 NA.A1.2(C) (Permanent favourable) [8]
Design Loads (Action A2)
Table A.NA.3 NA.A1.2(C) (Variable unfavourable) [9]
Surcharge: Lateral load calculations ȖQ = 1.3 Vd2s = 1.3 x 10 = 13kPa
Wall + Soil A: ȖG = 1.0 Vd2 = Vd1 = 267kN/m run, (Eqn 2)
Vd2=267 kN/m run
Vd2s =13kPa
Design Soil Parameters (M2) Table A.4
Table A.NA.4
[10] 9.5.1(5) [11]
Annex C.1(3) Fig C.1.1
CIRIA C641
Weight density ȖJ = 1.0 Design Jd = 20kN/m³ Angle of shearing resistance ȖI’ = 1.25 Design tanI’d2 = (tan32q)/1.25 = 0.5 I’d2 = tan-10.5 = 26.6q Soil A/soil interface, G Gd2 = 0
(Note: Rankine conditions adopted for the “virtual back of wall”)
Coefficient of horizontal active earth pressure Ka = 0.38 (for G/I’ = 0)
45
Annex C
Active lateral earth pressure at position Y-Y (‘virtual back of wall’)(A2) Lateral Design Earth Pressures (A2) Va2;d (z) = Ka (Jd x z x ȖG+Vd2s)
Table A.NA.3 NA.A1.2(C) (Permanent Unfavourable)
(Vd2s= 13kPa from Eqn 9)
ȖG = 1.0 Va2;d (0) = 0.38 x (20 x 0 x 1.0 + 13) = 4.94kPa Va2;d (6) = 0.38 x (20 x 6 x 1.0 + 13) = 50.54kPa Lateral Design Earth Load (A2)
[12]
H d2
= (4.94 x 6) + (50.54 – 4.94) x 6 x 0.5 = 29.64 + 136.8
H d2=166.4 kN/m run
Check for overturning of the wall
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Take moments about O M about O
=
[Soil A] + [Wall] + [Lateral earth load]
=
[2.25 x 165] + [(1.25 x 66) + (1.5 x 36)] – [(3 x 29.64) + (6/3 x 136.8)] [371.25] + [136.5] – [88.92] – [273.6] 507.75 – 362.52 = 145.23kNm
= =
[13]
Stabilising Moment (507.75kNm) Conclusion:
>
Overturning Moment (362.52kNm)
The stabilising moment is greater than the overturning moment at Point O therefore overall stability is satisfied. Wall of base width 3m is acceptable for Combination 2
Compare Combination 1 and 2 Ratio of stabilising to destabilising moments: Combination1: 507.75 / 397.4 = 1.28 Combination 2: 507.75 / 362.52 = 1.40 Conclusions: 1 Combination 1 governs overturning 2 Wall base of 3.0m is acceptable
Sliding
9.2 (2) & 6.5.3 Eqn 6.2
Require: H d Rd +Rp;d (in this case Rp;d | 0kN, negligible passive forces)
6.5.3 (8)
Design Resistance to Sliding by Factoring Ground Properties
Combination 2: Factored Ground Properties Lateral Load, H d2 = 166.4kN/m run (Eqn 12)
H d2 = 166.4 kN/m run
Design Resistance to Sliding Eqn 6.3a
[14] 46
Rd = V’d tanGd From Equation 8 V’d = Vd2 = 267
V’d = 267 kN/m run CIRIA C641
Angle of shearing resistance:
9.5.1 (7) [15]
From Equation (10), I’d2 = 26.6q Gd2 = K .I’d2 (For concrete cast against soil, K = 1) Resistance, Rd2 = 267 x tan26.6q Check if:
Rd2=133.7 kN/m run
H d2 < Rd2
166.4kN (Eqn 12) > 133.7kN (Eqn 15) Conclusion:
Cantilever wall of base width 3.0m is unacceptable to resist sliding for Combination 2.
Try wall with 4m wide base
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[16]
V’d = (5.5 x 2.5 x 20) + (5.5 x 0.5 x 24) + (0.5 x 4 x 24)+0 = 389kN/m run
Rd2 = 389 x tan26.6q
[17]
Check if:
V’d = 389 kN/m run Rd2=194.8 kN/m run
H d2 < Rd2
166.4kN (Eqn 12) < 194.8kN (Eqn 17) Conclusion:
Cantilever wall of base width 4.0m is acceptable to resist sliding for Combination 2.
Overall Conclusion 1 2 3 4
Wall Base of 3.0m is acceptable to resist o verturning but not sliding. Base width will have to be increased to 4m to resist sliding. Combination 1 governs overturning. Combination 2 adopted for sliding (Factored ground properties).
NOTE:
For design a bearing capacity assessment should also be undertaken in conjunction with these calculations.
Example 4 .4 is t he same desig n problem as Exam ple 4 .3 but with a wat er t able in t he retained m aterial. The example do es not f actor water pr essures as this would lead to unrealist ic circ umst ances. Instead it ado pts an adjustm ent to the wat er level to provide additio nal safet y.
CIRIA C641
47
Example 4.4
The design of a cantilever retaining wall with groundwater pressure acting
Project
Subject
Example 4.4
Cantilever wall with groundwater (safety margin approach) Retaining walls Note: Clause 2.4.6.1(8) identifies two ways in which design values of groundwater pressures are permitted to be derived. It is r ecommended, due to the potential to obtain unrealistic water pressures and contravene principles of effective stress , that applying factors to characteristic water pressures is not adopted. This example uses the safety margin approach, where the water level is increased from 2m below existing ground to 1m bgl.
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This example demonstrates how a retaining wall design should be approached using EC7. Stability and sliding calculations are included. However for a complete design, it will be n ecessary to consider other limit states discussed in the cod e such as bearing resistance and wall movement (Clause 9.2). Clauses
1 st trial, Base width = 4.0m Characteristic Vertical Load
2.4.2(4) & 9.3.1
[1]
Soil A Wt
=
5.5 × 2.5 × 20 = 275kN/m run Point of Action from O = 2.5/2 + 1.5 = 2.75m
Wall Stem Wt
=
5.5 × 0.5 × 24 = 66kN/m run PoA from O = 0.5/2 + 1 = 1.25m
Wall Base Wt
=
0.5 × 4 × 24 = 48kN/m run PoA from O = 4/2 = 2m Vk = 275 + 66 + 48
Vk = 389 kN/m run
Stability 2.4.7.3.4.2 (1)
2.4.6.1(11) 48
Combination 1: A1 “+” M1 “+” R1
Diagram Notes: The actions are assessed by assuming that the retaining wall and the soil above the h eel (Soil A) behave as a unit. The point O is at the toe of the bas e. Moments will be taken about this point to check for overturning. NOTE: A drainage system is installed behind the wall. CIRIA C641
Design Loads (A1) Table A.NA.3 NA.A1.2(B) (Permanent Favourable)
[2]
Design Vertical Load The weight of the wall and Soil A are favourable actions as increasing these loads increases the overall stability. JG = 1.0 Vd1 = 1.0 x 389
Vd1=389 kN/m run
Design Uplift Force Using the safety margin approach a water table 1m below existing ground is adopted.
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[3]
Table A.NA.3 NA.A1.2(B) (Variable Unfavourable) [4]
U b;d1
U b;d1=98.1 kN/m run
= 9.81 x 5 x 4/2 = 98.1kN/m run
Design Surcharge Load For lateral load calculations, surcharge is an unfavourable action as it increases the tend ency of the wall to overturn. ȖQ = 1.5 Vd1s = JQ x Vk Vd1s = 1.5 x 10
Vd1s = 15kPa
Design Soil Parameters (M1) Table A4
Table A.NA.4 [5]
Weight density ȖJ= 1.0 Design Jd = 20kN/m³ Angle of shearing resistance ȖI’ = 1.0 Design tan I’d1 = (tan32q)/1 = tan32q
9.5.1(5) [6]
Soil A/soil interface, G Gd1 = 0
Fig C.1.1
Coefficient of horizontal active earth pressure Ka = 0.32
(Note: Rankine conditions adopted for the “virtual back of wall”)
Active lateral earth pressure at position Y-Y (‘virtual back of wall’)(A1) Annex C Eqn C.1
V1a(z) = Ka (J.z + q) – 2cKa
(in this case c’ = 0)
Lateral Design Earth Pressures (A1) Va1;d (z) = Ka {[(Jd x z) – U w(z)] x ȖG + Vd1s} A lateral earth load is an unfavourable action as it causes the wall to have a greater tendency to overturn. Table A.NA.3 NA.A1.2(B) (Permanent Unfavourable)
CIRIA C641
ȖG = 1.35 Vd1s = 15kPa (From Eqn 4) V’a1;d (0) = 0.32 x [(20 x 0) x 1.35 + 15] = 4.8kPa V’a1;d (1) = 0.32 x [(20 x 1) x 1.35 + 15] = 13.44kPa V’a1;d (6) = 0.32 x {[(20 x 6) – (9.81 x 5)] x 1.35 + 15} = 35.5kPa
49
Lateral Design Earth Load (A1) H’d1
=
(4.8 x 1) + [(13.44 – 4.8) x 1 x 0.5] + (13.44 x 5) + [(35.5 – 13.44) x 5 x 0.5]} 4.8 + 4.32 + 67.2 + 55.15
=
[7]
H’d1= 131.47 kN/m run
Lateral Water Pressure (A1) As a safety margin approach has been adopted the water pressure is taken at 1.0m below ground level. U w(z) = (Jw x z) Lateral Design Water Load (A1)
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[8]
U d1 = 122.6 kN/m run
U d1 = 9.81 x 5 x 5 x 0.5 Passive Pressures The depth of excavation to allow for in front of the wall ('a) should strictly be limited to 10% of 6.0m ie 0.6m with a maximum of 0.5m (Clause 9.3.2.2(2)). In this example, the maximum excavation of 0.5m is taken. Therefore there are no passive pressures considered in this case. Check for overturning of the wall Taking moments about Point O: M about O
= [Soil A] + [Wall] - [Uplift water force] – [Lateral earth load] – [Lateral water load] = [275 x 2.75] + [(66 x 1.25) + (48 x 2)] – [2.67 x 98.1] – [(4.8 x 5.5) + (4.32 x 5.33) + (67.2 x 2.5) + (55.15 x 1.67)] – [122.6 x 1.67] = 756.25 + 82.5 + 96 – 261.9 – (26.4 + 23 + 168 + 92.1) – 204.7 = 934.75 – 776.1 = 158.65kNm
Stabilising Moment (934.75kNm) Conclusion:
>
Overturning Moment (776.1kNm)
The stabilising moment is greater than the overturning moment at Point O therefore overall stability is OK. Wall of base width 4m is acceptable for Combination 1
Combination 2: A2 “+” M2 “+” R1
NOTE: A drainage system is installed behind the wall. 50
CIRIA C641
Table A.NA.3 Design Loads (Action A2) NA.A1.2(C) (Permanent (Wall + Soil A): Favourable) ȖG = 1.0 [9] Vd2 = Vd1 = 389kN/m run, (from Eqn 3)
Vd2 = 389 kN/m run
Design Uplift Force The characteristic water level is 2.0m below ground level [10]
U b;d2
U b;d2 = 78.5 kN/m run
= 98.1 x 4 x 4/2 = 78.5kN/m run
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Table A.NA.3 Surcharge: NA.A1.2(C) Lateral load calculations (Variable ȖQ = 1.3 Unfavourable) Vd2s = 1.3 x 10 [11] Design Soil Parameters (M2) Table A4
Table A.NA.4
[12]
Vd2s = 13kPa
Weight density, ȖJ = 1.0 Design Jd = 20kN/m³ Angle of shearing resistance ȖI’ = 1.25 Design I’d2 = (tan32q)/1.25 = 0.5 I’d2 = 26.6q
9.5.1(5) [13]
Soil A/soil interface, G Gd2 = 0
Fig C.1.1
Coefficient of horizontal active earth pressure Ka = 0.38
(Note: Rankine conditions adopted for the “virtual back of wall”)
Active lateral earth pressure at position Y-Y (‘virtual back of wall’)(A2) Annex C Eqn C.1
V1a(z) = Ka (J.z + q)o – 2cKa
(in this case c’ = 0)
Lateral Design Earth Pressures (A2) Table A.NA.3 NA.A1.2(C) (Permanent Unfavourable)
Va2;d (z) = Ka {[(Jd x z) – U w(z)] x ȖG + Vd2s} A lateral earth load is an unfavourable action as it causes the wall to have a greater tendency to overturn. ȖG = 1.0 Vd2s = 13kPa (From Eqn 13) V’a2;d (0) = 0.38 x [(20 x 0) x 1.0 + 13] = 4.94kPa V’a2;d (2) = 0.38 x [(20 x 2) x 1.0 + 13] = 20.14kPa V’a2;d (6) = 0.38 x {[(20 x 6) – (9.81 x 4)] x 1.0 + 13} = 35.63kPa Lateral Design Earth Load (A2)
[14]
CIRIA C641
H’d2
= (4.94 x 2) + [(20.14 – 4.94) x 2 x 0.5] + (20.14 x 4) + [(35.63 – 20.14) x 4 x 0.5] = 9.9 + 15.2 + 80.5 + 31.0
H’d2 = 136.6 kN/m run 51
Lateral Water Pressure (A2) U w(z) = (Jw x z) Lateral Design Water Load (A2) [15]
U d2= 78.5 kN/m run
U d2 = 9.81 x 4 x 4 x 0.5
Passive Pressures The depth of excavation to allow for in front of the wall ('a) should strictly be limited to 10% of 6.0m ie 0.6m with a maximum of 0.5m (Clause 9.3.2.2(2)). In this example, the maximum excavation of 0.5m is taken. Therefore there are no passive pressures considered in this case. Check for overturning of the wall Take moments about O
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M about O
= [Soil A] + [Wall] – [Uplift water force] – [Lateral earth load] – [lateral water] = [275 x 2.75] + [(66 x 1.25) + (48 x 2)] – [2.67 x 78.5] – [(4.94 x 2 x 5) + ((20.14 – 4.94) x 2 x 0.5 x 4.7) + [(20.14 x 4) x 2] + ((35.63 – 20.14) x 4 x 0.5 x 4/3)] – (78.5 x 4/3) = 756.3 + 82.5 + 96 – 209.6 – (49.4 + 71.4 + 161.1 + 41.3) – 104.7 = 934.8 – 637.5 = 297.3
[16]
Stabilising Moment (934.8kNm) Conclusion:
>
Overturning Moment (637.5kNm)
The stabilising moment is greater than the overturning moment at Point O therefore overall stability is OK. Wall of base width 4m is acceptable for Combination 2.
Compare Combination 1 and 2 Ratio of stabilising to destabilising moments: Combination1:
934.8/776.1= 1.20
Combination 2: 934.8/637.5 = 1.47 Conclusions: 1 2
52
Combination 1 governs overturning. Wall base of 4.0m is acceptable.
CIRIA C641
Sliding
9.2(2) & 6.5.3(2) Eqn 6.2
6.5.3 (8)
Require: H d Rd + Rp;d (in this case Rp;d | 0kN, negligible passive forces)
Design Resistance to Sliding by Factoring Ground Properties
Combination 2: Factored Ground Properties [17]
Lateral Load, H d2 = H’d2 + U d2 = 215.1kN/m run (Eqn 14 + 15)
H d2= 215.1 kN/m run
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Design Resistance to Sliding Rd = V’d tanGd
Eqn 6.3a
V’d = 310.5 kN/m run
V’d = Vd2 - U b;d2 = 389 – 78.5
[18]
Angle of shearing resistance: I’d2 = 26.6q (From Equation (12)) Gd2 = K .I’d2 (for concrete cast against soil, K = 1) Rd2=155.5 kN/m run
Rd2 = 310.5 x tan26.6q
[19]
Check if:
H d2 < Rd2
215.1kN (Eqn 14 + 15) > 155.5kN (Eqn 19) Conclusion:
Cantilever wall of base width 4.0m is unacceptable to resist sliding for Combination 2.
Try wall with 6m wide base V’d
[20]
Rd2
[21]
=
=
Check if:
(5.5 x 4.5 x 20) + (5.5 x 0.5 x 24) + (0.5 x 6 x 24) – (9.81 x 4 x 6 x 0.5) = 515.3kN/m run 515.3 x tan26.6q
Rd2= 258 kN/m run
H d2 < Rd2
215.1kN (Eqn 14 + 15) < 258kN (Eqn 21) Conclusion:
V’d = 515.3 kN/m run
(satisfy Eqn 6.2)
Cantilever wall of base width 6.0m is acceptable to resist sliding for Combination 2.
Overall Conclusion 1 2 3 4
CIRIA C641
Wall with base width of 4.0m is acceptable to resist o verturning but not sliding. Base will have to be increased to 6m to resist sliding. Combination 1 governs overturning. Combination 2 governs sliding (Factored ground properties).
53
Example 4.5 is again designed to illustrate the handling of water pressures in an earth pressure calculation problem. In this case, the adjustment of water pressure to allow for uncertainties is dealt with differently. Firstly, the water pressure is factored and secondly water pressures on the active and passive sides are factored by the same partial factor value since it is not sensible to treat them as simultaneously both “unfavourable” and “favourable”. EC7-1 refers to this as the “single source principle” but does not explain it because there is continuing argument about what exactly the principle means.
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BS EN 1997-1 is rather ambiguous about the treatment of “favourable” earth pressures. Should the passive force be regarded as a favourable action or as a resistance? In the following examples, passive force has been taken to be a resistance. In doing so, the choice of resistance factor resides with the geotechnical engineer since to treat it as a favourable action leaves the choice of partial action factor with the NA to BS EN 1990.
54
CIRIA C641
Example 4.5
The design of an embedded retaining wall with groundwater pressures acting
Project
Subject
Example 4.5
Embedded Wall with Water at 2m Below Surface
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Em bedded Walls A contiguous bored pile wall with water level at 2m below surface
Soil A (Gravel) Js = 20kN/m³
Soil B (Clay) Js =18kN/m³
I ’ = 36q I cv = 33q
I ’ = 25q I cv = 25q
c’ = 0kPa
c’ = 5kPa
Diagram Notes: A contiguous bored pile concrete wall is to be designed to retain 6m of soil which consist of 4m gravel underlain by clay to depth. Water levels are at 2m and 1m below ground level behind and in front of the wall respectively. It is assumed that water pressure is equalised at the bottom of the wall, as illustrated in the diagram below.
The stability of the wall is analysed using Blum’s simplified method in which the reinforced concrete diaphragm wall is considered to rotate about the bottom of the wall (Point O). The overall length of the wall is determined so that the destabilising moment does not exceed the stabilising moment. The calculated embedd ed length of the wall is increased by 20% to take account of the simplification employed.
CIRIA C641
55
1 st trial, d = 14.0m
Stability Clauses 2.4.7.3.3(1) 2.4.7.3.4.2 (1) & 9.2
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Table A4
Design Approach 1 Combination 1: A1 “+” M1 “+” R1 Design Soil Parameters (M1) Weight density ȖJ= 1.0 Design Jd1 for gravel = 20/1 = 20kN/m³ Design Jd1 for clay = 18/1 = 18kN/m³
Table A.NA.4
Angle of shearing resistance ȖI’ = 1.0 Design tan I’d1 for gravel = (tan36q)/1 = tan36q Design tanI’d1 for clay = (tan25q)/1 = tan25q
Table A.NA.4
Effective cohesion Ȗc’ = 1.0 Design c’d1 for gravel = 0kPa Design c’d1 for clay = 5/1 = 5kPa
9.5.1 (6)
9.5.1 (7) 9.5.1 (6)
Soil/Wall interface, G Based on 9.5.1 (6), Gd = K. Icv;d and Icv;d = 33q (for gravel) and Icv;d = 25q (for clay). Based on 9.5.1 (7), for concrete cast against soil, K = 1.0 Design Gd1 for gravel = 33q Design Gd1 for clay = 25q
Fig C.1.1
Coefficient of horizontal active earth pressure For gravel, Ka = 0.22 (Gd1/I’d1 | 0.9) For clay, Ka = 0.34 (Gd1/I’d1 | 1)
Fig C.2.1
Coefficient of horizontal passive earth pressure For gravel, Kp = 8.9 (Gd1/I’d1 | 0.9) For clay, Kp = 4.0 (Gd1/I’d1 | 1) Design Loads (A1) Water Pressure at the back of the wall (A1)
[1]
U w(z) = H T
= (H 1 + H 2)/2 = (176.58 + 127.53)/2 =152kPa (at the toe of the wall)
Lateral Design Water Load at the back of th e wall (A1) A lateral water load at the back of the wall is an unfavourable action as it causes the wall to have a greater tendency to overturn. Table A.NA.3 NA.A1.2(B) (Permanent Unfavourable) [2]
56
ȖG = 1.35
U d1;a
= 1.35 x {(19.62 x 2 x 0.5) + (19.62 x 16) + [(152 – 19.62) x 16 x 0.5]} = 1.35 x [(19.62) + (313.92) + (1059.04)]
U d1;a = 1880 kN/m run CIRIA C641
Active lateral earth pressure at the back of the wall (A1) Va(z) = Ka (J.z + q) – 2cKa
Annex C Eqn C.1
(in this case q = 0)
Lateral Characteristic Earth Pressures (A1) V’a1 (z) V’a1 (0) V’a1 (2) V’a1 (4)
= = = = = V’a1 (4 +) = = V’a1 (6+14) =
Ka [(Jd x z) – (U w(z))] – 2cKa 0kPa 0.22 x (20 x 2) – 2 x 0 x 0.22 = 8.8kPa 0.22 x [(20 x 4) – 19.62] – 2 x 0 x 0.22 13.3kPa 0.34 x [(20 x 4) – 19.62] – 2 x 5 x 0.34 14.7kPa 0.34 x {(20 x 4) + [18 x (20 – 4)] – 152)] – 2 x 5 x 0.34 = 67.6kPa
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Lateral Design Earth Load (A1) Table A.NA.3 NA.A1.2(B) (Permanent Unfavourable)
A lateral earth pressure is an unfavourable action as it causes the wall to have a greater tendency to overturn. ȖG = 1.35 H’d1;a = 1.35 x {(8.8 x 2 x 0.5) + (8.8 x 2) + [(13.3 – 8.8) x 2 x 0.5] + (14.7 x 16) + [(67.6 – 14.7) x 16 x 0.5)} = 1.35 x (8.8 + 17.6 + 4.5 + 235.2 + 423.2)
[3]
H’d1;a = 930.56kN/m run
Water Pressure at the front of the wall (A1) U w(z) = H T = 152kPa
(at the toe of the wall)
Lateral Design Water Load at the front of the wall (A1) In examples 4 and 6 safety margins are applied to water levels, so that through all stages of the calculations water pressures remain unfactored. The alternative m ethod shown in this example entails two concepts being adopted: 1 2
Table A.NA.3 NA.A1.2(B) (Permanent Unfavourable) [4]
Water pressures are factored (Clause 2.6.4.1(8)) Factors applied to water pressures on both sides of the retaining wall are the same (2.4.2 (Note)). This is treating water as a “single source” – a concept not fully explained in the code.
ȖG = 1.35
U d1;p = 1.35 x [H T x (14 – 1) x 0.5] = 1.35 x 152 x 13 x 0.5kN/m run
U d1;p = 1333.8kN/m run
Passive lateral earth pressure at the front of the wall (R1) Annex C Eqn C.2
Vp(z) = Kp (J.z + q) + 2cKp
(in this case q = 0)
Lateral Characteristic Earth Pressures (R1)
CIRIA C641
57
9.3.2.2 (2)
(Allow for overdig at front of wall, d – 0.5) V’ p1 (z)
=
V’ p1 (0.5) V’ p1 (1) V’ p1 (14)
= 0kPa = 4 x {[18 x (1 – 0.5)] – 0} + 2 x 5 x 4 = 56kPa = 4 x {[18 x (14 – 0.5)] – 152} + 2 x 5 x 4 = 384kPa
Kp [Jd x (z – 0.5) - (U w(z))] + 2cKp
Lateral Design Earth Resistance (R1) A lateral earth pressure at the front of the wall is a resistance. ȖR;e = 1.0
Table A13 (Earth Resistance)
R’p;d1
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[5]
=
{[56 x 0.5 x 0.5] + [56 x 13] + [(384 – 56) x 13 x 0.5]} /1.0 = {14 + 728 + 2132} /1.0 = 2874
R’p;d1 = 2874kN/m run
Check for overturning of the wall Taking moments about Point O: Force/Re sistance
Lever arm
A
1.35 x 8.8 x 2 x 0.5 =11.9
B
1.35 x 8.8 x 2 = 23.8
2/3 + 18 =18.7 2/2 + 16 =17
C
1.35 x (13.3 – 8.8) x 2 x 0.5 = 6.1 1.35 x 14.7 x 16 = 317.5
2/3 + 16 = 16.7 16/2 =8
2540
1.35 x (67.6 – 14.7) x 16 x 0.5 = 571.3 1.35 x 19.62 x 2 x 0.5 = 26.5 1.35 x 19.62 x 16 = 423.8
16/3 = 5.3
3028
2/3 + 16 = 16.7 16/2 = 8
442.6 3390.4
16/3 = 5.3
7579
0.5/3 + 13 = 13.17 13/2 = 6.5
184.4 4732
13/3 = 4.3
9167.6
13/3 = 4.3
5735.3
D E F G H I J K L
1.35 x (152 – 19.62) x 16 x 0.5 = 1430 (56 x 0.5 x 0.5)/1 = 14 (56 x 13)/1 = 728 [(384 – 56) x 13 x 0.5]/1 = 2132 1333.8
Moment about Point O 222.5 404.6 101.87
M about O =
[Earth Resistance and water load] – [Active earth and water load] = [184.4 + 4732 + 9167.6 + 5735.3] – [222.5 + 404.6 + 101.87 + 2540 + 3028 + 442.6 + 3390.4 + 7579] = 19819.3 – 17709kNm
[6]
Stabilising Moment (19819.3kNm) Conclusion:
58
>
Overturning Moment (17709kNm)
The stabilising moment is greater than the overturning moment at Point O therefore overall stability is satisfied. Length of wall (6 + 14) = 20m is acceptable for Combination 1. CIRIA C641
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2.4.7.3.3(1) 2.4.7.3.4.2 (1) & 9.2
Design Approach 1 Combination 2: A2 “+” M2 “+” R1
Soil A (Gravel) Js = 20kN/m³
Soil B (Clay) Js =18kN/m³
I ’ = 36q I cv = 33q
I ’ = 25q I cv = 25q
c’ = 0kPa
c’ = 5kPa
Design Soil Parameters (M2) Table A4
Table A.NA.4
Weight density ȖJ= 1.0 Design Jd1 for gravel = 20/1 = 20kN/m³ Design Jd1 for clay = 18/1 = 18kN/m³ Angle of shearing resistance ȖI’ = 1.25 Design tanI’d1 for gravel = (tan36q)/1.25 = 0.58 Design I’d1 for gravel = 30q Design tanIcv;d for gravel = (tan33q)/1.25 = 0.52 Design Icv;d for gravel = 28q Design tanI’d1 for clay = (tan25q)/1.25 = 0.37 Design I’d1 for clay = 20q Design tanIcv;d for clay = (tan25q)/1.25 = 0.37 Design Icv;d for clay = 20q
Table A.NA.4
9.5.1 (6)
9.5.1 (7)
Effective cohesion Ȗc’ = 1.25 Design c’d1 for gravel = 0kPa Design c’d1 for clay = 5/1.25 = 4kPa Soil/Wall interface, G Based on 9.5.1 (6), Gd = K. Icv;d and Icv;d = 28q (for gravel) and Icv;d = 20q (for clay). Based on 9.5.1 (7), for concrete cast against soil, K = 1.0 Design Gd1 for gravel = 1 x 28q = 28q Design Gd1 for clay = 1 x 20q = 20q
Fig C.1.1
CIRIA C641
Coefficient of horizontal active earth pressure (Gd1/I’d1 | 0.9) For gravel, Ka = 0.28 For clay, Ka = 0.42 (Gd1/I’d1 | 1)
59
Fig C.2.1
Coefficient of horizontal passive earth pressure For gravel, Kp = 5.6 (Gd1/I’d1 | 0.9) For clay, Kp = 2.9 (Gd1/I’d1 | 1) Design Loads (A2) Water Pressure at the back of the wall (A2) From Equation [1] U w(z) = H T = 152kPa (at the toe of the wall)
Lateral Design Water Load at the back of th e wall (A2) Table A.NA.3 NA.A1.2(C) (Permanent Unfavourable)
A lateral water load at the back of the wall is an unfavourable action as it causes the wall to have a greater tendency to overturn. ȖG U d2;a
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[7]
= 1.0 = 1.0 x [(19.62) + (313.92) + (1059.04)] = 1392.58
U d2;a = 1392.58 kN/m run
Active lateral earth pressure at the back of the wall (A2) Annex C Eqn C.1
Va(z) = Ka (J.z + q) – 2cKa
(in this case q = 0)
Lateral Characteristic Earth Pressures (A2) V’a2 (z) = Ka {(Jd x z) – (U w(z))} – 2cKa V’a2 (0) V’a2 (2) V’a2 (4) V’a2 (4 +)
= = = = = V’a2 (6+14) =
0kPa 0.28 x (20 x 2) – 2 x 0 x 0.28 =11.2kPa 0.28 x {(20 x 4) – 19.62} – 2 x 0 x 0.28 =17kPa 0.42 x {(20 x 4) – 19.62} – 2 x 4 x 0.42 20.2kPa 0.42 x {(20 x 4) + (18 x 16) – 152} – 2 x 4 x 0.42 = 85.5kPa
Lateral Design Earth Load (A2) Table A.NA.3 NA.A1.2(C) (Permanent Unfavourable) [8]
A lateral earth pressure is an unfavourable action as it causes the wall to have a greater tendency to overturn. ȖG = 1.0 H’d2;a = 1.0 x {(11.2 x 2 x 0.5) + (11.2 x 2) + [(17 – 11.2) x 2 x 0.5] + (20.2 x 16) + [(85.5 – 20.2) x 16 x 0.5]}
H’ d2;a = 885 kN/m run
Water Pressure at the front of the wall (A2) From Equation [1] U w(z) = H T = 152kPa (at the toe of the wall)
Table A.NA.3 NA.A1.2(C) (Permanent Unfavourable) [9] 60
Lateral Design Water Load at the front of the wall (A2) ȖG = 1.0 U d2;p = 1.0 x [H T x (14 – 1) x 0.5] = 1.0 x [152 x 13 x 0.5]
U d2;p = 988kN/m run CIRIA C641
Passive lateral earth pressure at the front of the wall as resistance (R1) Annex C Eqn C.2
Vp(z) = Kp (J.z + q) + 2cKp
(in this case q = 0)
Lateral Characteristic Earth Pressures (R1) 9.3.2.2 (2)
(Allow for overdig at front of wall, d – 0.5) V’ p1 (z)
= Kp {Jd x (z – 0.5) – (U w(z))} + 2cK p
V’ p1 (0.5) V’ p1 (1) V’ p1 (14)
= = = =
0kPa 2.9 x {[18 x (1 – 0.5)] – 0} + 2 x 4 x 2.9 = 39.7kPa 2.9 x {[18 x (14 – 0.5)] – 152} + 2 x 4 x 2.9 277.5kPa
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Lateral Design Earth Resistance (R1) Table A.NA.13 (Earth Resistance)
A lateral earth pressure at the front of the wall acts as a resistance in resisting the overturning moment. ȖR;e = 1.0 R’p;d2
[10]
= {[39.7 x 0.5 x 0.5] + [39.7 x 13] + [(277.5 – 39.7) x 13 x 0.5]}/1.0 = {9.9 + 516.1 + 1545.7} /1.0 = 2071.7
Check for overturning of the wall Taking moments about Point O: Force/Re sistance
Lever arm
A
1 x 11.2 x 2 x 0.5 =11.2
2/3 + 18 =18.7
Moment about Point O 209.4
B
1 x 11.2 x 2 = 22.4
2/2 + 16 =17
380.8
C
1 x (17 – 11.2) x 2 x 0.5 = 5.8 1 x 20.2 x 16 = 323.2
2/3 + 16 = 16.7
96.86
16/2 = 8
2585.6
16/3 =5.3
2768.7
D E F
1 x (85.5 – 20.2) x 16 x 0.5 = 522.4 1 x 19.62 x 2 x 0.5 = 19.62
16.7
327.7
G
1 x 19.62 x 16 = 313.92
16/2 = 8
2511.4
H
16/3 = 5.3
5612.9
I
1 x (152 – 19.62) x 16 x 0.5 = 1059.04 (39.7x 0.5 x 0.5)/1 = 9.9
130.4
J
(39.7 x 13)/1 = 516.1
0.5/3 +13 = 13.17 13/2 = 6.5
K
[(277.5 – 39.7) x 13 x 0.5]/1 = 1545.7 988
13/3 = 4.3
6646.5
13/3 = 4.3
4248.4
L
[11]
M about O = [Earth resistance and water load] – [Active earth and water load] = [130.4 + 3354.7 + 6646.5 + 4248.4] – [209.4 + 380.8 + 96.86 + 2585.6 + 2768.7 + 327.7 + 2511.4 + 5612.9] = 14380 – 14493.36kNm Stabilising Moment (14380kNm)
CIRIA C641
3354.7
<
Overturning Moment (14493.36kNm)
61
Conclusion:
The stabilising moment is less than the overturning moment at Point O therefore overall stability is not satisfied.
Length of wall (6+14) = 20m is not acceptable for Combination 2. Therefore, length of wall has to be increased for Combination 2. A second trial for d = 15m works under Combination 2.
Conclusion: Length of wall (6+15) = 21m is acceptable for Combination 2. Overall Conclusions:
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1 2
Combination 2 governs overturning. Overall length of wall = 6 + (15 x 1.2) = 24m.
ie embedded length increased by 20% to account for simplification in the analysis.
Bending Moments and Shear Forces Note: Conventional practice in the UK for the determination of bending moments and shear forces, for cantilever or single propped walls, is to: 1 Determine the embedm ent length, using characteristic values, for a Factor of Safety of 1. 2 Calculate BM and SF along the wall length. If depth of wall requir ed is dc, then forces are as follows. Water Pressures (Active side) At gravel-clay interface (dc+2) U g;a = 19.62 At base of wall [Equalising water pressures at the toe] U b;a
= = =
([(19.62) + (dc+2) x 9.81] + [(dc – 1) x 9.81])/2 (19.62 + 9.81dc + 19.62 + 9.81dc – 9.81)/2 9.81dc + 14.71
Water Pressures (Passive side) At base of wall U b;p = 9.81dc + 14.71
Earth Pressures (Active side) As per diagram for Combination 1, except at base of wall, V’ad = 0.34 x {(20x4) + [18 x ((dc + b) – 4) – (9.81dc + 14.71)] – 2 x 5 x ¥0.34 = 0.34 {80 + (18dc + 36) – (9.81dc + 14.71)} – 5.83 = 34.44 + 2.78dc – 5.83 62
CIRIA C641
= 28.61 + 2.78dc Earth Pressures (Passive side)
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V’ pd
For Factor of Safety = 1
= 4{[18 x (dc – 0.5)] – 9.81dc + 14.71)} + 2 x 5 x ¥4 = 4 {18dc – 9 – 9.81dc – 14.71} + 20 = 32.76dc – 74.84
Check for overturning of the wall. Taking moments about Point 0 1 Force/Re sistance
Lever arm
A
8.8 x 2 x 0.5 = 8.8
B
8.8 x 2 = 17.6
2/3 + 4 + dc = dc + 4.67 dc = 3
C
(13.3) – 8.8) x 2 x 0.5 = 4.5
D
14.7 x (dc+2) = 14.7dc + 29.4 (28.61 + 2.78dc – 14.7) x (dc+2) x 0.5 = 1.39dc2 + 9.74dc + 13.1 19.62 x 2 x 0.5 = 19.62
E
F G
I
19.62 x (dc+2) = 19.62dc + 39.24 (14.71 + 9.81dc – 19.62) x (dc+2) x 0.5 = 4.91dc2 + 7.36dc – 4.91 56 x 0.5 x 0.5 = 14
J
56 x (dc-1) = 56dc – 56
K
[(32.76dc – 74.84)-56] x (dc1) x 0.5 = 16.38dc2 – 81.8dc + 65.42 (14.71 + 9.81dc) x (dc-1) x 0.5 = 4.91dc2 + 2.45dc – 7.36
H
L
2/3 + dc + 2 = dc + 2.67 (dc+2)/2 = 0.5dc+1 (dc+2)/3 = 0.33dc + 0.67 2/3 + dc + 2 = dc+2.67 (dc+2)/2 = 0.5dc+1 (dc+2)/3 = 0.33dc + 0.67 (dc-1) + 0.5/3 = dc-0.83 (dc-1)/2 = 0.5dc – 0.5 (dc-1)/3 = 0.33dc – 0.33 (dc-1)/3 = 0.33dc – 0.33
Moment about Point O1 8.8dc + 41.10 17.6dc + 52.80 4.5dc + 12.02 7.35dc2 + 29.4dc + 29.4 0.46dc3 + 4.15dc2 + 10.85dc + 8.78 19.62dc + 52.39 9.81dc2 + 39.24dc + 39.24 1.62dc3 + 5.71dc2 + 3.31dc – 3.29 14dc – 11.62 28dc2 – 56dc + 28 5.41dc3 – 32.40dc2 + 48.58dc – 21.59 1.62dc3 – 0.81dc2 – 3.24dc+ 2.43
For a factor of safety = 1, Moments (A + B + C + D + E + F + G + H) = Moments (I + J + K + L) CIRIA C641
63
From trial and error
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Depth of wall, dc = 9.6m
Maximum Bending Moment at 11.7m bgl (Shear Force = 0) Max Bending Moment = 1040kNm 2.4.7.3.2(2) Table A.NA.3 NA.A1.2(B) Permanent Unfavourable
In this situation, the partial factors are applied to the effects of actions. ȖE = 1.35 Design BM = 1.35 x 1040 = 1404kNm Design SF = 1.35 x 552 = 745kN Notes : 1 To determine the effects of variable actions, on the analysis for bending moments and shear forces, the characteristic values should be increased by 1.11 (Table A.NA.3: NA.A1.2(B) – Unfavourable, Variable ȖE=1.5;1.5/1.35 = 1.11). 2 The alternative approach, to using simple static analysis on a limiting depth of wall, is to analyse the depth of wall actually to be constructed. In this case characteristic values are used as previously. Analysis is undertaken using a soil continuum model (finite element or finite differ ence). Results obtained are strictly for a serviceability condition, however, to align Eurocode 7 methods with more traditional UK practice, the Bending Moment and SF are multiplied by the partial factor ȖE obtained from Table A.NA.3 to obtain design values. For multi-propped walls this alternative approach should generally be adopted.
Example 4 .6 is t he same problem as illustrated in Exam ple 4.5 . I n t his case, however , uncert ainty in the wat er pr essur e is dealt with using a safety marg in rather t han applying partial factoring. This met hod is seen to lead to a slight ly deeper wall.
64
CIRIA C641
Example 4.6
The design of a cantilever retaining wall with elevated groundwater pressures
Project
Subject
Example 4.6
Embedded wall with water level at 2m below surface: Safety margin approach
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Em bedded Walls A contiguous bored pile wall with water level originally at 2m below ground level (Safety Margin applied – raised water level to 1m b.g.l.)
Soil A (Gravel) Js = 20kN/m³
Soil B (Clay) Js =18kN/m³
I ’ = 36q I cv = 33q
I ’ = 25q I cv = 25q
c’ = 0kPa
c’ = 5kPa
Diagram Notes: A contiguous bored pile concrete wall is to be designed to retain 6m of soil which consist of 4m gravel underlain by clay to depth. Water level is originally at 2m below ground level behind the wall. However, instead of applying a partial factor to the water pressures, a safety margin is applied to the characteristic water level by assuming the water level is raised to 1m below ground level behind the wall. It is assumed that water pressure is equalised at the bottom of the wall, as illustrated in the diagram below.
The stability of the wall is analysed using Blum’s simplified method in which the reinforced concrete diaphragm wall is considered to CIRIA C641
65
rotate about the bottom of the wall (Point O). The overall length of the wall is determined so that the destabilising moment does not exceed the stabilising moment. The calculated embedded length of the wall is increased by 20% to take account of the simplification employed. 1 st trial, d = 14.0m
Stability Clauses 2.4.7.3.4.2 (1) & 9.2
Design Approach 1 Combination 1: A1 “+” M1 “+” R1 Design Soil Parameters (M1)
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Table A4
Weight density ȖJ= 1.0 Design Jd1 for gravel = 20/1 = 20kN/m³ Design Jd1 for clay = 18/1 = 18kN/m³
Table A.NA.4
Angle of shearing resistance ȖI’ = 1.0 Design tan I’d1 for gravel = (tan36q)/1 = tan36q Design tanI’d1 for clay = (tan25q)/1 = tan25q
Table A.NA.4
Effective cohesion Ȗc’ = 1.0 Design c’d1 for gravel = 0kPa Design c’d1 for clay = 5/1 = 5kPa Soil / Wall interface, G Based on 9.5.1 (6), Gd = K. Icv;d and Icv;d = 33q (for gravel) and Icv;d = 25q (for clay). Based on 9.5.1 (7), for concrete cast against soil, K = 1.0
9.5.1 (6)
Fig C.1.1
Fig C.2.1
Design Gd1 for gravel = 1 x 33q = 33q Design Gd1 for clay = 1 x 25q = 25q Coefficient of horizontal active earth pressure For gravel, Ka = 0.22 (Gd1/I’d1 | 0.9) For clay, Ka = 0.34 (Gd1/I’d1 | 1) Coefficient of horizontal passive earth pressure For gravel, Kp = 8.9 (Gd1/I’d1 | 0.9) For clay, Kp = 4.0 (Gd1/I’d1 | 1) Design Loads (A1) Water Pressure at the back of the wall (A1)
[1]
U w(z) = H T = (H 1 + H 2)/2 = (186.39 + 127.53)/2 =157kPa
(at the toe of the wall)
Lateral Design Water Load at the back of th e wall (A1) 2.4.6.1 (8)
66
A lateral water load at the back of the wall is an unfavourable action as it causes the wall to have a greater tendency to overturn. Instead of applying partial factors to the characteristic water pressures, a safety margin is applied to the characteristic water level by raising the water level behind the wall to 1m below ground level (originally at 2m below ground level). CIRIA C641
U d1;a [2]
= (29.43 x 3 x 0.5) + (29.43 x 16) + [(157 – 29.43) x 16 x 0.5] = 44.15 + 471 + 1020.6
U d1;a = 1535.75 kN/m run
Active lateral earth pressure at the back of the wall (A1) Annex C Eqn C.1
Va(z) = Ka (J.z + q) – 2cKa
(in this case q = 0)
Lateral Characteristic Earth Pressures (A1) V’a1 (z) = Ka [(Jd x z) – (U w(z))] – 2cKa
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V’a1 (0) V’a1 (1) V’a1 (4)
= = = = V’a1 (4 +) = = V’a1 (6+14) = =
0kPa 0.22 x (20 x 1) – 2 x 0 x 0.22 = 4.4kPa 0.22 x [(20 x 4) – 29.43] – 2 x 0 x 0.22 11.13kPa 0.34 x [(20 x 4) – 29.43] – 2 x 5 x 0.34 11.36kPa 0.34 x {(20 x 4) + [18 x (20 – 4)] – 157} – 2 x 5 x 0.34 66kPa
Lateral Design Earth Load (A1)
Table A.NA.3 NA.A1.2(B) (Permanent Unfavourable) [3]
A lateral earth pressure is an unfavourable action as it causes the wall to have a greater tendency to overturn. ȖG = 1.35 H’d1;a
= =
1.35 x {(4.4 x 1 x 0.5) + (4.4 x 3) + [(11.3 – 4.4) x 3x 0.5] + (11.36 x 16) + [(66 – 11.36) x 16 x 0.5]} 1.35 x (2.2 + 13.2 + 10.1 + 181.76 + 437.12)
H’d1;a= 870 KN/m run
Water Pressure at the front of the wall (A1) From Equation [1] U w(z) = H T = 157kPa (at the toe of the wall)
Lateral Design Water Load at the front of the wall (A1) 2.4.6.1(8)
[4]
A lateral water load at the front of the wall is a favourable action as it will resist the overturning moment. Instead of applying partial factors to the characteristic water pressures, a safety margin is applied to the characteristic water level by raising the water level behind the wall to 1m below ground level (originally at 2m below ground level). U d1;p = H T x (14 - 1) x 0.5] = 157 x 13 x 0.5 Passive lateral earth pressure at the front of the wall as resistance (R1)
Annex C Eqn C.2
Vp(z) = Kp (J.z + q) + 2cKp
U d1;p = 1020.5 kN/m run
(in this case q = 0)
Lateral Characteristic Earth Pressures (R1) 9.3.2.2 (2)
(Allow for overdig at front of wall, d - 0.5)
V’ p1 (z) = Kp [Jd x (z – 0.5) – (U w(z))] + 2cKp CIRIA C641
67
V’ p1 (0.5) V’ p1 (1) V’ p1 (14)
= = = =
0kPa 4 x {[18 x (1 – 0.5)] – 0} + 2 x 5 x 4 = 56kPa 4 x {[18 x (14 – 0.5)] – 157} + 2 x 5 x 4 364kPa
Lateral Design Earth Resistance (R1) Table A.NA.13 (Earth Resistance)
A lateral earth pressure at the front of the wall acts to resist the overturning moment. ȖR;e = 1.0 R’p;d1
= = =
[5]
{[56 x 0.5 x 0.5]+[56 x 13]+[(364 – 56) x 13 x 0.5]}/1.0 {14 + 728 + 2002}/1.0 2744
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Check for overturning of the wall
R’ p;dl = 2744kN/m run
Taking moments about Point O: Force/Re sistance
Lever arm
A
1.35 x 4.4 x 1 x 0.5 = 3.0
1/3 + 19 =19.3
Moment about Point O 57.9
B
1.35 x 4.4 x 3 = 17.8
3/2 + 16 =17.5
311.5
C
1.35 x (11.3 – 4.4) x 3 x 0.5 = 13.6 1.35 x 11.36 x 16 = 245.4
3/3 + 16 = 17
231.2
16/2 = 8
1963.2
16/3 = 5.3
3127
F
1.35 x (66 – 11.36) x 16 x 0.5 = 590 29.43 x 3 x 0.5 = 44.15
3/3 + 16 = 17
750.6
G
29.43 x 16 = 471
16/2 = 8
3768
H
(157 - 29.43) x 16 x 0.5 = 1020.6 (56 x 0.5 x 0.5)/1 = 14 (56 x 13)/1 = 728
16/3 = 5.3
5409.2
0.5/3 + 13 = 13.17 13/2 = 6.5
184.4 4732
D E
I J
[6]
K
[(364 – 56) x 13 x 0.5]/1 = 2002
13/3 = 4.3
8608.6
L
1020.5
13/3 = 4.3
4388.2
M about O = [Earth Resistance and water load] – [Active earth and water load] = [184.4 + 4732 + 8608.6 + 4388.2] – [57.9 + 311.5 + 231.2 + 1963.2 + 3127 + 750.6 + 3768 + 5409.2] = 17913.2 – 15618.6kNm Stabilising Moment (17913.2kNm) Conclusion:
68
>
Overturning Moment (15618.6kNm)
The stabilising moment is greater than the overturning moment at Point O therefore overall stability is satisfied. Length of wall (6 + 14) = 20m is acceptable for Combination 1.
CIRIA C641
Design Approach 1
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Combination 2: A2 “+” M2 “+” R1
Soil A (Gravel)
Soil B (Clay)
Js = 20kN/m³
Js =18kN/m³
I ’ = 36q I cv = 33q
I ’ = 25q I cv = 25q
c’ = 0kPa
c’ = 5kPa
Design Soil Parameters (M2) Table A4
Table A.NA.4
Weight density ȖJ= 1.0 Design Jd1 for gravel = 20/1 = 20kN/m³ Design Jd1 for clay = 18/1 = 18kN/m³ Angle of shearing resistance ȖI’ = 1.25 Design tanI’d1 for gravel = (tan36q)/1.25 = 0.58 Design I’d1 for gravel = 30q Design tanIcv;d for gravel = (tan33q)/1.25 = 0.52 Design Icv;d for gravel = 28q Design tanI’d1 for clay = (tan25q)/1.25 = 0.37 Design I’d1 for clay = 20q Design tanIcv;d for clay = (tan25q)/1.25 = 0.37 Design Icv;d for clay = 20q
Table A.NA.4
Effective cohesion Ȗc’ = 1.25 Design c’d1 for gravel = 0kPa Design c’d1 for clay = 5/1.25 = 4kPa Soil/Wall interface, G Based on 9.5.1 (6), Gd = K. Icv;d and Icv;d = 28q (for gravel) and Icv;d = 20q (for clay). Based on 9.5.1 (7), for concrete cast against soil, K = 1.0
9.5.1 (6)
Fig C.1.1
CIRIA C641
Design Gd1 for gravel = 1 x 28q = 28q Design Gd1 for clay = 1 x 20q = 20q Coefficient of horizontal active earth pressure (Gd1/I’d1 | 0.9) For gravel, Ka = 0.28 For clay, Ka = 0.42 (Gd1/I’d1 | 1) 69
Fig C.2.1
Coefficient of horizontal passive earth pressure For gravel, Kp = 5.6 (Gd1/I’d1 | 0.9) For clay, Kp = 2.9 (Gd1/I’d1 | 1) Design Loads (A2) Water Pressure at the back of the wall (A2) U w(z )
[7]
= = = =
H T = (H 1 + H 2)/2 [(13 x 9.81) + (18 x 9.81)]/2 (127.53 + 176.58)/2 152kPa (at the toe of the wall)
Lateral Design Water Load at the back of th e wall (A2) U d2;a [8]
= =
(19.62 x 2 x 0.5) + (19.62 x 16) + [(152 – 19.62) x 16 x 0.5] 1392.6kN/m run
U d2;a =1392.6 kN/m run
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Active lateral earth pressure at the back of the wall (A2) Annex C Eqn C.1
Va(z) = Ka (J.z + q) – 2cKa
(in this case q = 0)
Lateral Characteristic Earth Pressures (A2) V’a2 (z)
=
Ka [(Jd x z) – (U w(z))] – 2cKa
V’a2 (0) V’a2 (2) V’a2 (4)
= = =
V’a2 (4 +)
=
0kPa 0.28 x (20 x 2) – 2 x 0 x 0.28 =11.2kPa 0.28 x [(20 x 4) – 19.62] – 2 x 0 x 0.28 =16.9kPa 0.42 x [(20 x 4) – 19.62] - 2 x 4 x 0.42 = 20.2kPa 0.42 x [(20 x 4) + (18 x 16) – 152] – 2 x 4 x 0.42 85.5kPa
V’a2 (6+14) = =
Lateral Design Earth Load (A2) Table A.NA.3 NA.A1.2(C) (Permanent unfavourable) [9]
A lateral earth pressure is an unfavourable action as it causes the wall to have a greater tendency to overturn. ȖG = 1.0 H’d2;a = 1.0 x {(11.2 x 2 x 0.5) + [(11.2 x 2) + [(16.9 – 11.2) x 2 x 0.5] + (20.2 x 16) + [(85.5 – 20.2) x 16 x 0.5]} Water Pressure at the front of the wall (A2) From Equation [7] U w(z) = H T = 152kPa (at the toe of the wall)
H’d2;a = 884.9 kN/m run
Lateral Design Water Load at the front of the wall (A2) [10]
U d2;p =988 kN/m run
U d2;p = 152 x (14 - 1) x 0.5 Passive lateral earth pressure at the front of the wall as resistance (R1)
Annex C Eqn C.2 70
Vp(z)
=
Kp (J.z + q) + 2cKp
(in this case q = 0)
CIRIA C641
9.3.2.2 (2)
Lateral Characteristic Earth Pressures (R1) (Allow for overdig at front of wall, d - 0.5) V’ p1 (z)
= Kp [Jd x (z – 0.5) - (U w(z))] + 2cKp
V’ p1 (0.5) = V’ p1 (1) = V’ p1 (14) = =
0kPa 2.9 x {[18 x (1 – 0.5)] – 0} + 2 x 4 x 2.9 = 39.7kPa 2.9 x {[18 x (14 – 0.5)] – 152} + 2 x 4 x 2.9 277.5kPa
Lateral Design Earth Load (R1)
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Table A.NA.13 (Earth Resistance)
A lateral earth pressure at the front of the wall is a favourable action as it will resist the overturning moment. ȖR;e = 1.0 R’p;d2
[11]
= {[39.7 x 0.5 x 0.5] + [39.7 x 13] + [(277.5 – 39.7) x 13 x 0.5]}/1.0 = {9.9 + 516.1 + 1545.7}/1.0 = 2071.7
R’p;d2 = 2071.7 kN/m run
Check for overturning of the wall Taking moments about Point O:
A
Force/Re sistance
Lever arm
1.0 x 11.2 x 2 x 0.5 = 11.2
Moment about Point O 209.4
B
1.0 x 11.2 x 2 = 22.4
2/3 + 18 =18.7 2/2 + 16 =17
C
1.0 x (16.9 – 11.2) x 2 x 0.5 = 5.7 1.0 x 20.2 x 16 = 232.2
2/3 + 16 = 16.7 16/2 =8
2585.6
16/3 = 5.3
2768.7
F
1.0 x (85.5 – 20.2) x 16 x 0.5 = 522.4 19.62 x 2 x 0.5 = 19.62
327.7
G
19.62 x 16 = 313.9
2/3 + 16 =16.7 16/2 =8
2511.2
H
(152 – 19.62) x 16 x 0.5 = 1059 (39.7 x 0.5 x 0.5)/1.0 = 9.9 (39.7 x 13)/1.0 = 516.1
16/3 = 5.3
5612.7
0.5/3 + 13 = 13.2 13/2 = 6.5
130.7 3354.7
13/3 = 4.3
6646.5
13/3 = 4.3
4248.4
D E
I J K L
[(277.5 – 39.7) x 13 x 0.5]/1.0 = 1545.7 (152 x 13 x 0.5 = 988
380.8 95.2
M about O
[12]
= [130.7 + 3354.7 + 6646.5 + 4248.4] – [209.4 + 380.8 + 95.2 + 2585.6 + 2768.7 + 327.7 + 2511.2 + 5612.7] = 14380.3 – 14491.3kNm Stabilising Moment (14380.3kNm)
CIRIA C641
>
Overturning Moment (14491.3kNm)
71
Conclusion:
The stabilising moment is less than the overturning moment at Point O therefore overall stability is not satisfied. Length of wall (6+14) = 20m is not acceptable for Combination 2.
As the stabilising moment is marginally less than the overturning moment at Point O increase the length of wall by 1.0m.
Overall Conclusions: 1 2
Combination 2 governs overturning Overall length of wall = 6 + (15 x 1.2) = 24m (20% increase in embedded length to allow for simplification in the calculation)
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Note: Bending moment and shear force calculations would be the same as presented in Example 4.5.
Note t hat further examples of t he design of cantilever, st eel, sheet- pile walls (both wit h and witho ut anchor ages) may b e fo und in an OD PM G uide to Euroco des EC-3 and EC-4 ( see ODPM, 2005).
4.6
Embankments and slopes
4.6.1
Specific changes In Sect ion 11 Over all st abilit y, the Euroco de cover s nat ural and filled gro und, foundat ions, retaining structur es, embankm ents and exc avations, and natural slo pes. S ectio n 11 has no spec ific requirem ent s reg arding the type of any calculat ions that m ay be performed to check stabilit y. However , it does req uir e gro und param eters to b e design values, acquired in t he usual way ( wit h partial factors applied to charact erist ic values). Section 12 of E C7-1 deals, o nly in ver y g eneral term s, with embankments. As such, it introduces very little c hang e from t he mat erial co ver ed in BS 6031. It focuses on the use of light weight embankment mat erials: Where lightweight fill materials such as expanded polystyrene, expanded cl ay or fo amed concrete are used, the possibility of buoy ant effects shall be considered (C lause 12.5(3)P). Specif ic req uir ements r elate to the (obligator y) geot echnical desig n r eport: In cases where a supervision and monitori ng programme is required, the designer s hall present it in a Geotechnical Design Report (see 2.8). It shall be s pecified t hat t he monitoring records are to be evaluated and acted upon as necessary (C lause 12.7(3)P). Example 4 .7 ag ain illustrates t he import anc e of det ermining what ar e unfavour able as opposed to favour able earth act ions, since they will be factored b y differ ent values.
72
CIRIA C641
Example 4.7
The design of a stable slope
Project
Subject
Example 4.7
Slope Stability Slope Stability Soil J = 20kN/m³ c’ = 10kPa I ’ = 25q
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r u = 0.3kPa
11.5.1 (5)
2.4.7.3.2 (2)
Diagram Notes: The clay slope is assumed to be homogeneous and isotropic; circular failure surfaces are analysed. Where the soil mass is sliding along a circular slip surface, most of the weight of the sliding soil, Wu acts unfavourably, causing sliding, while some of the weight, Wf acts favourably, resisting sliding. For this type of problem partial factors are applied to the “effects of actions” ie the disturbing and restoring moments. Stability analysis for the slope is based on charts in Stability Coefficients for Earth Slopes by Bishop and Morgenstern (Geotechnique, 1960).
Clauses 2.4.7.3.4.2( 1) 11.5.1 (1)
Design Approach 1 is used.
Design Approach 1: Stability analysis for slopes Combination 1: A1 “+” M1 “+” R1 Design Actions (A1)
Table A.NA.3 NA.A1.2(B) (Permanent Unfavourable)
As stated the partial factor is applied to the effects of actions ie Moments. ȖEM = 1.35 Design Resistance (R1) The restraint to rotation is provided by shear resistance along the projected circular shear surface. The applicable partial factor is:
Table A.NA.14 (Earth Resistance)
CIRIA C641
ȖR;e = 1.0
73
Design Soil Parameters (M1) Table A4
Weight density ȖJ= 1.0 Design Jd = 20kN/m³
Table A.NA.4
Angle of shearing resistance ȖI’ = 1.0 Design tanI’d1 = (tan25q)/1 = 0.466 Design I’d1 = 25q
Table A.NA.4
Effective cohesion Ȗc’ = 1.0 Design c’d1 = 10/1 = 10kPa For slope stability analysis the equation is based on
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Mdst/Mstb Mdst = Destabilising moment Mstb = Stabilising moment
Consequently for Combination I using the partial factors on the effects of actions, ie moments, the stability equation becomes: Ȗ EM x Mdst;k = Ȗ R;e x Mstb;k 1.35 x Mdst;k = 1.0 x Mstb;k where Mdst;k – characteristic disturbing moment Mstb;k – characteristic restoring moment which is in essence the “traditional” form of a slope stability equation with a factor of safety of 1.35. Form of analysis Adopting charts by Bishop and Morgenstern (1960), the analysis takes the form of [1]
F = m – n.r u m,n – coefficients r u – pore pressure ratio Where F, for combination 1, must be greater than or equal to 1.35 for stability to be assured. Calculations c’/JH D.H D ru
= = = =
10/(20 x 10) = 0.05 12.5 12.5/H = 1.25 (where H = 10) 0.3
The following table is a summary of the various factor of safety, F based on Equation (1) for (c’/JH = 0.05) and (D = 1.25)
74
CIRIA C641
cot E m n F
2:1 1.822 1.595 1.344
2.1 : 1 1.850 1.600 1.370
2.5 : 1 2.000 1.720 1.484
3:1 2.222 1.897 1.653
4:1 2.705 2.287 2.019
5:1 3.211 2.690 2.404
Conclusion Based on the above calculation, the minimum factor of safety of 1.35 is achieved for cot E | 2.1 : 1 with a critical slope angle of about 25.5q.
Combination 2: A2 “+” M2 “+” R1 Design Actions (A2)
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As stated above th e partial factors are applied to the effects of actions, ie moments. Table A.NA.3 NA.A1.2(C) (Permanent Unfavourable)
ȖEM = 1.00
Design Resistance (R1) Table A.NA.14 (Earth Resistance)
ȖR;e = 1.00
Design Soil Parameters (M2) Table A4
Weight density ȖJ= 1.0 Design Jd = 20kN/m³
Table A.NA.4
Angle of shearing resistance ȖI’ = 1.25 Design tanI’d1 = (tan25q)/1.25 = 0.373 Design I’d1 = 20.5q
Table A.NA.4
Effective cohesion Ȗc’ = 1.25 Design c’d1 = 10/1.25 = 8kPa Form of Analysis As opposed to Combination 1, for Combination 2 the composite factor is 1.0 (ȖEM/ȖR;e), so stability is assured when F2 1.0 Calculations c’/JH D.H D ru
= = = =
8/(20 x 10) = 0.04 12.5 12.5/H = 1.25 (where H = 10) 0.3
Based on the charts (Bishop & Margenstern, 1960), there is no available chart for c’/JH = 0.04. Therefore, an interpolation for the factor of safety is calculated based on results derived from the two charts for c’/JH = 0.025 and c’/JH = 0.05 CIRIA C641
75
The following table is a summary of the various factor of safety, F based on Equation (1) for (c’/JH = 0.025) and (D = 1.25) cot E m n F
2:1 1.330 1.230 0.961
2.1 : 1 1.350 1.280 0.966
2.5 : 1 1.490 1.380 1.076
3:1 1.640 1.510 1.187
The following table is a summary of the various factor of safety, F based on Equation (1) for (c’/JH = 0.05) and (D = 1.25) cot E m n F
2:1 1.530 1.300 1.140
2.1 : 1 1.550 1.330 1.151
2.5 : 1 1.700 1.400 1.280
3:1 1.860 1.540 1.398
From the interpolation of the results from both tables for cot E = 2 : 1
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F2
= 0.961 + [(0.015 / 0.025) x 0.179] = 1.07
Conclusion Based on the above calculation, the minimum combination factor of 1.00 is achieved for cot E | 2 : 1 with a critical slope angle of 26.6q.
Overall Conclusions: 1 Combination 1 has a more critical slope angle of 25.5q based on the minimum required factor of safety.
76
CIRIA C641
4.7
Hydraulic failure
4.7.1
Introduction Unlike existing BS geotechnical codes, EC7-1 devotes an entire section (Section 10) to design to avoid hydraulic failure. It identifies four modes of failure:
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4.7.2
failure by uplift (buoyancy), given an unique ULS, “UPL”
failure by heave, given an unique ULS, “HYD”
failure by internal erosion
failure by piping.
UPL design (see Clause 2.4.7.4) This ULS has been introduced into EC7-1 primarily to cover what we know as the buoyancy of submerged structures, a subject not accorded a separate limit state in BS codes but rather treated as a design case for a specific structure such as a deep basement. While BS 8004 addresses uplift or buoyancy only in very general terms, EC7-1 introduces a specific design expression for achieving equilibrium: Vdst;d ≤ Gstb;d + Rd where Vdst;d is the sum of the permanent and temporary destabilising actions, Gstb;d is the sum of the permanent stabilising actions and Rd is the design value of any additional resistance to uplift, such as friction on the sides of a deep basement. Specific values of partial factor are given for use in this inequality (see Tables A.15 and A.16 in Annex A)92. Example 4.8 demonstrates how to satisfy the UPL limit state and the forces that are acting in an uplift situation.
CIRIA C641
77
Example 4.8
An excavation below the water table , showing design against uplift
Project
Subject
Example 4.8
Uplift of a hollow underground structure
Bac kfill soil c’ = 0kPa I ’ = 30q Jdry = 18kN/m³ Js at = 20kN/m³
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Concrete J = 24kN/m³
Diagram Notes: A circular hollow cast in situ reinforced concrete structure of internal diameter = 8m, depth = 5m and wall thickness of 0.5m is located in the ground with a groundwater table of 1m below existing ground surface level. The backfill soil around the structure has properties of c’ = 0 and I’ = 30q with J = 18kN/m3 a bove the water table and J = 20kN/m3 below the water table. This example demonstrates how the stability of an underground structure against uplift should be approached using EC7. In this case, the design is based on the calculation of the required base slab thickness to resist the uplift action induced by the water pressure under the structure. 1 st trial, Base slab thickness = 0.5m Characteristic Structure Load Weight of wall = [(S x 9 x 9)/4 – (S x 8 x 8)/4] x 5 x 24=1602.2kN Weight of base slab = (S x 8 x 8)/4 x 0.5 x 24 = 603.2kN [1] Clauses [2]
Total characteristic weight of structure Gstb,k = 1602.2 + 603.2 Characteristic uplift water force Uplift water force under the base slab U dst,k = 4 x 9.81x (S x 9 x 9)/4
2.4.7.4
Resistance to uplift Verification equation
Eqn 2.8
Vdst;d Gstb;d + Rd
Gstb,k = 2205.4kN U dst,k = 2496.3kN
Design Loads Design structure load (Wall + Base slab) Table A.NA.15 (Permanent Favourable) [3] 78
The weight of the structure is a permanent favourable action as increasing the load increases the resistance to uplift. ȖG;stb = 0.9 Gstb;d = 0.9 x 2205.4
Gstb;d = 1984.9kN CIRIA C641
Design uplift water force Table A.NA.15 (Permanent Unfavourable) [4]
The uplift water force is a permanent unfavourable action. ȖG;dst = 1.1 U dst;d = 1.1 x 2496.3
U dst;d = 2745.9kN
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Design Soil Parameters Table A.NA.16
Angle of shearing resistance ȖI’ = 1.25 Design tanI’d = (tan30q)/1.25 = 0.46 Design I’d = 24.7q
Table A.NA.16
Effective cohesion Ȗc’ = 1.25 Design c’d = 0/1.25 = 0kPa
9.5.1 (7)
9.5.2 (3)
Soil/Wall interface, G K = 1.0 Design Gd = K.I’d Design Gd = 24.7q In this case, no movement of wall relative to the ground should take place. Therefore the earth pressure is calculated from the at rest state of stress. Ko = (1 – sin.I’) x OCR (in this case, OCR = 1) Ko = (1 – sin24.7) = 0.58 Design Resistance The design resistance due to the friction between the backfill and the wall acts in a favourable manner as it causes a stabilising downward shear force; resisting the uplift force.
Annex C Eqn (C.1)
W a(z) = V’h; o.tanG + a
where a = c’ (in this case, c’d = 0kPa)
Design Shear Stress 2.4.7.4 (2) Table A.NA.15 (Permanent favourable)
W o;d (z) JG;stb W o;d (0) W o;d (1) W o;d (5)
= Ko(Jd x z x ȖG;st b).tanG Note: q = 0 (no surcharge) (below water table use J’ instead of Jd ) = 0.9 = 0.58 x (18 x 0 x 0.9) x tan24.7q = 0kPa = 0.58 x (18 x 1 x 0.9) x tan24.7q =4.3kPa = 4.3 + {0.58 x [(20 - 9.81) x 4 x 0.9] x tan24.7q} = 14.1kPa
Design Shear Load (Average)
[5]
Vd;W = [(0 + 4.3)/2 x x 9] + [(4.3 + 14.1)/2 x (4 x x 9)] = 60.8 + 1040.5
Vd;W = 1101.3 kN
Check for uplift of the structure Total destabilising vertical action = Vdst;d = U dst;d = 2745.9kN Total stabilising vertical action = Gstb;d = 1984.9kN Additional resistance to uplift = Rd = Vd;W = 1101.3kN
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Verification for uplift: Vdst;d d Gstb;d + Rd 2.4.7.4 (1)
2745.9kN d 1984.9 + 1101.3 (= 3086.2kN) Conclusion:
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4.7.3
The total stabilising force is greater than the total destabilising force therefore resistance to uplift is satisfied. Base sla b thickness of 0.5m is acceptable for the design to resist uplift force induced by the water pressure underneath the structure.
HYD ULS design (see Clause 2.4.7.5) This ULS has been introduc ed into EC7-1 to co ver failure by heave due to seepage in the ground. Again, while t his form of f ailur e is co ver ed in parts of BS 8002 and BS 8004, it is not accor ded it s o wn general lim it state. The E urocode offers two alternative forms of calculat ion for checking the stabilit y of a soil co lum n (Cla use 2.4.7 .5(1)P), for exam ple in an embankm ent or slope either b y a calculat ion in term s of pore water pressures and total stresses (Inequality 2.9a) or by a calc ulatio n in terms of seepag e forces and submerg ed weig ht (I neq uality 2.9 b). The design value of the pore wat er pressur e used in a ULS calc ulatio n is det ermined by applying a partial f actor to t he char acter istic value of the pressure using the usual definit ion of “charact erist ic”. Note that differ ent J values will apply depending o n whether the pressures ar e permanent, variab le, f avour able or unf avo urable.
4.7.4
Failure by internal erosion EC7-1 req uires t hat suitab le filter protect ion b e used to limit internal ero sio n.
4.7.5
Failure by piping To prevent the onset of piping , E C7-1 mak es mandator y t he use of pr escriptive measur es invo lving either filter s or t he control or b lockage of ground- wat er flo w. The code also requir es the reg ular inspect ion of susc ept ible str uctur es dur ing extrem e c ircumst anc es and the availab ilit y of mater ials for mit igat ion.
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5
Carrying out the construction
5.1
Summary
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5.2
1
Separation of design from construction.
2
Separate BS ENs dealing exclusively with the construction of individual geotechnical elements – ease of updating.
3
Mandatory for “appropriate” recording in the geotechnical design report of any supervision, construction, monitoring and maintenance requirements. The GDR must be updated as new information becomes available.
Construction requirements in EC7-1 A key difference between the BS codes and the new Eurocode suite of documents is that, in the latter, “design” is largely separated from “execution” (construction) in different documents93. Although EC7-1 is primarily about “design”, there are some recommendations about construction in it. These are complementary to the requirements of the set of BS ENs listed in Table 5.1, which also shows the approximate correspondence between the BS and BS EN documents. That part of EC7-1 that deals with construction matters, namely Section 4 Supervision of construction, monitoring and maintenance, introduces several requirements. The key difference here from BS codes is that these are mandatory, as is appropriate reporting in the geotechnical design report. For example, EC7-1 makes it mandatory for the GDR to include a plan of supervision activities94 and criteria for acceptable results. In Annex J the code provides a checklist of what should be considered for monitoring, particularly the flow of water and groundwater pressures.
5.3
BS EN “execution” standards discussed and compared with relevant BSs It is apparent from Table 5.1 that several topics (such as diaphragm walls and bored piles) are repeated in different BS codes. The publication of new BS ENs dedicated to a particular element may serve beneficially to concentrate information and should make future updating quicker and easier. A quick comparison of the numbers of pages95 in the BS ENs with those in the BSs suggests that the former contain more detail. However, BS 8006 and BS 8081 are likely to be the exception to this. As part of the work to draft the UK National Annex for EC7-1, possible conflicts between the BS ENs available at the time and BSs were identified. The results are shown in Appendix A4.
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Table 5.1
Correspondence between BS codes and standards and European codes and standards
Equivalent “execution” BS ENs Construction covered in BS code
BS 8004:1986
All of the execution BS ENs have a fairly standard contents list, a typical example being shown in this endnote96. BS EN 1538:2000
Diaphragm walls (55 pp)
Shallow foundations
BS EN 15237:2007
Vertical drainage (56 pp)
Deep and sub-aqueous foundations
BS EN 12715:2000
Grouting (57 pp)
Cofferdams and caissons
BS EN 1536:2000
Bored piles (91 pp)
Geotechnical processes: groundwater lowering, grouting and other methods of changing the ground characteristics in situ.
BS EN 12699:2001
Displacement piles (51 pp)
Foundations
Pile foundations
BS EN 12063:1999
Sheet pile walls (59 pp)
BS EN 14199:2005
Micropiles (50 pp).
Site preparation for foundation work Durability Safety precautions
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BS 8008:1996
Safety precautions and procedures for the construction and descent of machine-bored shafts for piling and other purposes
BS 8002:1994 (with amendments) Earth retaining structures (134 pp)
None
BS EN 1536:2000
Bored piles (91 pp)
BS EN 1538:2000
Diaphragm walls (55 pp)
Data for design
BS EN 12063:1999
Sheet pile walls (59 pp)
Design philosophy, design method and earth pressures
BS EN 14199:2005
Micropiles (52 pp)
BS EN 14475:2006
Reinforced fill (54 pp)
Design of specific earth retaining structures (includes some construction details) BS 8006:1995
Strengthened/reinforced soils and other fills (207 pp)
Information needed for execution
Concepts and fundamental principles
Geotechnical investigations
Materials
Materials and products
Testing for design purposes
Design considerations
Principles of design
Execution
Design of walls and abutments
Supervision, testing and monitoring
Design of reinforced slopes
Records
Design of embankments with reinforced soil foundations on poor ground
Specific requirements
Construction and maintenance BS 8081:1989
Ground anchorages (174 pp)
pr EN 14490
Soil nailing (54 pp)
BS EN 1537:2000
Ground anchors (65 pp)
BS EN 12715:2000
Grouting (57 pp)
Site investigation Design Materials and components Corrosion and corrosion protection Stressing equipment Construction Testing Maintenance Legal aspects BS 6031:1981
Earthworks (125 pp)
Site conditions and investigation
BS EN 12716:2001
Jet grouting (39 pp)
Cuttings
pr EN 14490
Soil nailing (54 pp)
Embankments and general filling
BS EN 14679:2005
Deep mixing (52 pp)
Excavation and filling
BS EN 14731:2005
Ground treatment by deep vibration (25 pp)
BS EN 15237:2007
Vertical drainage (56 pp)
Compaction The operation of construction plant for earthworks Maintenance and protection of slopes Trenches pits and shafts
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Implementing the new codes and standards in the UK
6.1
Summary
6.2
1
Initially, EC7 will apply only to the design and construction of buildings. In practice, the principles of EC7-1 are already being adopted by large public bodies for the design of highway schemes and other non-building applications.
2
Only Design Approach 1 will be permitted for projects in the UK.
3
The UK National Annex changes some of the partial factor values recommended in EC7-1, particularly for the design of piles.
4
The timing of the implementation of the Eurocodes remains uncertain.
Introduction97 To date within the UK the extent to which geotechnical design has been codified has been much less than in other sectors. Consequently the introduction of EC7-1 will represent some change in UK practice and the needs of geotechnical designers in order to permit them to adapt to the change may be significant. EC7-2 will represent a change in the way in which geotechnical parameters for design are developed from testing. Guidance such as presented in this book and elsewhere98 will be needed on the derivation and application of characteristic values and how these fit with the EC7-1 framework of design approaches, partial factors and ultimate and serviceability limit state design.
6.3
“National choice” and the National Annexes The setting of levels of safety for buildings and civil engineering works and parts thereof, including aspects of durability and economy are a matter for individual member states of the EU. Such national choice, as identified by “notes” in EC7, is made in the National Annex (NA). The NA for BS EN 1997-1 contains the following features99:
the stipulation that only Design Approach 1 will be used for GEO and STR ultimate limit state design calculations
the values of the partial factors to use in all design calculations. The NA makes some changes to the factor values given in Annex A of EC7-1, specifically for the design of piles
a statement that the informative Annexes in EC7-1 will not be withdrawn or substituted, although some comment is made on:
Annex C, on the determination of earth pressures
Annex D, concerning depth and ground inclination factors
In addition, the NA lists the current BS codes and other commonly used design documents such as CIRIA C580 that contain “non-contradictory complementary information” (NCCI) for use with EC7-1. The NA for BS EN 1997-2 is expected to be available in 2009.
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6.4
The retention of valuable national code and standards material The National Annex to BS EN 1997-1:2004 includes references to NCCI in addition to the current suite of BS Codes and Standard. These are PD 6694-1, CIRIA C580 (Gaba et al, 2003) and UK Design manual for roads and bridges (HA, 1997). Design aspects of some of these, or parts of them, might be in conflict with the design principles in BS EN 1997-1:2004. Until such time as “residual” documents are prepared to remove such conflicts and in the event that use of these documents presents a conflict, the Eurocode takes precedence.
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EN 1997-1 Geotechnical Design does not cover the design and execution of reinforced soil structures. In the UK, the design and execution of reinforced fill structures and soil nailing should be carried out in accordance with BS 8006, BS EN 14475 and pr EN 14490. The partial factors set out in BS 8006 should not be replaced by similar factors from Eurocode 7. Eventually, the intention is that all BS geotechnical codes will be withdrawn100. However, since much of the general, “best practice” advice given in them is not found in BS EN documents, and as this advice does not contradict any of the principles laid down in the latter, it is hoped that, once minor amendments have been made to remove conflicts such as those listed in Table A4.1, the BS codes will have their status reduced to “advisory” and will then be retained to “assist the user in applying the Eurocode”. Since most of the familiar BS material will still exist after the introduction of the BS ENs there may be some confusion about which documents take precedence. A general rule could be that, where there is overlap and unless otherwise stated in the NA, the BS EN governs and the BS material may be used only to supplement the design and construction activities.
6.5
Time-scale and processes for change The BS codes and standards have been developed and updated over many decades – indeed some of them are now showing their age. While not necessarily using them every day, many design and checking engineers will be familiar with their content. It will be no small task for engineers to replace the BS documents with the new BS ENs and to become familiar with the new concepts contained in EC7-1. The manner in which the change-over to the suite of new BS ENs will take place is not yet fully determined. It is however expected broadly to follow the sequence of activities, including any eventual withdrawal or amendment of BS codes and standards, that is illustrated in Figure 6.1. Note that while the design codes EC7-1 and 2 will co-exist as shown in Figure 6.1, the BS EN test standard documents can co-exist with existing BS documents for only six months from their publication date (see Powell and Norbury, 2007). The Eurocodes are particularly relevant for projects involving international cooperation or competition, especially on publicly-funded work, where it is likely to become a legal requirement to accept designs which satisfy the Eurocodes. It is anticipated that the BS ENs will be fully implemented by such public bodies as the Highways Agency and Network Rail, but the timing of this is not yet known.
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Figure 6.1
Possible implementation timetable
6.6
Guidance material To ease the change-over to the Eurocodes, much guidance material has been, and is being, prepared. In addition to this publication, the following documents exist or are expected:
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a suite of Designers’ guides, published by Thomas Telford Limited (eg Frank et al, 2004, for EC7-1)
Students’ guide to the Eurocodes, published by BSI (PP 1990:2004)
a series of Simple guides to the Eurocodes, published by ODPM, including a guide to EC7-1 (Driscoll et al, 2005).
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The impact of the geotechnical Eurocode system on UK practice
7.1
Summary
7.2
1
Provision of comprehensive lists, representing “good” practice.
2
Emphasis on risk identification, assessment and avoidance, leading to requirement for better awareness of ground variability.
3
Greater emphasis on ground deformation, impact on site investigation practice, including greater costs.
4
Costs of a) acquiring new documents, b) training and c) application.
The impact of EC7-1 on design practice It is anticipated that the use of EC7-1 will bring about the main changes, both positive and negative, to general design practice indicated in Table 7.1.
Table 7.1
Impact of EC7-1 on design practice Impacts
Impacts
EC7-1 contains many comprehensive lists of matters deemed to be “good geotechnical practice”. As reference to these lists, which are generally confined to what has to be done or achieved, rather than how to do it, is most often mandatory, the introduction of EC7-1 should do much to raise the quality of geotechnics generally101.
Overall, a greater number of documents to work with.
EC7-1 places emphasis on risk identification, assessment and avoidance ahead of construction. As such, it is the first geotechnical design code to embrace fully the CDM requirements.
May require more detailed and better quality site investigation.
There will be a need for greater clarity about where uncertainty (and hence risk) resides in a design problem. Engineers will need to think more carefully about the different geotechnical ingredients in a project and may have to demonstrate to a greater degree how they have been dealt with.
EC7-1 contains less detail on how design calculations are performed. A likely increase in design time, at least while engineers absorb the new documents and adjust to the changes in design methodology that they could bring.
Designers will need to specify any supervision and testing that is required during construction, with potentially greater supervision costs. Designers may be concerned that, unless steps are taken to convince Professional Indemnity Insurers that the change of codes will not lead to increased risk, their insurance premiums will rise.
There will be a need for greater clarity of thought about the variability of the ground behaviour and about how to deal with it102. There will be a need to think more specifically about limiting foundation and ground movements. In part, this implies that the geotechnical profession will need to improve its ability to calculate deformations and the Eurocode provides strong encouragement for continuing development of our understanding and handling of the deformation properties of the ground, and of the numerical methods that use them103.
More site investigation and design effort, resulting in greater costs.
It may be that site investigation practice will need to change to deliver a better understanding of the deformational properties of the ground.
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Theoretically, these changes should be technically beneficial, but only time will tell. The original purpose of the Eurocodes was to facilitate trade and fair competition in Europe. Whether the introduction of EC7 will assist in achieving this ambitious goal will not be known for some time after the implementation of the entire Eurocodes system.
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7.3
The impact of EC7-2 and associated documents on site investigation practice It was seen in Table 2.1 that EC7-2 and the associated testing standards and technical specifications will bring only small, but important, changes in the requirements of the site investigating industry. In practice, the geotechnical community will need to be aware that the mandatory reporting of ground information will become more prescribed and that appropriate communication of information will be obligatory. Furthermore, the greater general emphasis on the assessment of deformation is likely to lead to a greater need for SI providers to consider ground deformation parameters. These changes may, in turn, need to be reflected in changes to the procurement of site investigation services. There is also the potential for some initial confusion arising from the slight changes in soil description terms. Beyond these, any changes may be confined to a few small differences in some testing procedures, as shown in Table 2.1 (see Powell and Norbury (2007) for changes to SPT and DP tests).
7.4
The impact on geotechnical construction practice The BS EN and pr ENs execution standards listed in Table 1.1 were written for CEN by teams of engineers drawn from the geotechnical construction industry as represented by the European Federation of Foundation Contractors (EFFC). UK representatives were largely provided by the Federation of Piling Specialists, the UK member of EFFC. The opportunity was taken to produce up-to-date material reflecting the current state-of-the-art in Europe. As such, these documents should ensure that best practices are adopted throughout the UK and may serve to bring about general improvement in application.
7.5
Overall impact104 It is difficult to assess the immediate and long-term impact of a substantial change to the system of codes and standards that the Eurocodes will impose. The Institution of Structural Engineers has expressed concern over the cost implications for small-tomedium sized engineering practices in fully adopting all the structural Eurocodes (IStructE, 2004). The sums are not inconsiderable and it remains to be seen if clients will accept consequent increased costs unless they find there to be clear economic advantage in using the Eurocodes.
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References
BALDWIN, M, GOSLING, R and BROWNLIE, N (2007) “Soil and rock descriptions. A practical guide to the implementation of BS EN ISO 14688 and 14689” In: Ground Engineering, July 2007, pp 14–24 BAUDIN, C M (2001) “Design procedure according to Eurocode 7 and analysis of the test results” In: Proc symposium on Screw piles – installation and design in stiff clay, Brussels, Balkema, Rotterdam, pp 275–303
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BEREZANTSEV, V G, KHRISOFOROV, V S and GOLUBKOV, V N (1961) Load bearing capacity and deformation of piled foundations” In: Proc 5th int conf on Soil. Mech. and Found. Eng, vol 2, pp 11–15 BISHOP, A W and MORGENSTERN, N (1960) “Stability coefficients for earth slopes” Géotechnique, 10, pp 129–150 BJERRUM, L 1973 “Problems of soil mechanics and construction on soft clays and structurally unstable soils” General report In: In: Proc 8th lnt conf Soil. Mech. Found. Eng, Session 4, Moscow, 3, pp 111–159 BLUM, H (1931) “Einspannungsverhaltnisse bei Bohlwerken” In: Wilh. Ernst und Sohn, Berlin BURLAND, J B (1973) “Shaft friction of piles in clay – a simple fundamental approach” Ground Engineering, pp 30–42 CLARKE, B (1995) Pressuremeters in geotechnical design 1st edition, Routledge, Taylor & Francis Group, London (ISBN: 978-0-75140-041-0) DRISCOLL, R, POWELL, J and SCOTT, P (2005) A designers’ simple guide to BS EN 1997 Department for Communities and Local Government, London ENGLAND, M (1999) A pile behaviour model PhD Thesis, Imperial College FLEMING, W G K (1992) “A new method for single pile settlement prediction and analysis” Géotechnique, 42 (3), pp 411–425 FLEMING, W G K, WELTMAN, A, RANDOLPH, M F and ELSON, K (1986) Piling engineering Surrey University Press/Halsted Press, New York, NY FRANK, R, BAUDIN, C, DRISCOLL, R, KAVVADAS, M, KREBS OVESEN, N, ORR, T and SCHUPPENER, B (2004) Designers’ guide to EN 1997: Geotechnical design – Part 1: General rules ASCE, Thomas Telford Publishing, London (ISBN: 0-72773-154-8)
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GABA, A R, SIMPSON, B, POWRIE, W and BEADMAN, D R (2003) Embedded retaining walls: Guidance for economic design C580, CIRIA, London GULVANESSIAN, H, CALGARO, J-A and HOLICK, M (2002) Designers’ guide to EN 1990: Basis of structural design ASCE, Thomas Telford Publishing, London (ISBN: 0-72773-011-8) INSTITUTION OF STRUCTURAL ENGINEERS (2004) National strategy for implementation of the structural Eurocodes: Design guidance Report prepared for The Office of the Deputy Prime Minister, IStructE, London KÉRISEL, J and ABSI, E (1990) Active and passive earth pressure tables 3rd edition Balkema, Rotterdam (ISBN: 9-06191-886-3)
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KING, C (1981) The stratigraphy of the London Clay and associated deposits Tertiary Research Special Paper No.6 Rotterdam, Backhuys KULHAWY, F H (1985) “Uplift behavior of shallow soil anchors – an overview” In: Clemence (ed), special publication Uplift behavior of anchor foundations in soil, American Society of Civil Engineers, pp 1–25 LONDON DISTRICT SURVEYORS ASSOCIATION (2000) Foundations: Note 1: Guidance notes for the design of straight shafted bored piles in London Clay LUNNE, T, ROBERTSON, P and POWELL, J J M (1997) Cone penetration testing in geotechnical practice Spon Press, London (ISBN: 978-0-41923-750-1) NICHOLSON, D, TSE, C and PENNY, C (1999) The Observational Method in ground engineering: Principles and applications R185, CIRIA, London ODPM (2005) Steel and steel and concrete composite buildings: companion document to EN 1993 and EN 1994 Department for Communities and Local Government, London PADFIELD, C J and MAIR, R J (1984) Design of retaining walls embedded in stiff clay R104, CIRIA, London PATEL, D (1992) “Interpretation of results of pile tests in London Clay” In: Proc “Piling Europe” (piling, European practice and worldwide trends), 7–9 April 1992, Institution of Civil Engineers, pp 91–101 PECK, R B (1969) “Advantages and limitations of the Observational Method in applied soil mechanics” Géotechnique, 19 (2), pp 171–187 POWELL, J (2003) A simple guide to in-situ ground testing – in 7 parts. Part 2: Cone penetration testing 144065, BRE Bookshop, London POWELL, J J M and NORBURY, D R (2007) “An update on implementation of EC7” In: Ground Engineering, June 2007, pp 14–17
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SCHNEIDER, H R (1999) Determination of characteristic soil properties In: Proc 12th European Conf on Soil Mechanics and Foundation Engineering (ECSMFE), Amsterdam, June 7-12 1999. Balkema, Rotterdam, vol 1, pp 273–281 SIMPSON, B (2000) “Partial factors: where to apply them?” In: Proc LSD2000 Int workshop on limit state design in geotechnical engineering, ISSMGE, TC23, Melbourne SIMPSON, B and DRISCOLL, R (1998) Eurocode 7 a commentary BRE Press, London
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SKEMPTON, A (1951) "The Bearing Capacity of Clays," In: Proc. Building Research Congress, vol 1, pp 180–189 SKEMPTON, A (1959) “Cast in situ bored piles in London Clay” Géotechnique, 4, pp 153–173 SKEMPTON, A (1961) “Horizontal stresses in an overconsolidated eocene clay” In: Leonards, G A, and Ramiah, B K (eds) Proc 5th ICSMFE, 1960, ASTM STP 254, ASTM, pp 351–357 STROUD, M (1989) “The standard penetration test, its application and interpretation” In: Proc. Conf on Penetration testing in the UK, pp 29–50 TOMLINSON, M J (2001) Foundation design and construction 7th edition Prentice Hall, Pearson Higher Education (ISBN 978-0-13031-180-1) British standards BS EN ISO 14688-1:2002 Geotechnical investigation and testing – Identification and classification of soil Part 1: Identification of soil BS EN ISO 14689-1:2003 Geotechnical investigation and testing – Identification and classification of rock Part 1: Identification of rock BS EN ISO 14688-2:2004 Geotechnical investigation and testing – Identification and classification of soil Part 2: Classification principles of soil BS EN ISO 22476-2:2005 Geotechnical investigation and testing. Field testing. Standard penetration test BS EN ISO 22476-3:2005 Geotechnical investigation and testing. Field testing. Dynamic probing BS EN ISO 22475-1:2006 Geotechnical investigation and testing – Sampling methods and groundwater measurements. Part 1: Technical principles for execution DD EN ISO/TS 22475-2:2006 Geotechnical investigation and testing – Sampling methods and groundwater measurements. Part 2: Qualification criteria for enterprises and personnel DD EN ISO/TS 22475-3:2007 Geotechnical investigation and testing – Sampling methods and groundwater measurements. Part 3: Conformity assessment of enterprises and personnel by third party BS 1377:1990 British Standards methods for soils for civil engineering purposes
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BS 5930:1999 Code of practice for site investigations BS 6031:1981 Code of practice for earthworks BS 8004:1986 Code of practice for foundations BS 8002:1994 Code of practice for earth retaining structures (with amendments) BS 8006:1995 Code of practice for strengthened/reinforced soils and other fills BS 8008:1996 Guide to safety precautions and procedures for construction and descent of machine-bored shafts for piling and other purposes BS 8081:1989 Code of practice for ground anchorages BS 8110-1:1997 Structural use of concrete BS EN 1536:2000 Execution of special geotechnical works – bored piles BS EN 1537:2000 Execution of special geotechnical works – ground anchors BS EN 1538:2000 Execution of special geotechnical works – diaphragm walls
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BS EN 12063:1999 Execution of special geotechnical works – sheet pile walls BS EN 12699:2001 Execution of special geotechnical works – displacement piles BS EN 12715:2000 Execution of special geotechnical works – grouting BS EN 12716:2001 Execution of special geotechnical works – jet grouting BS EN 14475:2006 Execution of special geotechnical works – reinforced fill BS EN 15237:2007 Execution of special geotechnical works – vertical drainage PD 6694-1 (in preparation) Recommendations for the design of structures subject to trafiic loading to BS EN 1997-1 PP 1990:2004 Structural Eurocodes. Guide to the structural Eurocodes for students of structural design pr EN 14490:2007 Execution of special geotechnical works – soil nailing Eurocodes DD ENV 1997-1 Eurocode 7 (1995) Geotechnical design – Part 1 general rules NA to BS EN 1997-1:2004: UK National Annex to Eurocode 7:Geotechnical design – Part 1: General rules BS EN 1990:2002 Eurocode: Basis of structural design BS EN 1991-1-1:2002 Eurocode 1 Actions on structures. Part 1-1: General actions. Densities, self-weight, imposed loads for buildings BS EN 1993-5:2007 Eurocode 5 Design of steel structures Part 5 – piling BS EN 1997-1:2004 Eurocode 7 Geotechnical design – Part 1: General rules BS EN 1997-2:2007 Eurocode 7 Geotechnical design – Part 2: Ground investigation and testing BS EN 1998-5:2004 Eurocode 8 Design of structures for earthquake resistance – Part 5: Foundations, retaining structures and geotechnical aspects
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A1
Examples of the selection of characteristic ground property values using all available site information
Figure A1.1 shows the results of a series of undrained shear strength measurements on samples taken from three boreholes in London clay. The measurements were made in unconsolidated undrained triaxial tests. A statistical mean line has been drawn through the data and it is clear that undrained strength increases with depth. A characteristic line is required, and this should depend on how the characteristic values will be used – what is the limit state being considered? For example, if the undrained strength is used to calculate ground movements around a retaining wall, a value such as the “cautious (average)” value shown on the figure could be used. However, for a problem in which failure might take place in a small zone of soil, such as an isolated foundation placed at a deep level, a more cautious value – the “cautious (local)” value – should be adopted. Results from standard penetration tests were also available for these boreholes, as shown in Figure A1.2. In London clay, there is usually a fairly constant correlation factor between standard penetration and undrained shear strength results – the factor varies between about 4.5 and 5. However, if the mean line from the SPT results is transferred onto the undrained strength plot, as in Figure A1.3, it appears that the correlation factor does not fit the normal range – values larger than five would be necessary to fit the undrained strength results. In fact, the measured undrained strengths are remarkably high and are consistent with very low measurements of water contents. Figure A1.3 also shows lines representing mean values through data from other nearby sites, both for undrained shear strength and SPT results. The usual correlation factor value between SPT “N” and undrained strength applies to these, and it is confirmed that the undrained strengths for the new site are remarkably high. On the basis of these inconsistent data sets, what profile should be taken as the characteristic profile of undrained strength? The values measured in the triaxial tests should not be ignored, but the SPT results and the data from adjacent sites should also affect the decision. The characteristic profile proposed for all data is shown on Figure A1.4. This is less than the cautious (average) assessment in Figure A1.1, which was based on the triaxial results only, and is closer to the cautious (local) assessment for this particular set of triaxial results. Engineers often need to follow this sort of process when trying to interpret real data. It may be that statistical methods could trace a similar logical sequence. However, this would require quite advanced methods and any statistical approach which failed to take account of the diverse array of data that is sometimes available would be detrimental to the design process. Characteristic values depend on failure mode To illustrate how the words “affecting the occurrence of the limit state” (in the definition of characteristic value) might apply, consider that the characteristic value of a parameter in one ground stratum is not necessarily the same for two different limit states. It may depend on the extent to which a particular failure mode averages out the variable properties of the stratum, as illustrated in Figure A1.5. This shows a small industrial building, founded on pad footings near a long slope. The underlying materials are estuarine beds, mainly of sands with some impersistent lenses of clay
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occurring at random. In this type of situation, the designer could, for designing the footings, assume that all of them are founded on clay, the most adverse circumstance105. When the designer considers the overall stability of the building, and the possibility of a slope failure along the large slip surface illustrated, it seems inconceivable that this surface will lie entirely, or even mainly in clay. In this example, there could be more than one characteristic value for strength parameters of the same site, with a selection for the footing design that is different from that of the slip surface.
Figure A1.1
UU txl. strengths (U100) for a site with 3 b/hs (courtesy Simpson and Driscoll, 1998)
Figure A1.2
Corrected SPT “N” values for the site (courtesy Simpson and Driscoll, 1998)
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Figure A1.3
SPT inferred strengths (courtesy Simpson and Driscoll, 1998)
Figure A1.4
Assessed “characteristic” strength profile (courtesy Simpson and Driscoll, 1998)
Figure A1.5
Small building on estuarine beds near slope
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A2
Statistical methods
EC7-1 states that, if statistical methods are used, they should differentiate between local and regional sampling and should allow the use of a priori knowledge of comparable experience with ground properties. This demands a high order of statistical expertise available only from a few designers. Attempts by statisticians to tackle geotechnical design have often failed and it is very difficult to have a sufficient grasp of both disciplines that can be combined sensibly.
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Nevertheless, some pointers to more general rules for assessment of data might be obtained from statistical analysis. Schneider (1999) has proposed that, where a spatial mean is relevant, the characteristic value might be taken as half a standard deviation from the mean. Simpson and Driscoll (1998) suggest that this rule could be a useful guide. A very simplistic form of statistical manipulation has been adopted in EC7-1 for the design of piles using either sets of pile load test results or profiles of ground strength data. In this, allowance is made for: 1
The amount of data, using a “correlation factor”.
2
The variability of data by assessing maximum and minimum values and taking the more pessimistic outcome (see Section 4.4.1).
Further guidance on the use of statistics in determining characteristic values may be found in Frank et al (2004).
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A3
Design Approach 1 for GEO and STR limit state calculations
A3.1
Introduction
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In some European countries there are perceived to be different and equivalent ways to account for the coupling between geotechnical actions and resistances, so the expressions 2.6 and 2.7 shown in Section 3.7.5 can be applied using different combinations of γF, γE, γM and γR. This leads to a plethora of different factor values, and combinations of values106. To help with understanding how to apply these values in the alternative forms of expressions 2.6 and 2.7, EC7-1 simplifies them into sets of values shown symbolically as follows: A “+” M “+” R where:
the symbol A represents the sets of partial factors for the actions γF (expression 2.6a)or for the effects of actions γE (expression 2.6b)
the symbol M represents the sets of partial factors γM for strength (material) parameters of the ground
the symbol R represents the sets of partial factors for resistance γR (expression 2.6b)
the symbol “+” means “used in combination with”.
While Section 3.7.5 of this book shows only one form of the expressions 2.6 and 2.7, EC7-1 in fact provides three alternatives for use in calculations to check that GEO and STR ultimate limit states will not be exceeded. These are the three design approaches (1, 2 and 3) shortened to “DA-1” etc. As mentioned in Section 3.7.5, only DA-1 is permitted in BS EN 1997-1, for the reasons discussed below. The other two approaches are not given any further coverage in this book107. In EC7-1, “recommended” values for the sets of factors for the different design approaches are given in Annex A. This annex is “normative” which means that the factors and sets of factors should be used, but the values of the factors are “informative” which means that alternative values may be given in the National Annex (hence the word “recommended”). The National Annex to BS EN 1997-1 provides the partial factor values for DA-1 to be used in design in Britain.
A3.2
Design Approach 1 For actions, partial factors are usually applied directly to the representative values of the actions (expression 2.6a), except when doing so would lead to physically impossible situations108. For resistances, partial factors are applied to the characteristic values of the ground strength parameters (c′k and tan ϕ′k or cu;k) using expression 2.7a. An important
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exception to this general rule applies for the design of piles and anchorages. This is because their designs have most often evolved from measurements of their resistance to load, in site tests. Checks for the designs of piles and anchorages are usually undertaken by applying partial resistance factors γR (expression 2.7b) to measured or calculated characteristic pile and anchorage resistances. In principle, DA-1 requires separate checks to be performed for failure in the soil and in the structure, using two combination sets of partial factors109. The two distinct combinations of factors used for the separate checks are now explained. For simplicity, factor values for DA-1 and the GEO and STR limit states have been assembled from the Annex A tables into Table A3.1. Combination 1110 This is expressed symbolically as:
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A1 “+” M1 “+” R1 in which the sets of partial factor values are used in combination. This combination, by using γG = 1.35 and γQ = 1.50 (where γG and γQ are, respectively, the factors for permanent and variable structural actions111 – see set A1 in Table A3.1), is intended to provide safe design against unfavourable deviations of the actions, or of their effects, from their characteristic values. Calculations of ground resistance are performed using design values of ground properties that are equal to their characteristic values, ie γϕ′ = γc′ = γcu = γcu = γγ = 1 (set M1, Table A3.1). The R1 term is ignored since, except for piles and anchorages, resistances are not factored112. Combination 2113 This is expressed symbolically as: A2 “+” M2 “+” R1 In this combination, it is assumed that the permanent actions are very close to their expected, representative values (γG = 1, set A2, Table A3.1) and that the variable actions from the structure may deviate in an unfavourable way (γQ = 1.3, set A2, Table A3.1). Calculations of ground resistance are performed using design values where γϕ′ = γc′ = 1.25, γcu = γqu = 1.4 and γγ = 1 (set M2, Table A3.1). This combination is intended to provide safe design against unfavourable deviations of the ground strengths from their characteristic values. Uncertainty about the calculation model’s real behaviour is also assumed to be covered by the partial factor values of combination 2. As resistances are not generally used, the R1 term is ignored114. The design of piles and anchorages For the design of piles and anchorages, combination 2 becomes: A2 “+” (M1 or M2) “+” R4 If the resistance of piles and anchorages is calculated using ground properties, a value of resistance is calculated by applying γϕ = γc = γcu = 1 (set M1, Table A3.1) to the
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ground properties and then by applying a partial resistance factor γR > 1 (set R4, Table A3.1). Note that values of γR vary depending on the type of pile and the type of mobilised resistance. In certain specific instances, such as where piles are unfavourably loaded by the ground, for example where downdrag occurs, set M2 is applied to the characteristic values of ground properties to calculate the unfavourable forces and R4 is applied to the favourable resistance of the pile in the stable ground.
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A suggested initial procedure is firstly to perform the combination 2 calculation to find the size of the substructure and then to check that its strength is sufficient to carry the internal forces and moments found using combination 1. Obviously, where the strength of the foundation is not in question, this second step will be unnecessary unless very tight cost controls are in place. Where it is obvious that one combination governs the design, it is usually not necessary to perform full calculations for the other combination. However, which of the two combinations will prove critical may not always be obvious. For example, in the design of a pad footing supporting a tower that is subjected to significantly large lateral forces, it may be found that a combination 1 calculation governs the pad dimensions, rather than combination 2. For this reason, in principle, both combinations require checking, though a designer will, with experience, know that calculation using only one combination will suffice for a straight-forward design problem. Further information on DA-1 may be found in Simpson (2000). The principal reasons why only DA-1 is permitted for design in the UK are:
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DA-1 is felt to be a logical and comprehensive approach. During the development of the ENV, there had been broad agreement that all designs should, in principle, be checked separately for three cases A, B and C115, so it is appropriate to adopt DA-1 in BS EN 1997-1
there is a concern that in certain, often rather complex designs of, for example, flexible sheet pile walls with substantial interactions between the ground and the walls, the application of DA-2 and DA-3 might lead to uncertain and possibly erroneous results
there is a fundamental point of principle concerning the place in a design calculation where the partial factors are applied. DA-1 is the approach that most consistently applies partial factors where the greatest uncertainties lie, that is to actions and material properties rather than to the effects of (geotechnical) actions, with the exception of the design of piles and anchorages. This matter has been more fully discussed by Simpson (2000).
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Values of partial factors recommended in EC7-1 Annex A
Factors on actions* (or the effects of actions)
Anchors
CFA piles
1.1 (1.0)
1.1 (1.0)
Temporary
γa;p
1.25 (1.0) γa;t
Shaft in tension
Permanent
1.1 (1.0)
γs
1.0
γs
Shaft (compression)
γs;t
1.1 (1.0)
γb
Base
Total/combined (compression
1.15 (1.0) 1.25 (1.0)
γs;t
Shaft in tension
1.0
γs
Shaft (compression) Total/combined (compression)
1.25 (1.0)
γb
Base
γs
1.25 (1.0)
Shaft (compression)
γs;t
γs
Shaft in tension
γb
Base
1.0
1.0
1.0
γγ
Weight density
γt
1.0
γqu
Unconfined strength
Total/combined (compression)
1.0
1.0
1.0
γcu
Effective cohesion
R1*
Undrained strength
1.0
0
fav
γc′
1.5
unfav
γ φ′
1
fav
M1
Combination 1
tanϕ′
Variable – γQ
1.35
unfav
A1
Note: * Figures in bold are the changed values required by the UK National Annex to BS EN 1997-1:2004. Refer to NA for details. ** Bold figures in last two columns are the values required by the UK National Annex to BS EN 1997-1:2004. Refer to NA for details.
“R” (resistance values) (γR)
Bored piles
Driven piles
“M” (material values) Factors on ground (γM) properties
“A” (Action values) (γF)
Permanent – γG
Note: Care is required when using these factors in computer modelling. See Section 3.7.3. The spaces shown in grey contain factor values = 1, as may be seen in the Tables in Annex A of EC7-1.
Table A3.1
0.0
1.3
1.0
1.0
A2
1
1.4
1.4
1.25
1.25
M2
R1
0.0
1.3
1.0
1.0
A2
1.0
1.0
1.0
1.0
1.0
M1 or
1.0
1.4
1.4
1.25
1.25
M2
1.1
1.1
1.6
1.4
1.3
1.45
1.6
1.5
1.3
1.6
1.6
1.3
1.3
1.3
2.0
2.0
1.6
2.0
2.0
2.0
1.6
2.0
2.0
1.7
1.5
1.7
R4**
Piles and anchors, Combination 2
Design Approach 1 Combination 2
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1.7
1.7
1.4
1.7
1.7
1.7
1.4
1.7
1.7
1.5
1.3
1.5
A4
Conflicts of construction practice and requisite amendments
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A systematic examination, commissioned by BSI and the then DETR116, of the BS EN execution codes that were available117 in 2000 found very little conflict with construction guidance in BS codes. Table A4.1 lists the conflicts that were found, while Chapter 6 indicates how the solutions to them will be implemented when the full system of BS EN standards supersedes BSs.
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BS 8008
BS 8004
BS 8002
BS Code
Table A4.1
Conflicts of values
Conflict of quantities of cement and slump
Lateral link spacings
Rates differ
BS 8004 has no requirement
Pile positional and verticality tolerances
Concrete
Pile reinforcement
CRP pile testing
Noise
References to other British standards
Differences
Driven pile reinforcement
Out-of-position (on land) values
Driven piles
Change range from “150-200 mm” to “160–220 mm”
fresh concrete slump values
“where reference is made in this British standard to other British standards, the requirements of the corresponding BS EN shall take precedence”.
Add general statement:
Refer to BS EN 12063:1999 requirement for European and national standards to be met.
Change BS 8004 rate of 0.75 mm/min for friction piles in clay and 1.5 mm/min for end-bearing in sand and gravel to BS EN 1536 requirement for ~1 mm/min119
Amend to BS EN 12699:2001 requirement of a min spacing of 100 mm or 80 mm with aggregate ≤ 20 mm.
BS 8004 range of slump (75 mm to collapse) to be changed to 130–180 mm of BS EN 1536
BS 8004 cement quantity of 300 kg/m³ conflicts with min required in BS EN 1536.
Amend BS 8004 to comply with BS EN 12699 and BS EN 1536
Amend BS 8004 to comply with requirement of BS EN 12699 for a minimum amount of reinforcement of 0.5 % of the pile cross-section or 4 no. 12 mm bars in the top 4 m of bearing pile.
Replace BS 8004 value of £75 mm with BS EN 12699 value of £ 100 mm.
Replace (withdrawn) DFCP 4 with ISO 13500:1998 Petroleum and natural gas industries, Drilling fluid materials, Specifications and tests.
Change to reflect EN 1538 requirement for verticality within 1 %.
Verticality tolerances
Specification
Change to refer to BS EN 1993-5:2007
BS EN 12063 refers to ENV 1993-5:1998
Bentonite
Cast in situ diaphragm walls used as retaining walls
Section modulus of steel sheet piling
“where reference is made in this British standard to other British standards, the requirements of the corresponding BS EN shall take precedence”.
Add general statement:
Change to refer to BS EN 1538:2000
BS EN 1538:2000 has some different tolerances118
References to other British standards
Change to refer to BS EN 12063:1999
Refers to manufacturers’ recommendations
“where reference is made in this British Standard to other British standards, the requirements of the corresponding BS EN shall take precedence”.
Add general statement:
Solution
Tolerances bored pile walls and secant piles
References to other British standards
Conflict
Section modulus of steel sheet piling
Topic
Note: These BS EN geotechnical execution standards currently have no authority since they are not referenced in current design codes and will only have such authority when BS EN 1997-1 is published and implemented.
Conflicts between BS Codes and those BS EN execution standards available in January 2005
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BS 8081
Delete option in BS 8081 Table 11.
BS 8081 permits a class of protection: “temporary without protection”. This option is not permitted in BS EN 1537 (Clause 6.9.2)
Protection of temporary anchors
Amend BS 8081 accordingly.
Amend BS 8081
BS 8081 – +/- 2.5°, BS EN ≤ 2° BS 8081 refers to BS 5896, whereas BS EN 1537 refers to manufacturers’ specifications. BS 8081 – 5° (strand), 2.5° (bar) v. BS EN 1537 - ≤ 3°
Angular tolerance of drill holes
Radii of tendon coils during storage
Limiting angular tolerances at anchor head
Testing of rock bolts
Some conflict
In BS 8081 delete “…when a higher proportion (50 – 100 %) should be subject to such tests” and put “… when all should be subject to such tests”.
BS EN 1537:2000 –“…shall be calibrated at intervals not exceeding six Amend BS 8081 months and …. prior to use on contract..”
BS 8081 – “prior to use on contract”
Amend BS 8081 to comply.
BS 8081 dos not mention accuracy during creep tests and otherwise gives values different from BS EN 1537
Accuracy of readings during short and long-duration load testing
Equipment calibration
Amend BS 8081 to comply.
BS 8081 permits min thickness of 0.8 mm, but in BS EN 1537 this is 1 mm
Thickness of plastic sheath walls and corrugated ducts
Amend BS 8081 to comply.
Amend BS 8081 requirement for < 0.1 mm to < 0.2 mm.
Change BS 8081 value of +/- 5° to BS EN 1537:2000 value of ≤ 3° at 97 % characteristic load capacity.
Amend BS 8081 to increase design stress from 80 %of characteristic tendon force to 100 % of characteristic load capacity required in BS EN 1537:2000.
Amend BS 8081 to increase cover thickness from 5–10 mm required in BS EN 1537
Different values (BS EN 1537:2000 value for TAM type anchor)
Tolerance of tendon angular position
Capacity differences
Cover thickness differs
“where reference is made in this British standard to other British standards, the requirements of the corresponding BS EN shall take precedence”.
Add general statement:
Limiting crack width in grout between tendon and duct
Stressing head
Grout cover to tendon or encapsulation, and to centralisers.
References to other British standards
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A5
Case study using EC7
Preface The final version of EC7-1 was published in December 2004. While several designs undoubtedly have been undertaken by some in the UK’s geotechnical community using BS EN 1997-1, the Steering Group for this book was able to acquire this very interesting ENV design using the “trial” ENV version (BSI, 1995), which employs many of the principles embodied in BS EN 1997-1.
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This case study illustrates the following specific benefits to be obtained from EC7-1 by virtue of:
pile tests (the more the better)
adopting partial factors that lead to lower equivalent overall factors than used in “traditional” design but with care taken to ensure that settlements are acceptable.
A specific objective in the ground investigation for the project was to assess ground variability using reliable and repeatable CPT in order to adopt the (lower) partial factors.
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Pile test interpretation for the new Wembley National Stadium Introduction The new Wembley Stadium required the design and construction of about 4000 piles, of varying diameters between 0.45 m and 1.5 m, and varying lengths between 10 m and 40 m. All the piles were rotary bored, cast in situ straight shafted piles. Some of the piles were subject to complex combinations of vertical, horizontal, moment and torsional loads. Prior to and during the works a comprehensive pile test programme was undertaken including compression, tension and lateral load tests with some piles loaded to failure. This case history summarises the relevant data from the ground investigations, the interpretation of tests on preliminary piles subject to compressive loading only and the application of the draft (BSI, 1995) EC7 rules for deriving factor of safety. This application of EC7 resulted in significant cost and programme savings.
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Site geology and ground properties The site is underlain by a thick deposit of London Clay. Across most of the site, there is a layer of made ground about 1 m thick which overlies the London Clay surface. Locally beneath parts of the old stadium made ground increased in thickness to about 5 m. The site is on a hill and ground surface level varies by about 10 m. To the south of the site is a railway cutting up to 13 m deep. The upper 8 to 10 m of London Clay is weathered, and at depth the macro fabric of the unweathered London Clay changes with silt and sand partings becoming more frequent. This macro fabric change is believed to be associated with a transition between stratigraphic unit B to A (King, 1981) and occurs at a level of about 20 to 25 m OD. At 10 to 15 m OD, the Lambeth Group underlies the London Clay, and the chalk is encountered at a level of about -2 to -4 m OD. The site geology and topography are shown in Figure A5.1. For calculating the ULS pile capacity a conventional total stress approach was adopted (eg Fleming et al, 1986), hence an undrained strength profile was required for design calculations. Seven separate methods were used to assess undrained strength, su: i
empirical correlation between SPT “N” and su (Stroud, 1989)
ii
“quick” unconsolidated undrained triaxial tests on 100 mm diameter specimens, from U100 sampling “quick” unconsolidated undrained triaxial tests on 38 mm diameter specimens, from U100 sampling self-boring pressuremeter tests (Clarke, 1995) empirical correlation between static cone penetration test, qc, and su (Lunne et al, 1997)
iii iv v vi
unconsolidated undrained triaxial tests carried out at a slow strain rate on 100 mm diameter specimens, using high quality samples from rotary coring or push-in thin wall sampling. Mean effective stress prior to shear, p′k, was measured and su in situ = (su lab)p′ in situ, with p′ in situ derived from a range of methods. p′k
(denoted TW/RC on Figure A5.2). vii su derived from effective stress parameters (c′, ϕ′), excess pore pressures at failure (Af) and in situ stress conditions (Ko , σv′) as described by Skempton (1961). Data from all the above methods, except method (iii) are shown on Figure A5.2. As expected the tests on 38 mm diameter samples, method (iii), gave considerable scatter, and gave strengths about 30 to 50 per cent higher than equivalent tests on 100 mm diameter samples, method (ii). Perhaps more noteworthy is the observation that the undrained strengths from method (ii) were markedly higher than all the other methods within the weathered London Clay, and at greater depth in the unweathered London Clay the rate of increase of strength with depth was relatively small. There could be several reasons for this including sample disturbance effects and strain rate effects. Historically, empirical pile capacity methods based on a total stress, su, approach (Skempton, 1959) were based on a mean profile through 38 mm diameter undrained strength data, method (iii), and an alpha, α, value of 0.4. This was discounted at Wembley due to the large scatter making the derivation of a reliable “characteristic” profile for design problematic. More recently, this empirical approach has been modified (Patel, 1992), based on 100 mm diameter undrained strength data, method (ii), and α equal to 0.6 (if capacity based on CRP tests, as carried out historically), or α equal to 0.5 (if capacity based on maintained load, ML, tests, as current conventional UK practice). The decision was made to derive an undrained strength profile from the SPT “N” values, method (i), and the more advanced test methods, methods (iv) to (vii) inclusive, for two main reasons: (a) there was less scatter in the data, hence derivation of a “characteristic” strength profile was deemed to be more reliable and representative of the in situ mass soil strength and its variation with depth, and (b) there would be a tendency to over-predict the ULS capacity of shallow piles and under-predict the ULS capacity of deep piles if method (ii) was used, the former being unsafe and the latter uneconomic. The characteristic su profile is also shown on Figure A5.2. For this profile an α of 0.6 was used, this gave equivalent pile capacities for intermediate length piles (about 25 m long) as method (ii) with an α of 0.5, and also with
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alternative effective stress methods (Burland, 1973). For geotechnical design an important consideration is an assessment of intrinsic ground variability, eg potential variation in undrained shear strength at a particular depth between different parts of the site. Routine laboratory shear strength testing is usually not suitable for assessment of variability, because of the scatter introduced during sampling and specimen preparation stages, and the variable influence of soil fabric at the laboratory scale (eg fissure orientation, silt/sand lenses, gravel particles). London Clay is normally considered to be relatively homogenous laterally, however at Wembley there were particular concerns due to the site’s variable topography and the presence of the railway cutting adjacent to the southern site boundary. To assess this a large number of static cone penetration tests were carried out, and the cone resistance, qc , profiles are summarised on Figure A5.3. At any particular depth qc only fluctuates by about +10 per cent around the mean (except when claystone bands are encountered).
Compressive load tests on preliminary piles Seven tests were carried out on preliminary piles which were loaded to failure, the geometrical and load test data are summarised in Table A5.1.
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Table A5.1
Summary of vertical pile tests
Pile test no.
Pile dia (m)
Ground surface (mOD)
Pile length (m)
Predicted capacity (kN)
Ultimate load (kN)
1
0.45
2
0.45
46.8
15.2
1408
1462
46.8
25.0
2811
2905
3
0.75
46.8
11.5
1852
2030
4
0.60
46.8
25.0
3864
3864
5
0.60
46.6
20.0
2839
2698
6
0.60
53.1
20.0
2839
3267
7
0.60
50.2
20.6
2974
3123
Note: “Ultimate” load obtained at displacements of 7-10 per cent of pile diameter.
Four of the pile tests were distributed across the site, to check if there were systematic variations in behaviour in different parts of the site, with the remaining three tests carried out in a predefined area to verify variation in capacity with depth. The tests were maintained load tests carried out in accordance with the ICE Specification for piling and embedded retaining walls (1996) and particular attention was paid to ensuring that rates of pile head movement were low prior to applying the next load increment. One departure from normal practice was that unloadreload stages were not carried out, which follows recommendations by England (1999), since this was expected to simplify interpretation of pile test data. The load-settlement data is summarised in Figure A5.4. The ultimate pile capacities are compared with predicted values in Figure A5.5 and Table A5.1, measured pile capacities are on average about 5 per cent higher than the predicted pile capacities, with a range of -5 per cent (capacity over-predicted) and +15 per cent (capacity under-predicted).
Interpretation of compressive load tests Fleming’s method (Fleming, 1992) was used to interpret the pile tests, this modern empirical method is based on the back analysis of several thousand pile tests and considers the main components of pile load-settlement behaviour (pile concrete compressibility, base resistance, shaft capacity) within a robust framework. A particular benefit is the ability to interpolate and extrapolate pile load-settlement behaviour for piles of different diameters and lengths. EC7120 allows pile capacity to be predicted from load tests on piles with a diameter of no less than 50 per cent of the working piles. To extrapolate safely a robust analytical framework is required, which Fleming’s method provides, together with preliminary test piles which are loaded to large displacements so that pile behaviour can be properly analysed. At Wembley, working pile diameters were a maximum of 1.5 m, whereas the maximum pile diameter for preliminary test piles was only 0.75 m. The pile head displacement of the preliminary test piles at “failure” was between 7 and 10 per cent of the pile diameter, with a final test displacement of at least 35 mm to ensure that shaft capacity had been fully mobilised. The first stage of the pile test interpretation was to extrapolate the settlement time response at each load increment to infinite time (England, 1999). Because the test stages were held at constant load for sufficient time this extrapolation could be carried out reliably and for this site the corrected pile load displacement curve was only slightly beneath the measured curve, for example the “timeset” corrected curve for the most flexible pile, TP2, is shown on Figure A5.4. Based on the pile tests, characteristic, upper and lower bound sets of parameters were derived for
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subsequent predictions using Fleming’s method, summarised in Table A5.2. Table A5.2
Fleming’s analyses (CEMSET), input parameters
“Characteristic” values
Upper bound
Lower bound
Shaft capacity, Us
t = α u, α = 0.6 su, see Figure 2
10% increase
10% decrease
Base capacity, Ub
qb = 9 su, su as for shaft
10% increase
30% decrease
Shaft length through made ground, Lo (zero friction transfer)
Best estimate
5% decrease
5% increase
Shaft length through London clay, Lf (friction transfer)
Best estimate
5% increase
5% decrease
Shaft flexibility, Ms
0.001
0.0008
0.0015
Base deformation modulus, Eb
250 su
500 su
100 su
40
60
30
Parameter
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concrete Young’s modulus, Ec (GN/m²)
Note: 1. Friction Transfer Coefficient, KE equal to 0.45 for all analyses. 2. Refer to Fleming (1992) for description of parameters.
The range of parameters was intended to reflect potential variations in likely pile load-settlement behaviour for the main working piles due to ground variability and construction tolerances, eg made ground thickness (Lo), shaft length in London Clay (Lf), pile concrete stiffness (Ec), shaft interface compressibility (Ms), shaft capacity (Us) and, in particular, base capacity (Ub) and base stiffness (Eb). The relatively large variations in Ub and Eb reflect the intrinsic difficulty of forming a clean competent pile base. It should be noted that this range of parameter values is site specific and reflects the site geology, thoroughness of the ground investigations and the level of site supervision. Other sites, pile types and construction environments may demand different parameter sets. The parameter variations were not intended to accommodate gross workmanship errors. Figure A5.6 compares the load test data with the predicted load-settlement response from Fleming’s method, for Test Piles 1 and 4. Typically the actual load-settlement response is between the characteristic and upper bound profiles. Fleming’s method was then used to predict the loadsettlement response of larger diameter piles. Figure A5.7 compares the data from a load test on a 1.5 m diameter working pile carried out during the main works with the predictions from Fleming’s method and using the characteristic, upper and lower bound parameter sets given in Table A5.2. It can be seen that the “characteristic” parameters derived from the preliminary pile tests, give a remarkably accurate prediction of the behaviour of a much larger pile subjected to significantly larger loads. Figure A5.8 compares the normalised load-settlement response of test piles at Wembley with pile test data reported in the technical literature. The influence of L/d (pile length divided by diameter) ratio is worth noting. TP2, which is a very long slender pile, is more compressible than short stubby piles, eg TP1 and 3. TP2 is, however, not representative of the L/d ratios of the working piles (typically between 13 and 45, whereas TP2’s L/d equals about 56), and TP7 is perhaps more representative of the more flexible working piles (L/d equal to 34). Normalised load-settlement curves from the Fleming’s method predictions for a representative range of pile lengths and diameters are shown in Figure A5.9.
Derivation of factor of safety The factors of safety usually adopted for UK pile design are between 2 and 3 (BS8004). In contrast, the combination of partial factor values recommended in the ENV version of EC7 can give equivalent overall factors of safety as low as 1.43 (1.1 × 1.3, for ULS case C)121, 122 on the lower bound of shaft capacity from pile test results (assuming there are three or more pile tests). It is worth examining the main factors that need to be considered when deriving an appropriate factor of safety. These are summarised in Table A5.3 with specific comments for the Wembley Stadium site.
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Table A5.3
Factors which may affect choice of factor of safety
Risk factor
Site specific comments
Site specific risk
Geology uncertainty
Relatively uniform conditions London Clay main bearing strata.
Very low
Variable strength and deformation characteristics
Comprehensive investigations, both in situ and laboratory testing.
Very low
Applied loading
Extremely complex structure, some potential uncertainty.
Medium
Reliability of geotechnical calculations
Several different methods of analysis, verified by site specific load tests to failure.
Very low
Pile installation workmanship
Some variability inevitable, but top quality piling contractor and independent supervision.
Low
Excessive settlement
Specific SLS analyses, verified by site specific tests.
Low
For this site the main uncertainty was judged to be the applied loading, due to the overall complexity of the superstructure. For many sites the loading may be simple, but the geology, strength parameters and reliability of geotechnical calculations may be much more uncertain. For this site the overall factor of safety set for piles subject to compressive loading was 1.7, based on the characteristic soil shear strength profile (Figure A5.2) and an α value of 0.6. Design in accordance with EC7 would require a factor of safety of 1.69 (1.3 × 1.3) on mean shaft resistance from pile tests and 2.08 (1.3 × 1.6) on mean base resistance from pile tests for ULS Case C123. For most pile geometries at Wembley the shaft capacity comprised more than 85 per cent of the overall pile capacity, hence the above simplification changed predicted pile safe working load by less than 5 per cent. This was deemed to be suitably conservative given the uniformity of the site geology and the detailed nature of the ground investigations and subsequent geotechnical analyses. The influence of a change in factors of safety on pile load-settlement behaviour is explored in Figure A5.9 and A5.10. From Figure A5.9, using the lower bound normalised Fleming curve for a relatively compressible pile (27 m long, 0.6 diameter), the settlement at an overall factor of safety of 2 would be 0.61 per cent of pile diameter, ie 3.7 mm. At an overall factor of safety of 1.7 (used for the Wembley project) normalised settlement for the same pile is only increased to 0.77 per cent, ie 4.6 mm. Hence, the reduction in overall factor of safety from a “conventional” value to those recommended in EC7 (ENV) would have a negligible impact on pile settlement at this site. Although it should be noted that for different ground conditions and/or pile types changes in factors of safety may have a more significant impact on settlement particularly when end bearing resistance is important at working loads. Figure A5.10 shows the test data for TP7 (representative of a relatively flexible working pile) compared with the characteristic, upper and lower bound curves from the Fleming analyses. Curve M is the measured test data, curve A is the “characteristic” load-settlement plot (obtained by dividing curve M by 1.3) used for SLS design and ULS case B, and curve B is derived from curve A, shaft resistance is divided by 1.3 and base resistance by 1.6. Curve C is curve M divided by 1.7. Curve C is very similar to curve B, particularly for loads less than the predicted pile safe working load. Curve A is similar to the lower bound “Fleming” load-settlement curve.
Conclusions A combination of in situ testing together with modern sampling and laboratory testing methods enabled a reliable “characteristic” undrained strength profile to be developed. Conventional sampling and routine laboratory testing methods generated strength data with significant scatter. This scatter was not representative of intrinsic ground variability, but merely indicative of sampling disturbance and inherent problems with routine laboratory test methods. Ground variability was best assessed from static cone penetration testing, due to its high repeatability. This indicated a variability of undrained strength of only about +10 per cent about a mean profile. Preliminary pile tests to failure confirmed that the characteristic design parameters were appropriate for assessing pile ultimate capacity under compressive loads derived from the ground investigation. The potential effects of loading rate and displacement-time behaviour need to be considered in interpreting pile test data. The application of Fleming’s method for single pile analysis, provided a rational framework for extrapolating pile load settlement behaviour from small diameter piles to larger diameter piles, and for piles with different L/d ratios. The application of EC7 (ENV) rules for reductions in partial factor values (equivalent to a reduction in overall factor of safety) on the basis of the results of pile tests, and the number of them, facilitated significant cost and programme savings for this element of the works.
Acknowledgements A large number of individuals and organisations contributed to the success of the piling engineering at Wembley Stadium. However, Dr Ed Ellis, former colleague at Mott MacDonald, and Mr Nigel Brooks, chief design engineer at Stent Foundations warrant particular mention.
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Figure A5.1
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Wembley Stadium site geology and topography
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Figure A5.2
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Undrained shear strength
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Figure A5.3
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CPT cone resistance profiles
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Figure A5.4
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Preliminary pile load tests
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Figure A5.5
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Pile tests, observed versus predicted failure loads
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Figure A5.6
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Pile load settlement behaviour (observed versus predicted)
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Figure A5.7
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1.5 m diameter pile predicted load settlement (from load tests on 0.45 m to 0.75 m diameter piles)
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Figure A5.8
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Wembley pile load test data compared with previous published results
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Figure A5.9
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Predicted pile load settlement characteristics
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Figure A5.10
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Test pile 7 measured, characteristic and factored load settlement curves, compared with predicted behaviour
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A6
The provenance of BS EN standards
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In the provision of the structural Eurocodes, member state national standards bodies (NSBs – BSI in our case) adopt documents that are published by CEN, the European standards body. CEN acquires its documents primarily from two sources: its own committees and from ISO. Under the auspices of Technical Committee 250 – Structural Eurocodes, CEN drafting committees, panels and working groups produce design standards. TC250 has subcommittees that deal with each of the Eurocodes, so CEN TC250-SC7 has written and is writing the two parts of the geotechnical Eurocode, EC7. At the time that SC7 was set up, there was no provision for the writing of standards other than for design, ie there was no resource to produce ground testing standards or, for example, standards for the testing of geotechnical elements such as piles. Furthermore, the TC250 SC7 committee’s remit was to cover design, not construction. Consequently, a number of separate technical committees were set up under CEN to produce Geotechnical “execution” (construction) standards (TC288) and Ground testing standards (TC341). Co-incidentally, several ISO committees have been working to produce International Standards of similar, if not identical, scope. The most significant is ISO TC 182 for the identification and classification of soil and rock. CEN and ISO came to an agreement that they would try to avoid duplication of effort by, where appropriate, adopting each others’ documents. The upshot is that all documents emanating from CEN committees are published by them with the reference “EN…”, and any relevant ISO material (having firstly been accepted - after any requisite editing – by the relevant CEN committee) are published with the reference “EN ISO …”. The published document is then issued by CEN to each NSB which must publish locally within six months of receipt. All CEN documents published by BSI have the reference “BS EN…”
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Endnotes
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1
These are documents such as CIRIA guides, design manuals of the Highways Agency and Network Rail, engineering standards of London Underground Ltd. (now Transport for London) and design guides of the London District Surveyors Association.
2
Several guides to Eurocode 7 are anticipated. Thomas Telford Ltd has already published one (Frank et al, 2004) while ODPM have produced an electronic simple guide to Eurocode 7 (Driscoll et al, 2005). BSI has published students’ guides to the entire suite of structural Eurocodes, including EC7 (PP 1990:2004).
3
The final text of EC7-1 was published by BSI in December 2004.
4
International Standards Organisation. ISO and CEN have an agreement to share a common set of codes and standards.
5
“Traditional” design practice has gathered several sources of uncertainty into one, global, “factor of safety”, whereas the Eurocode requires more precise identification and quantification of the separate uncertainties when performing design calculations.
6
This change implies clarity in specifications concerning “who” shall do “what”. Specifications will need to state where responsibilities lie and how information shall be reported and communicated.
7
The code makes frequent use of the term “verification” – see Glossary.
8
Informative Annexes offer recommended methods and calculation models that may be substituted by others if so wished.
9
These are documents such as CIRIA guides, design manuals of the Highways Agency and Network Rail, engineering standards of London Underground Ltd (now Transport for London) and design guides of the London District Surveyors Association.
10
UK representation on the relevant committees seeks to ensure that the best of UK practice appears in the European documents.
11
The use of expressions such as “if relevant” or “if possible” in some Principle clauses serves to weaken the impact of “shall”.
12
However, EC7-2 does not state who shall perform the investigation beyond repeating the requirement of EC7-1 regarding appropriate qualification and experience.
13
It might be implied that items of this type are “obligatory” and if not done could be construed as poor practice.
14
These categories express the ability of the sampling process to obtain samples in a range of quality classes. BS EN ISO 22475-1 also specifies the required handling, transport and storage of samples.
15
This may be regarded as “good” practice but is now rather more prescriptive than in UK BSs.
16
Clause 2.1.1(12)P of EC7-2 requires that An appropriate quality assurance system shall be in place in the laboratory, in the field and in the engineering office, and quality control shall be exercised competently in all phases of the investigations and their evaluation. There is no guidance on what is meant by “appropriate”. It is envisaged that quality assurance schemes such as NAMAS will continue to operate using the EC7-2 requirements.
17
EC7 does not use the term “site investigation”.
18
This should be appropriate to the nature and scale of the project and to the limit states that apply to the project.
19
The selection of the characteristic value cannot be made without sufficient knowledge of the nature of the site and design requirements of the project. It cannot be made in isolation in a GIR.
20
It is unfortunate that the word “derived” has been used here since it is not connected with the obtaining of a “derived value” shown in Figures 2.1 and 2.2
21
See for example Simpson & Driscoll, 1998.
22
This is as true for “undisturbed” ground material as it is for fill for earthworks or for backfill behind a retaining wall.
23
See for example Frank et al, 2004.
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24
Note that EC7 uses the term “weight density” instead of “unit weight”.
25
The context of this definition is ULS structural design, in which values of stiffness are needed for analysis, but they rarely play a dominant part in determining the occurrence of a limit state.
26
Assumptions about any elastic behaviour and suitable ground deformation parameters depend crucially on such matters as the nature of the ground eg the degree of any over-consolidation and appropriate correlations between strength and stiffness. For example, for stiff, heavily over-consolidated clay or weak rock, elastic behaviour is often adopted using a conservative value of Eu/su, based on mean su values. Conversely, an assumption of elastic behaviour would not be appropriate for normally and lightly over-consolidated soil.
27
If a serviceability limit state is being considered and displacements are being calculated, partial load factors will be unity, so displacements calculated using a true “mean value” stiffness would be best estimates, with no reserve of safety. In design practice, engineers rarely take this approach, preferring to make a more pessimistic estimate using correlations that recognise the background of the settlement calculation method and case history evidence of comparisons between observation and prediction.
28
EC7 consists of clauses that are either normative, that is they contain the word “shall”, or informative, that is they use words such as “should” or “may”. The former are called “Principle” clauses and are numbered using the letter “P”, while the latter are called “Application Rules”, ie they indicate means by which the Principles may be fulfilled.
29
Adopting in EC7-1 a design system that is fully compatible with structural design has introduced some additional complexity. For example, in contrast to most structural design, geotechnical actions and resistances often cannot be separated, as in the case of a flexible retaining wall where it is not immediately apparent which actions (forces) are stabilising (favourable) for the structure and which are destabilising (unfavourable). EC-1 embodies a general design methodology that caters for this complexity. Notwithstanding this, it is beneficial for the superstructure and the substructure to be designed using the same basic system for handling uncertainty and safety. By using BS geotechnical codes, there has been an uncomfortable change in philosophy at the design interface between the sub- and superstructure.
30
While “earlier” BS Codes such as BS 8004 do not do this, BS 8002 and BS 8081, having been written during the early development of the Eurocodes, do define ultimate and serviceability limit states.
31
This is in general contrast to BS codes such as BS 8004 that adopt “global” factors. Note, however, that BS 8081 uses partial factors while BS 8002 adopts a strength “mobilisation” factor akin to a partial factor, but only on the resistance side of the equation.
32
In contrast, BS codes such as BS 8002 and BS 8006 use partial factors (or their equivalent) that relate more to the avoidance of an SLS.
33
Prescriptive measures adopt a familiar, established and proven design, so the ground and loading conditions must be well-defined. These measures involve conventional and generally conservative details in the design, and attention to specification and control of materials, workmanship, protections and maintenance ... when calculations are not available or not necessary.
34
This method is particularly useful for the design of piles and anchors.
35
Formal use of the Observational Method post-dates the current suite of British geotechnical codes. It is discussed more fully in Clause 2.7 of EC7-1.
36
Simple structures involving negligible risk, and for which the requirements can be satisfied using prescriptive measures such as local experience, fall into GC 1. Most structures will be in GC 2, while complex problems fall into GC 3. EC7-1 concentrates on GC 2 designs and lists examples of typical design problems (Clause 2.1(10)).
37
EC7-1 uses the term “verification” or “verify”. In this book, the term “check” is used instead of “verification”. This means that the Eurocode may be seen as presenting means for checking that a design is satisfactory, not for conducting the full design itself.
38
For example, the prescribed, minimum depth of a footing in a shrinkable clay soil already contains an allowance for uncertainty (see Approved Document A to the Building Regulations).
39
EC7-1 describes “comparable experience” as documented or other clearly established information related to the ground being considered in design, involving the same types of soil and rock and for which similar geotechnical behaviour is expected, and involving similar structures. Information gained locally is considered to be particularly relevant.
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40
An example would be the table of minimum widths of strip foundations given in Approved Document A to the Building Regulations. These widths are based on allowable bearing pressures for the different ground conditions cited. The calculations of allowable pressure have an implicit “factor of safety” based on “comparable experience”.
41
Prescriptive measures and calculation models may be used concurrently in a project design, but not for the same elements in the design.
42
Such measures are also found in, for example, BS 8004. Problems for which prescriptive solutions are appropriate will normally fall into Geotechnical Category 1.
43
Water pressures frequently cause problems and require particular attention.
44
Readers familiar with the “trial” version of EC7-1 (DD ENV 1997-1, 1995) will recall the Cases A, B and C. These are associated, respectively, with the EQU, STR and GEO limit states, while UPL and HYD have been added in EC7-1.
45
In fact, as γ = 1 for SLS, the design value is equal to the characteristic value. It is important to bear in mind that not only will the γ values be different for ULS and SLS conditions: characteristic values may also differ. For example, a highly cautious value of strength might be taken to satisfy the ULS requirement while a less cautious value might be used in a correlation with Young’s Modulus to determine that the SLS requirement is satisfied.
46
The different design situations for ULS and SLS are defined in BS EN 1990 and discussed in Gulvanessian et al (2002). It is important to note that the partial factor values proposed in EC7-1 (Annex A) for ULS design are only valid for these design situations.
47
For saturated ground with relatively low mass permeability, the time required for the dissipation of excess positive or negative pore water pressures generated by construction activities is large compared to the time of construction. Loads of short duration, eg impact loads, may need to be assessed using different strength and stiffness values from those for persistent conditions (eg undrained strength for impact loads, drained strength under dead weight conditions). For this reason, both transient (the undrained condition with excess pore water pressures) and persistent (the drained condition when the pore water pressures have dissipated) design situations have to be considered when checking that an ultimate limit state will not occur. Transient conditions apply particularly to temporary works design and to construction in ground the strength of which is changing over time.
48
EC7-1 lists only those matters that affect durability and does not indicate how to design to reduce their effects or to eliminate them. Means for eliminating or reducing durability issues may be found in the relevant structural Eurocode such as that for concrete or steel.
49
Note, however that BS 8004 recognises a similar problem: “It should, however, be realised that existence of an adequate factor of safety against shear failure will not necessarily ensure that foundation settlements will be sufficiently small. In particular, the allowable bearing pressure for a large foundation on granular soil may have to be much smaller than the ultimate bearing capacity divided by a conventional factor of safety of two or three.” (BS 8004 Clause 2.1.2.3.2.1, p 9).
50
The term action means either an imposed load or an imposed displacement.
51
For the design situations pertaining, the calculation model is required to predict the resistance (eg the bearing capacity of a pad footing), the effects of the action (eg bending moments and shear forces in the pad footing) and/or the deformations of the ground.
52
This may require reference to more than one Eurocode.
53
Note that BS EN 1991-1-1:2002 applies only where the superstructure is a building.
54
It may be noted that in EC7-1 different interpretations may be made of what are “effects of actions”, depending on the design approach adopted.
55
One small but potentially significant consequence of regarding favourable passive pressure as a resistance not an action is that, rather than applying γG (fav. = 1.0, Table A.3) one would apply a resistance factor γR;e (= 1.0, Table A.13). While γG is fixed in value by BS EN 1990 the geotechnical community has the freedom to vary the value of γR;e if it so wishes. In fact, the UK NA does not change the value of γR;e.
56
For example, bearing capacity theory is adopted to calculate the resistance of a spread footing using the design value of ground strength. In the case of a bored pile, the calculation “model” may be based on an empirical relationship derived from the measurement of pile resistance in a load test. Such a model would be the so-called “α method” using undrained shear strength cu.
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57
Specific forms of this inequality are developed for the EQU (see Section 3.7.7) and UPL (see Section 3.7.8) limit states.
58
The term Xk/γM introduces into the expressions the influences of ground properties such as strength and weight density on geotechnical actions such as earth pressures, and on geotechnical resistances such as the bearing pressure from the ground.
59
The characteristic value of an action, Fk, is its primary, representative value. An action, particularly if it is variable, may have other representative values, for example, when the variable structural actions are combined.
60
Note that γF is used where the factor applies to an action and γE where the factor applies to the effect of the action.
61
Sometimes the resistance in the ground includes a contribution from the representative actions, Frep, such as in the case of a spread foundation subjected to an inclined load.
62
Note that the term Xk has been used only because γM = 1.
63
Use of a simple elastic model for settlement calculation is acceptable for overconsolidated clay, though the selection of a suitable modulus value requires care. However, simple elasticity is not appropriate for normally- and lightly overconsolidated clay. Settlement calculations for granular soil typically use correlations with SPT and CPT data.
64
Clause 4.2(8) of BS EN 1990 states that characteristic values of structural stiffness parameters are equal to their mean values while EC7-1 requires that the characteristic value of the ground stiffness is selected as a cautious estimate of the mean value. This cautious estimate should be consistent with all empirical data for the site.
65
Annex H of EC7-1 gives some indications of limiting values of differential settlements which can be used as guidelines, in the absence of specified limiting values of structural deformations.
66
It should be noted that settlements occurring during construction may not affect the serviceability of the structure or parts of it. These settlements should not be included when comparing with the specified limiting values.
67
Although BS 8004 makes more than 300 references to “settlement”, it does not make any recommendations about limiting values beyond mentioning that the presumed bearing resistances for pad foundations on rock assume a limiting settlement of 0.5 per cent of the pad width. These bearing resistances are reproduced in Annex G of EC7-1.
68
BS 8002 adopts mobilisation factors ranging from 1.2, for effective stress design calculations, to 3 to limit the movement of retaining walls in total stress calculations. This is akin to the global factor value of 3 recommended in BS 8004 for limiting foundation settlements.
69
“For cohesive soils it is always necessary to ensure that the foundation is safe against shear failure by taking a presumed bearing value not greater than the ultimate bearing capacity divided by a suitable factor of safety, conventionally of the order of 2 to 3” (BS 8004 Clause 2.2.2.3.3.4, p 24).
70
“In general, an appropriate factor of safety for a single pile would be between 2 and 3. Low values within this range may be applied where the ultimate bearing capacity has been determined by a sufficient number of loading tests or where they may be justified by local experience; higher values should be used when there is less certainty of the value of the ultimate bearing capacity” (BS 8004 Clause 7.3.8, p 117).
71
“The factor of safety for anchorage system and individual members should not be less than 2 in accordance with BS 8081” (BS 8002 Clause 4.6.5.2.3, p 107).
72
“The factor of safety should be sufficiently large to cater for uncertainties in the parameter values and to satisfy serviceability requirements by preventing unacceptable deformations under working conditions…” (BS 8002 Annex B.1, p 122).
73
Historically, the limit state method became popular at about the time that partial safety factors began to be adopted. The two are often linked, although there is no fundamental connection between them. A calculation using a global factor of safety or directly assessed pessimistic design values could be sufficient to demonstrate that limit states will not occur. The limit state method does not necessarily require calculations as the basis of design.
74
Note that the formulae in the “informative” Annex D apply to homogeneous ground and allowance will need to be made for layering and other inhomogeneities, but it omits depth and ground inclination factors. The NA to BS EN 1997-1 permits the alternative use of depth and ground inclinations as appropriate.
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75
See Annex E of BS EN 1997-1.
76
Results from the standard penetration test (SPT) have been related to the allowable bearing pressure (assuming a 25 mm settlement) for footings on sand (see Tomlinson, 2001). Results from the cone penetration test (CPT) have been similarly used (see Lunne et al, 1997).
77
This definition is the same as that used in BS 8004.
78
BS EN 1997-1 does not define failure for a laterally-loaded pile.
79
In the Fleming method pile shaft and base capacity and base stiffness parameters are used with empirical factors to estimate pile settlement. The method relies on a definition of pile capacity as the asymptote of the load-settlement graph since the model does not give sensible answers if reduced shaft and base capacities are used. If ultimate pile and shaft capacity are taken as the values mobilised at a settlement of 10 per cent pile diameter, the method will over-predict settlement.
80
This will depend on the type of pile, for example, small lightly-loaded piles in stiff clay should experience only small movements and the SLS should not require separate checking. However, large, heavily-loaded piles may experience movements that do require a separate SLS check.
81
Correlation factors have been introduced from some European practices in which piles are designed using the results of many profiles of, for example, CPT tests. Their function is to make allowance for the quantity of data known about test pile performance and the ground. The more data available the lower the factor, leading to a smaller pile, while the more variable the data the more pessimistic is the value that is used in design. There is a clear design benefit from carrying out more testing. Some background information on the values for the ξ factors recommended in Annex A of BS EN 1997-1 can be found in Bauduin (2001).
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82
In the context of BS EN 1997-1, a “profile” may be thought of as a linear sequence of information, such as from ground surface to required maximum depth, on such items as undrained shear strength or blow count in a standard penetration test.
83
The procedure is similar to the one using the results of static pile load tests, that is by using ξ factors to account for the quantity of profiles of ground test data.
84
The first method employs a correlation factor the value of which depends on the quantity of testing and which varies, in the UK National Annex, between 1.15 (many test results or profiles) and 1.55 (only one test result or profile). Consequently, this alternative procedure, which will accord with what many designers might do, requires the application of an additional model factor in order to deliver a design comparable with the first method. The UK National Annex states that the value of this model factor should be 1.4, except that it may be reduced to 1.2 if the resistance is verified by a maintained load test taken to the calculated, unfactored ultimate load.
85
Many small gabion walls are still design using CP2.
86
See BS 8002 and CIRIA R104 – Table 5
87
BS 8002 requires a minimum surcharge loading on the retained ground surface of 10 kPa unless the wall is less than 3 m in height.
88
The National Annex to BS EN 1997-1 permits the use of Annex C and provides some explanatory information on its use.
89
Wall deflections and ground displacements may be calculated using numerical analyses (eg finite element models) of the ground-structure system including the complete wall construction with any supports. Alternatively, the wall may be modelled as a beam supported on elasto-plastic springs. This method does not predict supported ground movements. When performing detailed deflection calculations, characteristic earth resistance and ground parameter values should be selected as cautious estimates of the values governing the SLS in question. Clause 4.2(8) of BS EN 1990 states that characteristic values of structural stiffness parameters are equal to their mean values while this Book recommends that the characteristic value of the ground stiffness is selected as a cautious estimate of the mean value. If using a relationship of the type Eu/su or E′/su to determine an elastic stiffness modulus, the chosen correlation should reflect the nature of the empirical database behind the correlation and the likely strain level.
90
BS 8002 uses values of M of 1.5 (total stress calculations) and 1.2 (effective stress calculations). M may have different values, depending on the circumstances.
91
BS EN 1997-2:2007 defines “low” as Ip < 10 per cent and wL ≤ 35 per cent but does not define “high”, which is left to the designer to judge if “high” means Ip ≥ 10 per cent and wL > 35 per cent.
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Although EC7-1 does not require this, it may be more sensible to check that: Vtemp ≤ Gtemp + Rtemp
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93
As has been said already, BSs make recommendations using the word “should” while “Principle” clauses in the “execution” BS ENs use the word “shall”.
94
Indeed, this is a CDM requirement anyway.
95
The number of pages in a standard is only an indication of level of detail.
96
1 2 3 4 5 6 7 8 9 10 11
97
The following text is adapted from the National strategy for implementation of the structural Eurocodes: Design guidance, a report prepared for The Office of the Deputy Prime Minister by The Institution of Structural Engineers, April 2004.
98
See ODPM Simple guide (Driscoll et al, 2005).
99
Note that the UK National Annex was published in 2007.
100
For testing standards, withdrawal of BS documents has to be carried out within six months of publication of the equivalent BS EN.
101
It will be interesting to see if clients will be willing to pay for better quality.
102
It is considered likely that statistical methods, well applied, could add to the geotechnical profession’s understanding of uncertainty and safety in design. At worst, it is important that geotechnical engineers ensure that their work is not damaged by spurious, but plausible, uses of statistics which they are unable to challenge through lack of knowledge.
103
It should not be inferred however that BS EN 1997-1 encourages reliance on numerical analysis where it is not needed. Our understanding of deformations is based mainly upon case histories, leading either to simple empirical rules or to more complex back-analysis. The collection, categorisation and simple interpretation of case histories will remain of paramount importance.
104
The following text has been adapted from IStructE, 2004.
105
Alternatively, he could require an inspection and probing at each footing location, so avoiding this most conservative assumption.
106
Annex A of BS EN 1997-1 contains 17 tables of sets of factor values to cover the three design approaches, the different limit states, EQU, GEO, STR, UPL and HYD and different geotechnical elements.
107
Some explanation of DAs 2 and 3 is given in the informative Annex B of BS EN 19971, while additional discussion and comparisons of results using the 3 DAs may be found in Frank et al (2004).
108
For example, when, to calculate the action resulting from water pressure, a factor greater than 1 is applied to the known depth of a free water surface, leading to a design value of the free water surface that would be above ground level, a physically impossible situation. In such cases, partial factors γE are applied to the effects of the action using expression 2.6b.
109
This is akin to the ENV requirement that both Cases B and C were in principle required to be checked.
110
For readers familiar with ENV 1997-1, Combination 1 is the old Case B.
111
These are the action factor values given by BS EN 1990.
112
In Annex A of BS EN 1997-1, R1 takes values of 1. These are not shown in Table A3.1, except for piles and anchorages for which the R values are > 1.0, except for driven piles.
113
For readers familiar with ENV 1997-1, Combination 2 is the old Case C.
Scope. Normative references. Terms, definitions and symbols. Specific needs. Site investigation. Materials and products. Design considerations. Execution. Testing, supervision and monitoring. Records. Special requirements. Annexes.
125
Licensed copy:Royal Haskoning, 21/05/2014, Uncontrolled Copy, © CIRIA
126
114
In fact, R1 takes the value 1 but, for simplicity, this is not shown in Table A3.1
115
In most instances, Case A would not be applicable and the need to carry out two checks, for Cases B and C, would be eliminated if the result was obvious in advance. However, in some specific instances, it would not be immediately obvious which of Cases B and C took precedence and two checks would be required. For a discussion on the Cases A, B and C in the ENV 1997-1, see Simpson & Driscoll (1998).
116
Department for the Environment, Transport and the Regions.
117
The published and draft BS ENs that were examined for conflict are shown below with the BS codes with which they were compared. BS EN or draft EN (“pr…”)
Subject
Equivalent BS
BS EN 1536
Bored piles
BS 8004, BS 8008
BS EN 1537
Ground anchors
BS 8081, BS 8002
BS EN 1538
Diaphragm walls
BS 8002, BS 8004
BS EN 12063
Sheet pile walls
BS 8004, BS 8002
BS EN 12699:2001
Displacement piles
BS 8004
BS EN 12715
Grouting
BS 8004
BS EN 12716:2001
Jet grouting
BS 8004
118
EN 1538 tolerances relate to diaphragm walls constructed with guide walls, whereas BS 8002 remarks that it is not usual practice to form guide walls for bored pile walls.
119
This change could impact on data used in correlations. BS EN 1536 states that other rates may be agreed.
120
See BS EN 1997-1 Clause 7.6.2.2(4).
121
Note that this number applies for the ENV version of EC7-1. In BS EN 1997-1 it reduces to 1.37 (1.05 × 1.3), for 3 pile tests, using the recommended partial factor values for DA-1 Combination 2 in Table A.7 and the correlation factors in Table A.9. The value reduces to 1.3 (1 × 1.3) for five or more pile tests.
122
A tacit assumption in directly comparing combinations of partial factors with an equivalent overall (or lumped) factor of safety is that the characteristic strength adopted for use with the former is the same as the strength adopted when applying the latter.
123
Note that this number applies for the ENV version of EC7-1. In BS EN 1997-1 the recommended factor values would give 1.56 (1.2 × 1.3) for mean shaft resistance and 1.92 (1.2 × 1.6) for mean base resistance, assuming three pile tests. Even greater reductions result for five and more pile tests (see Table A.9).
CIRIA C641
