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

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.

ii

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 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

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. 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 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

vi

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 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

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vii

Contents

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ii Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .iv Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .vi List of figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .x List of tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .x Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .xi Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .xii 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

viii

CIRIA C641

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 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

6

7

Carrying out the construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .81 5.1

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 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

CIRIA C641

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

Examples

CIRIA C641

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

xi

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. Action

xii

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.

xiii

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.

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.

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. 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.

4

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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. 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 (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

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

Table 1.2 The content of BS codes and testing standards and their correspondence with the European documents

7

8

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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

General rules (permeability) 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

Plate Loading Test

pr EN ISO 22477-4

pr EN-ISO 22476-12

Mechanical cone penetration test

Pile Load tesr – rapid axial loaded compression test

DD CEN-ISO/TS 22476-11:2005

Flat dilatometer test

pr EN ISO 22477-3

CEN-ISO/TS 22476-10*

Weight sounding test

Pile load test – dynamic axially loaded compression test

pr EN-ISO 22476-9

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*

Fall cone test

CEN ISO/TS 17892-10*

Unconsolidated triaxial test

CEN ISO/TS 17892-7*

CEN ISO/TS 17892-11

Direct shear test

Unconfined compression test on fine grained soils

Permeability test

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 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