CIRIA C641 London, 2008

EC7 implications forUK practice

Eurocode 7 Geotechnical design

Richard Driscoll BRE

Peter Scott Buro Happold

John Powell BRE

Classic House, 174180 Old Street, London EC1V 9BPTEL: +44 (0)20 7549 3300 FAX: +44 (0)20 7253 0523

EMAIL: enquiries@ciria.org WEBSITE: www.ciria.org

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Summary

The introduction of the Eurocodes represents for most civil and structural engineers asignificant challenge in adapting to a very extensive set of new design and constructionrequirements. This is particularly so for geotechnical engineers in that Eurocode 7 andits associated new standards present some profound departures from traditionalpractice. The aim of this publication is to provide geotechnical engineers with anunderstanding of how the new documents will affect their day-to-day activities. Muchinformation on the detail of the new Eurocode system already exists, so this bookfocuses on changes to common practice and their implications.

The book takes the reader through a logical sequence of activities, from site andground investigation to geotechnical element design, to construction practicesintroduced by the new European Execution Standards. It then concludes with anindication of the likely timing of full implementation and a prediction of the effect thatthe 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 inappendices such detail of the Eurocode system that is necessary to understand thesechanges. It illustrates the changes with a set of design examples covering mainstreamdesign challenges such as piles, retaining walls, embankments and slopes, and hydraulicfailure.

The book is authored by three specialists who have worked closely with thedevelopment 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 geotechnicalpractitioners.

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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.

Published by CIRIA, Classic House, 174180 Old Street, London, EC1V 9BP

This publication is designed to provide accurate and authoritative information on the subject mattercovered. It is sold and/or distributed with the understanding that neither the authors nor the publisher isthereby engaged in rendering a specific legal or any other professional service. While every effort hasbeen made to ensure the accuracy and completeness of the publication, no warranty or fitness isprovided or implied, and the authors and publisher shall have neither liability nor responsibility to anyperson 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 anymeans, 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 beobtained 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 otherCIRIA publication for use in other documents or publications, please contact the Publishing Departmentfor more details on copyright terms and charges at: publishing@ciria.org Tel: +44 (0)20 7549 3300.

CIRIA C641 iii

Keywords

Ground engineering, Eurocode, foundations, geotechnical design, geotechnicalinvestigation, ground investigation and characterisation, in situ testing andinstrumentation, piling, soil structure interaction

Reader interest

Design of geotechnicalstructures, limit state design,Eurocodes replace BritishCodes and Standards

Classification

AVAILABILITY Unrestricted

CONTENT Advice/guidance

STATUS Committee-guided

USER Client organisation, consultants,contractors , geotechnicalengineers, project managers,structural design engineers

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Foreword

The creation of the structural Eurocodes has been in progress for many years. Thesenew EU standards have now advanced to a stage that warrants serious preparation fortheir implementation and the consequences of withdrawal of corresponding nationaldocuments. For a complex engineering discipline such as geotechnics, used to thepiecemeal and evolutionary introduction of national codes and testing standards, theintroduction of a significantly different design philosophy for dealing with engineeringuncertainty and the relatively rapid replacement of national documents representmajor changes for the industry.

A recent report (Institution of Structural Engineers, 2004) has highlighted thechallenges facing engineers in adapting to the Eurocodes and has advocated thepreparation of guidance to ease their passage into practice. This publication has beenproduced to assist in this process by indicating the most important differences thatgeotechnical engineers will encounter when implementing the new suite of geotechnicalEurocode documents. It is not intended that this publication teaches the reader how touse the Eurocode since other referenced documents are available for this. However, acertain amount of explanation for some of the features of Eurocode design has beenfound necessary to assist in understanding the differences to practice that the Eurocodewill bring.

The book lists all the documents that will eventually comprise the full suite ofeuronorms covering geotechnical engineering. Many of these documents are still inpreparation in several CENa committees and working groups. However the maindesign code, EC7-1, and several executionb standards have now been published byBSI. This mixture of published and unfinished documents leads to a rather confusingreference 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 documentfor the two parts of Eurocode 7. EC7-1 concerns geotechnical design and EC7-2 refersto 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 discusseschanges that may occur in site investigation practice before concentrating on how theEurocode may affect general geotechnical design philosophy in the UK, with likelyconsequences, in Chapter 3. Chapter 4 focuses on changes that are specific to the maingeotechnical elements that require designing, such as piles, retaining walls and slopes,with several worked examples demonstrating how the EC7-1 design methodologymight differ from conventional practice. Chapter 5 briefly discusses differences ingeotechnical construction practice that the new execution standards may introduce.

Precisely how the new Eurocode suite of documents will be implemented in the UK isstill a matter for debate. The intention is for packages of Eurocodes including, forexample, loading, geotechnical, concrete, masonry and timber all necessary to design acomplete building structure, to be available for full implementation and consequentwithdrawal of national documents. It may be obvious that the timing for this

CIRIA C641iv

a Comit Europen de Normalisation.

b Execution is defined as all activities carried out for the physical completion of the work includingprocurement, the inspection and documentation thereof .

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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 andstandards will apply within it.

Finally, Chapter 7 comprises a short piece on the likely overall effect of the Eurocodeon geotechnical investigation, design and construction practice in the UK. Theappendices provide more detail and further information. The intention is to keep thisbook as simple and succinct as possible in discussing what is a complex system of linkeddocuments and which introduces a partial factor design philosophy to geotechnics. Thishas been carried out in several ways:

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, whileclause references are indicated in bold italics.

3 Key conclusions from each chapter are summarised in a table at the beginning ofthe chapter.

4 The examples have been formatted so that appropriate code clauses are apparent.

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Acknowledgements

Research contractor

This publication is the main output from CIRIA research project 701. It was preparedby 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. Richardworked at BRE for 27 years before retiring as the head of ground engineering. Hespent many years as a BSI representative developing EC7 and has co-authored a bookon the subject.

Peter Scott BSc MSc CEng FICE MASCE FGS

Peter Scott is the technical head of the geotechnical group at Buro Happold ConsultingEngineers. Peter has extensive experience in geotechnical design for major projects inthe 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 Technologyat BRE. He chairs the BSI committee for BS 5930 and 1377 that is the mirrorcommittee for EC7 Part 2. He represents BSI on the committee responsible for thedrafting of EC7 Part 2 and is the national technical contact for associated technicalspecifications.

David Poh of Buro Happold Consulting Engineers assisted in the preparation of theworked examples.

Following CIRIAs usual practice, the research project was guided by a steering group,which comprised:

Steering group

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 OBrien (chair) Mott MacDonald

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Dr M Pedley Cementation Foundations Skanska

Mr S G Smith Bechtel

Dr J Wilson Atkins

CIRIA managers

CIRIAs research managers were Mr Chris Chiverrell and Dr Andrew Pitchford.

Project funders

This project was funded by:

The DTIs Partners in Innovation scheme

The Highways Agency

Network Rail

CIRIAs Core Programme Sponsors

Technical organisations

CIRIA and the authors gratefully acknowledge the support of those fundingorganisations, the technical help and advice provided by the members of the steeringgroup, and colleagues and specialists for reviewing the document and for assisting theauthors in co-ordinating and collating all the technical contributions.

Contributions do not imply that individual funders necessarily endorse all views expressed inpublished outputs.

CIRIA C641 vii

Front cover photo: The piled wall for the new Wembley Stadium (courtesy StentFoundations Ltd, a Balfour Beatty company). See Case study in Appendix A5

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Contents

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ii

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .iv

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .vi

List of figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .x

List of tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .x

Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .xi

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .xii

1 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

2 Site characterisation and determination of ground property designvalues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .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

3 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 The difference between DA-1 and traditional design calculations . . .26

4 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 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 withrelevant BSs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .81

6 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

7 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 investigationpractice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .87

7.4 The impact on geotechnical construction practice . . . . . . . . . . . . . . . .87

7.5 Overall impact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .87

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .88

A1 Examples of the selection of characteristic ground property values using allavailable 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

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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 andstructural codes and standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

Figure 2.1 Processing test measurements into design values of groundparameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .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 groundproperties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .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 publishedresults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .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

Table 1.1 The content of BS codes and their correspondence with the Europeandocuments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

Table 1.2 The content of BS codes and testing standards and theircorrespondence 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 codesand 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 standardsavailable in January 2005 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .102

Table A5.1 Summary of vertical pile tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .106

Table A5.2 Flemings analyses (CEMSET), input parameters . . . . . . . . . . . . . . .107

Table A5.3 Factors that may affect choice of factor of safety . . . . . . . . . . . . . . . .108

Examples

Example 4.1 Design of a vertical, pre-cast concrete pile driven into sand andgravel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .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 pressuresacting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .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 groundwaterpressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .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. Thefollowing table indicates what meaning these are intended to convey to the reader. Theinterpretations of the terminology are largely those of the authors, often using text inBS EN 1990: 2002 unless they include direct quotations from EC7.

Action 1 Set of forces (loads) applied to a structure (directaction).

2 Set of imposed deformations or accelerations caused,for example, by temperature changes, moisturevariation, uneven settlement or earthquake (indirectaction).

Characteristic value Clause 2.4.5.2(2)P states that: The characteristic value of ageotechnical parameter shall be selected as a cautiousestimate of the value affecting the occurrence of the limitstate. A fuller discussion may be found in Section 2.4.

Code Published guidance from a national standards body onhow activities should be undertaken to achieve a requiredresult using recommended best practice.

Comparable experience Documented or other clearly established informationrelated to the ground being considered in design,involving the same types of soil and rock and for whichsimilar geotechnical behaviour is expected, and involvingsimilar structures. Information gained locally is consideredto be particularly relevant.

Derived value Value of a geotechnical parameter obtained by theory,correlation or empiricism from test results. A fullerdiscussion is found in Section 2.2.

Design situation Set of physical conditions representing the real conditionsoccurring during a certain time interval for which thedesign will demonstrate that relevant limit states are notexceeded.

Design value Value of a variable used in the calculation of thedimensions 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 wholestructure (eg deflection, rotation).

Execution All activities carried out for the physical completion of thework including procurement, the inspection anddocumentation thereof.

Geotechnical action Action transmitted to the structure by the ground, fill,standing water or groundwater (definition adapted fromClause 1.5.3.7 of BS EN 1990).

Limit states States beyond which the structure no longer fulfils therelevant design criteria.

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Nominal value Value fixed on non-statistical basis, for instance onacquired experience or on physical conditions.

Partial factor A factor to either increase or decrease a variable used inpart of the determination of the dimensions of or forceson or in the structure to be built.

Representative value Value used for the verification of a limit state. Aof an action representative value may be the characteristic value.

Resistance Capacity of a member or component, or cross-section of amember or component of a structure, to withstand actionsor their effects without mechanical failure, eg bendingresistance, buckling resistance, tension resistance.

Serviceability limit States that correspond to conditions beyond whichstates specified service requirements for a structure or structural

member are no longer met.

Standard Published instructions from a national standards body onhow activities must be undertaken to achieve a requiredresult.

Technical specification Published instructions from a standards body on howactivities should be undertaken to achieve a requiredresult.

Ultimate limit states States associated with collapse or with other similar formsof 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 willin due course largely replace British codes and standards. This book has been writtento identify and explain to the general geotechnical practitioner in the UK the keydifferences between the incoming and outgoing system and to indicate what othercommonly used design documents1 will be retained. The book does not provide aclause-by-clause commentary on the main design Eurocode, EC7 Part 1 (this may befound elsewhere2), nor is it intended to be a manual of good practice in geotechnicaldesign. Rather, it highlights the important features of the new Eurocode system andseeks to show how they may affect practice. With accompanying illustrations in workedexamples, some guidance is given on how to apply the systems Principles to ensurethat designs will conform to the new requirements, and will be built and maintained asthe Eurocodes intend.

The main changes to geotechnical practice introduced in the Eurocodes areconcentrated in Eurocode 7 Geotechnical design Part 1: General rules, which this bookconcentrates on3, and Eurocode 7 Geotechnical design Part 2: Ground investigation andtesting. It is important to appreciate that the new European suite of geotechnicaldocuments is a comprehensive, linked system of codes, standards and technicalspecifications. These indicate how information on the ground is to be acquired, how itis to be interpreted and transformed into design parameters and the geometry ofgeotechnical structures, and how these structures are to be built and maintained, withsuitable monitoring and quality assurance.

There is a confusing plethora of alphanumeric references within many of the newEuropean documents. For the purposes of simplicity, this book refers to the two partsof Eurocode 7 as EC7-1 and EC7-2. It should be understood that all Euronormspublished by CEN have the prefix EN, those produced by ISO4 and adopted by CENhave the prefix EN-ISO and all these documents, when published by BSI as UKversions will be prefixed by BS EN etc. Further complication is introduced by the useof pr EN to signify documents that are in preparation.

Figure 1.1 illustrates the system of new European documents while Tables 1.1 and 1.2show the current BS codes and standards and their approximate relationships withthose European documents that exist or are anticipated. There is direct correspondencefor some documents (for example, some parts of BS 1377 are being and will continue tobe replaced by an equivalent standard from CEN Technical Committee 341, see Powelland Norbury, 2007 for examples) while in most other cases there is limited overlapbetween 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. Inparticular it:

presents, for the first time, a unified set of Principles for all geotechnical design

bridges the philosophical divide between geotechnical design and superstructuredesign that has existed since BS 8110, explicitly employing limit state design andpartial factors, was introduced in the UK

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makes a clear distinction between the avoidance of an ultimate limit state (failure ofthe ground and collapse of all or part of a ground-supported structure) and of aserviceability limit state (undue movement and its consequences). Much routinegeotechnical design has historically blurred these two requirements. The Eurocodeshould prompt greater thought about designing to prevent unacceptablemovement, which should be beneficial

requires more systematic thought about the degree of uncertainty in the values ofgeotechnical material parameters for use in design calculations5

introduces a degree of compulsion by indicating that certain (Principle) activitiesshall be undertaken in both design and ground investigation6.

EC7-1 is not only about carrying out design but is also about checking7 that a designwill not reach a limiting condition in prescribed design situations. The code does nottell the reader how to design, rather it lays down a set of guiding design Principles, liststhe many physical conditions that the ground and the structure it supports mayexhibit, and states how the constructed outcome must behave.

In common with the other structural Eurocodes, the foreword to EC7-1 indicates that itserves as:

a means to prove compliance with the essential requirement of mechanicalresistance and stability

a basis for specifying contracts for construction works.

Unusual forms of construction or design conditions are not covered and additionalexpert consideration will be required by the designer in such cases.

It is explicitly stated that appropriately qualified personnel are to provide the inputdata for geotechnical designs and that the design and ground investigations are to beperformed 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 willneed to address when checking that their geotechnical design conforms with theEurocode

to describe briefly the range of information presented in the Eurocode suite, toclarify the meanings of some new terms, to describe briefly the new design methodsand to present easy-to-understand explanations of how the new methods workusing design examples and a case study

to indicate the likely effect on geotechnical practice in the UK of the move to theEurocode suite of documents, including how use of the Eurocode will comply withthe requirements of the Building Regulations and any other local regulations, suchas the London District Surveyors rules.

The book has been written primarily for three groups of readers:

1 The general geotechnical engineer who may often not have routine recourse tocodes but who will, nevertheless, need to be assured that a design complies with thecode requirements.

2 The non-geotechnically qualified engineer who carries out simple design for smallprojects 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 whereother 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 tounderstand what the geotechnical engineer is doing.

This book is intended to be a companion to the suite of European geotechnicaldocuments 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 currentBS codes and standards. It is expected that all references to BS documents in theBuilding Regulations and other regulatory documents such as those of the HighwaysAgency and Network Rail will be replaced by references to the new BS ENs. TheEurocodes 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 usedto 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 andthe 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 theassessment of existing structures.

EC7-1 and EC7-2 also apply primarily to greenfield sites, and while clean fill iscovered, contaminated land is not.

Limit state design

Two different types of limit state are identified, each having its own designrequirements:

ultimate limit states (ULS), defined as states associated with collapse or with othersimilar forms of structural failure (eg exceeding the bearing resistance of thefoundation). For geotechnical design, it is particularly important to note thatultimate limit states include failure by excessive deformation, leading to ... loss ofstability of the structure or any part of it

serviceability limit states (SLS), defined as states that correspond to conditionsbeyond which specified service requirements for a structure or structural memberare no longer met (eg excessive settlement leading to cracking in the structure).

Limit states are generally avoided by considering design situations in which adverseconditions apply (see Section 3.6.3). The need to identify these design situations shouldhelp 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 ofpartial factors to characteristic values of actions, ground parameters and resistances inplace of global factors for dealing with all uncertainty and safety.

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Movement

Because of the explicit requirement to check serviceability conditions, greater attentionwill need to be paid to settlements and other movement. However, note that the codedoes not provide explicit guidance on how to calculate movement. As will be discussedlater, the separation of bearing capacity (a ULS) from settlement (an SLS) means thatpartial factors applied in a ULS calculation may not guarantee that settlements aresufficiently small, particularly on soft ground. Clients should be confident thatappropriately qualified and experienced personnel have been involved in any EC7-1design calculations.

Compulsory reporting of information

The production and communication of the Geotechnical design report and Groundinvestigation report are requirements of EC7. Minimum contents for these reports arespecified 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 othergeotechnical structures but the code does not specify which soil mechanics theories orsoil behaviour models to use, although it does suggest, in informative annexes8, meansto determine, for example, the earth pressure acting on a retaining structure or thestability of a slope.

A unifying set of design Principles

EC7-1 presents a unified set of Principles for design (see Appendix A3). In contrast, BScodes have emerged over many years in a rather piecemeal fashion, with a collection ofdifferent design philosophies.

Terminology

EC7-1 introduces terms that are not widely used or defined in the UK, at least by thegeotechnical 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 withEC7-1. For ground investigation, including laboratory and field testing, EC7-2 dealswith basic ground data and its interpretation with the resulting derived values beingpassed to EC7-1 for conversion into a characteristic and hence design value. Thedifferences from current practice in these processes are briefly outlined.

Chapter 3 deals with the key differences in the general Principles of design betweenEC7-1 and the BS codes of practice. The alternative methods of design permitted inthe code are briefly described after which design by calculation is discussed in somedetail 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 suitablyaccurate input parameters. The chapter also highlights important new concepts forarriving at suitably conservative values of input parameters so that the design will avoidthe occurrence of a limit state. The concept of characteristic value of a parameter andhow it is acquired, starting with the elements of a site investigation, is discussed, afterwhich the obtaining of a design parameter value is considered.

Finally, the adoption in the UK of Design Approach 1 is outlined (three alternativedesign approaches are permitted in the Eurocode).

Chapter 4 briefly describes specific differences for common design problems andillustrates them in typical worked examples and a case history.

Chapter 5 describes the key differences involved in moving from BS codes to the BSEN standards for execution (construction). The resolution of any conflicts identifiedbetween the documents is outlined.

Chapter 6 deals with the manner in which the Eurocodes will be implemented in theUK. It briefly discusses how national preferences for safety are incorporated into theNational Annexes for EC7-1 and EC7-2 and explains how and when the Eurocodes arelikely to replace the BS codes as references in Building Regulations and otherregulatory and widely-adopted design documents9.

Chapter 7 discusses the manner in which the move to the Eurocodes might affectgeotechnical practice in the UK, from changes in site and ground investigation,through design calculations to construction activities on site. Brief mention is made ofany consequences for the economics of geotechnical works and any effect onconstruction programmes.

There are a number of appendices that contain specific details that have beenseparated 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 asomewhat disconnected manner by several different CEN committees, the emergingsuite 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 constitutedesign as normally practiced in the UK. So this book does not follow the order ofpresentation of material in the Eurocodes. Throughout, an attempt has been made tokeep the narrative simple and focused on how the Eurocode may introduce changes topractice.

1.6 Consultation

During the writing of this book, consultation has taken place with a group ofgeotechnical design, construction and site investigation specialists. While several in thegroup are familiar with EC7-1, a concerted attempt has been made to address thisdocument to people who have little or no knowledge of EC7.

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Table 1.1 The content of BS codes and their correspondence with the European documents

CIRIA C6416

New

Eur

opea

n do

cum

ents

Stan

dard

s fo

r th

e ex

ecut

ion

of s

peci

alge

otec

hnic

al w

orks

(CEN

TC2

88

)

B

S EN

14

475

:20

06

Rein

forc

ed fi

ll

pr

EN

14

49

0So

il na

iling

B

S EN

12

06

3:1

99

9Sh

eet p

ile w

alls

B

S EN

15

36

:20

00

Bore

d pi

les

B

S EN

12

69

9:2

001

Disp

lace

men

t pile

s

B

S EN

141

99

:20

05

Mic

ropi

les

B

S EN

15

37:2

00

0G

roun

d an

chor

s

B

S EN

15

38

:20

00

D

iaph

ragm

wal

ls

B

S EN

12

06

3:1

99

9

Shee

t pile

wal

ls

B

S EN

127

15:2

00

0G

rout

ing

B

S EN

127

16:2

001

Jet g

rout

ing

pr

EN

14

49

0So

il na

iling

B

S EN

14

67

9:2

00

5D

eep

mix

ing

B

S EN

147

31:2

00

5G

roun

d tr

eatm

ent b

yde

ep v

ibra

tion

B

S EN

15

237

:20

07Ve

rtic

al d

rain

age

B

S EN

14

475

:20

06

Rein

forc

ed fi

ll

Sect

ion

Ti

tle

1G

ener

al

2Pl

anni

ng o

f gro

und

inve

stig

atio

ns

3So

il an

d ro

ck s

ampl

ing

and

grou

ndw

ater

mea

sure

men

t

4Fi

eld

test

s in

soi

ls a

nd r

ocks

5La

bora

tory

test

s on

soi

ls a

nd r

ocks

6G

roun

d in

vest

igat

ion

repo

rt

Anne

x B

Plan

ning

str

ateg

ies

for

geot

echn

ical

inve

stig

atio

ns

EC7

-2

Gen

eral

issu

es c

over

ed

Ove

rall

appr

oach

Gro

und

inve

stig

atio

n

Des

ign

aspe

cts

ofco

nstr

uctio

n ac

tiviti

es

Des

ign

of s

peci

ficel

emen

ts

EC7

-1

Sect

ion

Ti

tle

1G

ener

al

2B

asis

of g

eote

chni

cal d

esig

n

3G

eote

chni

cal d

ata

4Su

perv

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n of

con

stru

ctio

n,m

onito

ring

and

mai

nten

ance

5Fi

ll, d

ewat

erin

g, g

roun

dim

prov

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t an

d re

info

rcem

ent.

(NNot

e: E

C7-1

doe

s no

t co

ver

the

desi

gn o

fre

info

rced

soi

ls o

r gr

ound

str

engt

hene

d by

naili

ng e

tc).

6Sp

read

foun

datio

ns

7Pi

le fo

unda

tions

8An

chor

ages

9R

etai

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str

uctu

res

10H

ydra

ulic

failu

re

11O

vera

ll st

abili

ty

12

Emba

nkm

ents

BS

code

BS

59

30

:19

99

Site

inve

stig

atio

n

Som

e of

tho

se b

elow

BS

60

31:1

981

Eart

hwor

ks

BS

80

06

:19

95

Stre

ngth

ened

/rei

nfor

ced

soils

and

othe

r fill

s

BS

80

04

:19

86

Foun

datio

ns

BS

80

04

:19

86

Foun

datio

ns

BS

80

08

:19

96

Safe

ty p

reca

utio

ns a

ndpr

oced

ures

for t

heco

nstr

uctio

n an

d de

scen

t of

mac

hine

-bor

ed s

hafts

for

pilin

g an

d ot

her p

urpo

ses

BS

80

81:1

98

9

Gro

und

anch

orag

es

BS

80

02

:19

94

Eart

h re

tain

ing

stru

ctur

es

Som

e

BS

60

31:1

981

Eart

hwor

ks

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

CIRIA C641 7

New

Eur

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cum

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CEN

ISO

sta

ndar

ds(s

ee A

ppen

dix

A6 fo

r an

exp

lana

tion

of t

hepr

oven

ance

of t

he d

iffer

ent

stan

dard

s)

BS

EN IS

O 1

46

88

-1:2

00

2

BS

EN IS

O 1

46

88

-2:2

00

4

pr E

N IS

O 1

46

88

-3

BS

EN IS

O 1

46

89

-1:2

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3

pr E

N IS

O 1

46

89

-2

pr E

N IS

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22

82

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pr E

N IS

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82

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pr E

N IS

O 2

22

82

-3

pr E

N IS

O 2

22

82

-4

pr E

N IS

O 2

22

82

-5

pr E

N IS

O 2

22

82

-6

BS

EN-IS

O 2

247

5-1

:20

06

DD

EN

-ISO

/TS

224

75

-2:2

00

6

DD

EN

-ISO

/TS

224

75

-3:2

007

CEN

ISO

/TS

178

92

-1*

CEN

ISO

/TS

178

92

-2*

CEN

ISO

/TS

178

92

-3*

CEN

ISO

/TS

178

92

-4*

CEN

ISO

/TS

178

92

-12

*

Not

e: T

here

app

ear

to b

e B

S EN

s in

exi

sten

ceth

at h

ave

been

dra

fted

by

com

mitt

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conc

erne

d w

ith a

ggre

gate

s. T

hese

will

nee

dto

be

revi

ewed

to a

sses

s th

eir

appl

icab

ility

toso

ils.

CEN

ISO

/TS

178

92

-5*

Labo

rato

ry a

nd fi

eld

test

ing

stan

dard

s an

d te

chni

cal s

peci

ficat

ions

(CEN

TC3

41

).N

ote:

Te

chni

cal S

peci

ficat

ions

ar

e id

entif

ied

by

TS

in t

he r

efer

ence

num

ber.

Geo

tech

nica

l inv

estig

atio

n an

d te

stin

g

Iden

tific

atio

n an

d cl

assi

ficat

ion

of s

oil:

Part

1: I

dent

ifica

tion

and

desc

riptio

n

Part

2: C

lass

ifica

tion

prin

cipl

es

Part

3: E

lect

roni

c da

ta e

xcha

nge

- soi

l

Roc

ks

Part

1: I

dent

ifica

tion

and

desc

riptio

n

Part

2: E

lect

roni

c da

ta e

xcha

nge

r

ock

Geo

hydr

aulic

test

ing

Gen

eral

Rul

es

Wat

er p

erm

eabi

lity

test

s in

a b

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CIRIA C6418

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Note The different sources of these documents are explained in Appendix A6.

Figure 1.1 Diagrammatic representation of the suite of EU geotechnical and structural codes andstandards

CIRIA C641 9

Geotechnicalprojects

Eurocodes:BS EN 1990:2002

Basis of structural designBS EN 1991-1-1:2002Actions on structures

European standards forthe Execution of special

geotechnical works

Other structuralEurocodes

eg BS EN 1993-5:2007

ISO/CENStandards for

identification andclassification

Test standard fortechnical specificationsfor ground properties

Geotechnical designEurocodes:

BS EN 1997-1:2004BS EN 1997-2:2007

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2 Site characterisation and determinationof ground property design values

2.1 Summary

2.2 Ground investigation and testing

The processes for obtaining ground parameters to use in design with EC7-1 arespecified in several parts of the suite of European codes and standards, as indicated inTables 1.1 and 1.2. It is evident that change over to the BS EN suite will entail theacquisition of many more standards and a period of adjustment to those documentsthat will replace BS 5930 and parts of BS 1377.

Table 1.1 lists the standards and technical specifications (TS) that are being producedby CEN and ISO, which will replace UK codes, standards and practice, covering thesame subject matter. Most of the standards and TS documents are to be finalised or, insome cases, drafted, so it is not yet possible to fully identify conflicts with UKprocedures10.

The main areas in which EC7-2 differs from current BS codes and standardsrequirements are listed in Table 2.1, from which it can be seen that a fundamentalchange involves the introduction of some compulsory activities (the word shall isused). This change has consequences for the procurement of site investigation, for theclear specification of who does what and for how information is disseminated to allappropriate parties.

In fact, there are only two major changes to existing British site investigation practice and itcan be expected that much if not all of the good practice guidance contained in, forexample, BS 5930 will continue to apply in the future (see Section 6.4). The first majorchange concerns the provision of a geotechnical investigation report discussed later. The secondconcerns the effect of the requirement in EC7-1 for much greater consideration ofsettlement and deformation. This will entail much more attention being paid to thedetermination of the deformational properties of the ground, a difficult subject.

CIRIA C64110

1 EC7-2 makes compulsory the provision of a ground investigation report to all relevant parties.

2EC7-2 is more prescriptive than BS 5930 in its planning and execution requirements for groundinvestigation.

3

The emphasis in EC7-1 on better prediction of settlement and deformation raises the importance of grounddeformation properties. While the option is available in EC7-1 to use a reduced strength value, akin to thestrength mobilisation factor used in BS 8002, the need for better knowledge of the deformation propertiesof the ground from additional and specific testing should presage a profound change in UK geotechnicalpractice.

4Some parts of BS 1377 and BS 5930 have been and will continue to be replaced by new BS ENdocumentation.

5 Some departures from BS 5930 terminology apply for soil and rock descriptions.

6Design values of ground properties may be assessed directly as an alternative to applying partial factors tocharacteristic values.

7Procurement processes may need to be clearer about who does what, quality assurance, professionalindemnity implications and communication between interested parties.

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Table 2.1 Some of the changes introduced by EC7-2

CIRIA C641 11

EC7-2 novel feature Impact on practice as embodied in BS codes and standards

Use of shall in Principle clauses,

rather than should11 (examplesfrom Section 2 Planning of groundinvestigations)

if the main ground investigations do not supply the necessaryinformation, complementary investigations shall be undertaken.Clients may come to appreciate that, if they fund morecomprehensive, initial investigations, they can avoid the expense offurther investigation at a later stage

it is stated that investigations shall be planned and data shall beadequate to manage risks12

the document states that a visual inspection shall be undertakenbefore planning the investigation programme and used in conjunctionwith a desk study

it also says that quality assurance systems shall be in place for allaspects of the work13

the necessary number of specimens to be tested shall bedetermined. Recommended numbers are contained in informativeannexes but the status/validity of these will need to be discussed inthe NA for EC7-2. It is unclear what the implications might be if therecommendations are ignored and things go wrong

a table of applicability of various field tests is also presented.

Section 3 on soil and rock samplingintroduces categories of samplingmethod based on BS EN ISO 22475-114.

this implies that only certain sampling methods can be used to obtainsamples of a certain quality class

the quality class relates to use in specific laboratory tests in order togive the test results required for the selection of characteristicvalues15.

Section 4 Field tests in soils androcks specifies that CEN standardsshall be used when specifying tests.Conversion of test results intoderived values is introduced

existing BS 1377 and BS 5930 sections specifying test methods willbecome redundant where a corresponding standard exists. If atechnical specification is listed for a particular test then either this orthe BS can be used. See Table 1.2 for the tests affected.

Section 5 Laboratory tests on soilsand rocks

general statements occur with shall throughout the section. Much issimply good practice but if things go wrong then decisions takenrelating to clauses saying shall will need to be justified

checks shall be made that the laboratory equipment used isadequate, fit for purpose, is calibrated and within the calibrationrequirements

there is a requirement that all test methods and procedures shall bereported

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 ISO14688-2:2004

for the laboratory tests listed in Table 1.2 there will be no withdrawalof corresponding BS documents as the CEN documents are all onlytechnical specifications (TS) and EC7-2 allows the NA to adoptNational Standards in preference. This will be the case in the UK (onlythe TS for the fall cone will be adopted) (see NA to EC7-2 in 2009).

Section 6 Ground investigation reportdeals with what shall be in the report

the report shall form part of the geotechnical design report

it shall state known limitations of the results

it shall include a presentation of all available information andgeological features and a geotechnical evaluation of the information

all methods shall be documented in accordance with the relevantstandards

it shall include all relevant information on how the derived valueswere arrived at.

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EC7-2 identifies an explicit hierarchy of investigations that is also found in BS 5930:

geotechnical investigations17, which comprise the gathering of all relevantinformation about the site18 and a ground investigation

ground investigations, which comprise field investigations, laboratory testing anddesk studies of geotechnical and geological information

field investigations, which comprise direct investigations (drilling, sampling and trialpits) and indirect investigations (in situ tests, such as the CPT).

The code further distinguishes between investigations for the purposes of design and forcontrol.

In a clear departure from most current practice, EC7-2 makes compulsory theprovision of some form of ground investigation report (GIR) as part of the geotechnicaldesign report. The code specifies that the GIR should include:

a presentation of all available geotechnical information including geological featuresand relevant data

a factual account of all field and laboratory investigations

a geotechnical evaluation of the information, stating the assumptions made in theinterpretation 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 derivedvalues (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 froma 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 ofgeotechnical parameters for a suitable design, it could be argued that EC7 places toomuch emphasis on the manipulation of test results and not enough on desk studies andother means to determine information on such matters as:

site geology, geomorphology and overall stability

Mans 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 theground properties for design using the Eurocode suite could be seen as a logicalsequence 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 therequirements 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, theterm derived value is used EC7-2 defines this as the value of a geotechnicalparameter 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 goodpractice 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 fromwhich to determine a design value. The selection is made in EC7-1 because, in the finalanalysis, it should be the designers responsibility. This may generate difficulty for theground investigation contractor in some instances. For example, if the contractor isasked to provide characteristic rather than derived values of ground parameters, it maybecome difficult or risky unless there is sufficient information about the designsituations and the limit states pertaining to the project.

2.3 Ground identification and classification

For soil and rock descriptions (ISO-EN 14688-1, 14688-2 and 14689-1) somedifferences from BS 5930 terminology are apparent. Examples that have required arevision of our current terminology are shown in Table 2.2.

Table 2.2 Some terminological changes

(Fuller details can be found in Powell and Norbury, 2007 and Baldwin, Gosling andBrownlie, 2007).

Some of the new field testing standards are likely to cover not only equipmentspecification, sizing and operation but also to introduce requirements for what might betermed fitness for purpose. In these, the test specification is related to its applicationand the required accuracy for the ground conditions, and for intended use of theresults (this will become clearer as the documents are completed).

As the various documents become available, detailed comparisons with BS documentswill have to be undertaken to identify any potential conflicts.

2.4 Determining the design values of geotechnicalparameters

One of the biggest changes for UK practice is the formalised process in the Eurocodefor determining the design values of ground properties using partial factors andcharacteristic values.

EC7-1 states that design values of geotechnical parameters (Xd) shall either bederived20 from characteristic values using the following equation:

Xd = Xk / M (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 M is the partial material factor.

CIRIA C641 13

Old terminology New terminology

Slightly organic Low-organic

There are changes in the boundariesbetween the classes

Organic Medium-organic

Highly organic High-organic

Very soft, soft, firm, stiff and very stiff(terms used in BS 5930 to describeshear strength)

Now used to describe the consistency of silts and clays. Shear strengthdescriptors become: vvery low, low, medium, high, very high and extremelyhigh, (though maintaining the same strength ranges used in BS 5930)

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Although the Eurocode clearly permits the direct assessment of a design value, it placespriority on the use of factored characteristic values. There are situations in which it ismore appropriate to assess, for example, a strength where the critical state strengthvalue will be used in the design. This may also apply to the design values ofdeformation properties since these are rarely measured and are commonly deducedfrom correlations with strength.

In selecting the characteristic value, account should be taken of a number of mattersthat are listed in EC7-1, which then defines characteristic value as the characteristicvalue of a soil or rock parameter shall be selected as a cautious estimate of the valueaffecting the occurrence of the limit state.

Design values of parameters may be required for both ultimate and serviceability limitstate considerations. It is important to appreciate that, while the partial factor used toobtain the design value will have different values for ULS and SLS, so also may thecharacteristic value itself differ in calculations for these limit states.

The meaning and selection of a characteristic value have been debated for manyyears21. It is important to realise that, despite the formalisation in the Eurocode, theselected value(s) is for the judgement of the designer, having considered all relevantinformation, including prior knowledge of the particular site and all ground testingand assessment data. As the values of the partial material factor M in Equation 2.2 arefixed in the National Annex (see Section 6.3), the designer has control of the designvalue through the selection of the characteristic value22.

Much has also been said23 about the merits or otherwise of using statistics in thedetermination of characteristic values and it is important to appreciate that theEurocode does not require their use.

Note: It is very important that the chosen correlation is appropriate for the prevailing geologicalcondition.

Figure 2.1 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 inFigure 2.2. An example of how a characteristic profile of ground strength values mightbe obtained for a particular site is given in Appendix A1. It is important to appreciatethat the selections made in Appendix A1 are quite subjective, so a risk-averse or risk-taking designer might make a rather different selection which could be seen as aretention 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 structuralstiffness parameters (eg moduli of elasticity ) should be represented by a meanvalue25.

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In problems involving ground structure interaction the stiffness of the ground is oftena very important parameter26. In these cases, the use of a mean value for groundstiffness is questionable27. As EC7-1 does not define a characteristic value of groundstiffness, it is suggested that its definition should follow that for strength ie a cautiousestimate and not a mean value.

EC7-1s definition of characteristic strength value might be assumed also to apply to theweight density of soil and rock. However, the uncertainty about weight density isusually sufficiently low that there is no need to make a distinction between mean andcautious values.

Other attempts to deal with uncertainty in ground parameters

It is useful to compare the acquisition of conservative ground parameters in EC7-1 withapproaches in other geotechnical design codes and guidance for ensuring the necessarycaution 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 theoccurrence of the limit state which compares closely with the EC7-1 definition. CIRIAC580 (Gaba et al, 2003) discusses these definitions.

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3 The new principles of geotechnicaldesign in Eurocode 7

3.1 Summary

3.2 Introduction

There are several key features of EC7-1 that make it different from the current BSgeotechnical codes, these are:

unlike current codes, EC7 states that Principles shall be honoured. BS codes stateonly that things should be done28

EC7-1 embodies a design calculation methodology that makes sub-structure designfully 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 factorsapplied to characteristic values of parameters, to account more directly foruncertainty in the values of parameters used in the calculations31 and to achievecompatibility with structural codes that also conform to the basis of structural designlaid 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 ofthe geotechnical design report.

Together, these provide a single set of guiding principles for all geotechnical designsthat is absent in the current, diverse set of BS design codes.

CIRIA C641 17

1 Clear separation of ultimate (failure) condition from serviceability (settlement and comfort) condition.

2Need to be aware of the important distinction between permanent and variable actions, since differentvalues of partial factors apply to each. Similarly for favourable and unfavourable actions.

3Use of characteristic to define values of ground properties for use with partial factors to form designvalues.

4Application of separate partial factors to several aspects of uncertainty, rather than a single lumped factorof safety applied to cover all uncertainty.

5Partial factor values have been largely selected to avoid failure and are not necessarily sufficient to ensureacceptable movement. A check on movements will often be required.