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
CIRIA C641
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
CIRIA C641
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
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Table A4.1
Conflicts between BS codes and those BS EN execution standards
available in January 2005 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .102
Table A5.1
Summary of vertical pile tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .106
Table A5.2
Fleming’s analyses (CEMSET), input parameters . . . . . . . . . . . . . . .107
Table A5.3
Factors that may affect choice of factor of safety . . . . . . . . . . . . . . . .108
Examples
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Example 4.1
Design of a vertical, pre-cast concrete pile driven into sand and
gravel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32
Example 4.2
Pile design incorporating negative skin friction (downdrag) . . . . . . . .37
Example 4.3
The design of a cantilever retaining wall without groundwater
pressures acting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43
Example 4.4
The design of a cantilever retaining wall with groundwater pressures
acting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .48
Example 4.5
The design of an embedded retaining wall with groundwater
pressures acting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .55
Example 4.6
The design of a cantilever retaining wall with elevated groundwater
pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .65
Example 4.7
The design of a stable slope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .73
Example 4.8
An excavation below the water table, showing design against uplift . .78
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:
CIRIA C641
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
CIRIA C641
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.
CIRIA C641
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
CIRIA C641
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.
CIRIA C641
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
CIRIA C641
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