EC7 – implications for
Eurocode 7 Geotechnical design
Classic House, 174–180 Old Street, London EC1V 9BP TEL: +44 (0)20 7549 3300 FAX: +44 (0)20 7253 0523 EMAIL: email@example.com WEBSITE: www.ciria.org
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.
EC7 – implications for UK practices. Eurocode 7 Geotechnical design
Driscoll, R, Scott, P, Powell, J
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, 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: firstname.lastname@example.org Tel: +44 (0)20 7549 3300.
Ground engineering, Eurocode, foundations, geotechnical design, geotechnical investigation, ground investigation and characterisation, in situ testing and instrumentation, piling, soil structure interaction
Design of geotechnical structures, limit state design, Eurocodes replace British Codes and Standards
USER Client organisation, consultants,
contractors , geotechnical engineers, project managers, structural design engineers
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 CENacommittees and working groups. However the main design code, EC7-1, and several “execution”bstandards 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
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 ”.
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:
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.
This publication is the main output from CIRIA research project 701. It was prepared by BRE in association with Buro Happold.
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
Following CIRIA’s usual practice, the research project was guided by a steering group, which comprised:
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
Dr M Pedley Cementation Foundations Skanska
Mr S G Smith Bechtel
Dr J Wilson Atkins
CIRIA’s research managers were Mr Chris Chiverrell and Dr Andrew Pitchford.
This project was funded by:
The DTI’s Partners in Innovation scheme The Highways Agency
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
ContentsSummary . . . .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 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
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
3.7.9 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 188.8.131.52) . . . .77
4.7.3 HYD ULS design (see Clause 184.108.40.206) . . . .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 with relevant 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 investigation practice . . . .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 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
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 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 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 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
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 1 Set of forces (loads) applied to a structure (direct
2 Set of imposed deformations or accelerations caused, for example, by temperature changes, moisture variation, uneven settlement or earthquake (indirect action).
Characteristic value Clause 220.127.116.11(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 18.104.22.168 of BS EN 1990).
Limit states States beyond which the structure no longer fulfils the
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 Value used for the verification of a limit state. A
of an action 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 that correspond to conditions beyond which
states 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.
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 documents1will 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 ISO4and 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:
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
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 checking7that 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:
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
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.
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.
Important features of EC7
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.
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
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.
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.
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.
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
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.
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.
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.
Table 1.1 The content of BS codes and their correspondence with the European documents N e w E u ro p e a n d o c u m e n ts S ta n d a rd s f o r th e e xe c u ti o n o f s p e c ia l g e o te c h n ic a l w o rk s ( C E N T C 2 8 8 ) BS EN 1 4 4 75:2006 R e inf o rced f ill pr EN 1 4 490 Soil nailing BS EN 12063:1 999 Shee t pile w a lls BS EN 1 536:2000 Bored piles BS EN 12699:200 1 Displacement piles BS EN 1 4 1 99:2005 Micr opiles BS EN 1 5 3 7 :2000 Gr ound anc h or s BS EN 1 538:2000 Diaphragm w a lls BS EN 12063:1 999 Shee t pile w a lls BS EN 12 7 1 5:2000 Gr outing BS EN 12 7 1 6:200 1 Je t gr outing pr EN 1 4 490 Soil nailing BS EN 1 4679:2005 Deep mixing BS EN 1 4 73 1:2005 Gr ound treatment b y deep vibration BS EN 1 523 7 :200 7 V e rt ical drainage BS EN 1 4 4 75:2006 R e inf o rced f ill S e c ti o n – T it le 1 G eneral 2 P lanning of gr ound in v e stigations 3 S oil and r o ck sam p ling and gr oundw at er measurement 4 F ield t e sts in soils and r o cks 5 Laborat or y t e sts on soils and r o cks 6 G ro und in v e stigation repor t Anne x B P lanning strat e gies f o r geo technical in v e stigations E C 7 -2 G e n e ra l is s u e s c o v e re d Ov erall appr oach Gr ound in v e stigation
Design aspects of construction activities Design of specif
ic elements E C 7 -1 S e c ti o n – T it le 1 G eneral 2 B asis of geo technical design 3 G eo te chnical data 4 S uper vision of construction, monit o
ring and maint
e nance 5 F ill, de w a te ring, gr ound im pr o v
ement and reinf
o rc ement. (NN o te : EC7-1 does no t co v e r the design of reinf o rc ed soils or gr ound strengthened b y nailing etc). 6 S pread f oundations 7 P ile f oundations 8 A nchorages 9 R etaining structures 1 0 Hydraulic f a ilure 1 1 Ov erall stability 12 Embankments B S c o d e BS 5930:1 9 99 Sit e in v e s tigation
Some of those belo
w BS 603 1:1 9 8 1 Ear thw orks BS 8006:1 995 S trengthened/reinf orced soils and o ther f ills BS 8004:1 9 86 F oundations BS 8004:1 986 F oundations BS 8008:1 996 Saf e ty precautions and pr ocedures f o r the cons
truction and descent of
mac h ine-bored shaf ts f o r piling and o ther purposes BS 808 1 :1 989 Gr ound anc h orages BS 8002:1 994 Ear th re taining s tructures Some BS 603 1 :1 98 1 Ear thw orks
Table 1.2 The content of BS codes and testing standards and their correspondence with the European documents N e w E u ro p e a n d o c u m e n ts C E N I S O s ta n d a rd s (s e e A p p e n d ix A 6 f o r a n e xp la n a ti o n o f th e p ro v e n a n c e o f th e d if fe re n t s ta n d a rd s ) BS EN ISO 1 4688-1:2002 BS EN ISO 1 4688-2:2004 pr EN ISO 1 4688-3 BS EN ISO 1 4689-1:2003 pr EN ISO 1 4689-2
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 BS EN-ISO 22
4 7 5-1:2006 DD EN-ISO/TS 22 4 75-2:2006 DD EN-ISO/TS 22 4 75-3:200 7 CEN ISO/TS 1 7 892-1 * CEN ISO/TS 1 7 892-2 * CEN ISO/TS 1 7 892-3 * CEN ISO/TS 1 7 892-4 * CEN ISO/TS 1 7 892-12 * N o te : There appear t o be BS ENs in e xist e nce that ha v e been draf ted b y committ ees
concerned with aggregat
es. These will need
to be re vie w ed t o
assess their applicability t
soils. CEN ISO/TS 1
7 892-5 * L a b o ra to ry a n d f ie ld t e s ti n g s ta n d a rd s a n d t e c h n ic a l s p e c if ic a ti o n s ( C E N T C 3 4 1 ). N o te : “T e c h n ic a l S p e c if ic a ti o n s ” a re i d e n ti fi e d b y “T S ” in t h e r e fe re n c e n u m b e r. Geo technical in v e stigation and t e sting – Identif
ication and classif
ication of soil:
t 1: Identif
ication and description
Par t 2: Classif ication principles Par t 3: Electr onic data e xchange - soil Ro c k s Par t 1: Identif
ication and description
Par t 2: Electr onic data e xchange – r o ck Geoh ydraulic t e sting General R u les W a te r permeability t e
sts in a borehole without pack
er W a te r pressure t e st in r o ck Pum p ing t e sts Inf iltr omet er t e st W a te r permeability t e sts with pack er and pulse-lik e stimulation Sam p ling – principles Sam p ling – q u alif ication crit eria Sam p ling – conf ormity assessment W a te r cont ent Density of f
ine grained soils
Density of solid par
ticle size distribution
Incremental loading oedomet
er t e st E C 7 -2 1 G eneral planning of gr ound in v e stigations 2 S oil and r o ck sam p ling and gr oundw at er measurements 3 F ield t e sts in soil and r o ck 4 Laborat or y t e sts on soil and ro c k 5 G round in v e stigation repor t Anne x B P lanning of geo technical in v e stigations Anne x A L ist of t e st results of geo technical t e st standar d s Anne x L D etailed inf o rmation on
preparation of soil specimens for t
e s ti n g Anne x M Detailed inf o rmation on t e sts fo r classif ication, identif ication
and description of soils
Anne x N D etailed inf o rmation on chemical t e sting of soils Anne x R D etailed inf o rmation on com p action t e sting of soils Anne x Q D etailed inf o rmation on com p ressibility t e sting of soils B S c o d e BS 5930:1 999 Sit e in v e s tigation BS 1 3 7 7 :1 990 Me thods of t e s t f o r soils f o r
civil engineering purposes
uirements and sam
p le preparation Par t 2 C lassif ication t e sts Par t 3
Chemical and electr
o-chemical t e sts Par t 4 C om paction-relat ed t e sts Par t 5 C om pressibility , permeability and durability t e sts
CEN ISO/TS 1 7892-1 1 CEN ISO/TS 1 7892-7 * CEN ISO/TS 1 7892-1 0 * CEN ISO/TS 1 7892-8 * DD CEN ISO/TS 1 7892-6:2009 * CEN ISO/TS 1 7892-9 pr EN-ISO 22 4 7 6-1 BS EN-ISO 22 4 7 6-2:2005 BS EN-ISO 22 4 7 6-3:2005 pr EN-ISO 22 4 7 6-4 pr EN-ISO 22 4 7 6-5 pr EN-ISO 22 4 7 6-6 pr EN-ISO 22 4 7 6-8 pr EN-ISO 22 4 7 6-9 CEN-ISO/TS 22 4 7 6-1 0 * DD CEN-ISO/TS 22 4 7 6-1 1 :2005 pr EN-ISO 22 4 7 6-12 pr EN-ISO 22 4 7 6-1 3
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 pr EN ISO 22
4 7 7-1 pr EN ISO 22 4 7 7-2 pr EN ISO 22 4 7 7-3 pr EN ISO 22 4 7 7-4 pr EN ISO 22 4 7 7-X pr EN ISO 22 4 7 7-5 pr EN ISO 22 4 7 7-6 pr EN ISO 22 4 7 7-7 P e rmeability t e st Unconf ined com p ression t e st on f
ine grained soils
Direct shear t e st Unconsolidat e d triaxial t e st F a ll cone t e st Consolidat ed triaxial t e st
Electric cone penetration t
e st Dynamic Pr obing Standar d P e netration T e st Menar d pressuremet e r t e st Fle xible dilat o met e r t e st Self-boring pressuremet e r t e st F u ll-displacement pressuremet e r Field v a ne t e st W eight sounding t e st Flat dilat o met e r t e st
Mechanical cone penetration t
e st Plat e Loading T e st
General rules (permeability) Pe
rmeability t e sts in a borehole W a te r pressure t e sts Pum p ing t e st Inf iltr omet er t e sts Closed syst ems pack er t e sts Pile load t e
st – static axially loaded com
p ression t e st Pile load t e
st – static axially loaded t
e nsion t e st Pile load t e st – static transv e rs ely loaded t e nsion t e st Pile load t e
st – dynamic axially loaded com
p ression t e st Pile Load t e sr –
rapid axial loaded com
p ression t e st Te sting of anchorages Te sting of nailing Te sting of reinf o rc ed f ill Anne x S Detailed inf o rmation on permeability te sting of soils Anne x O D etailed inf o
rmation on strength inde
x te sting of soils Anne x P D etailed inf o rmation on strength t e sting of soils Anne x C Exam p le of gr ound w a te r pressure deriv a
tions based on a model and long
one and piezocone penetration t
e sts Anne x E P ressuremet e r t e st Anne x F S tandar d penetration t e st Anne x G D ynamic pr obing Anne x H W eight sounding t e st Anne x I Field v a ne t e st Anne x J Flat dilat o met e r t e st Anne x K Plat e Loading T e st Anne x T P reparation of specimens f o r t e sting of ro ck mat e rial Anne x U C lassif ication t e sting of r o ck mat e rial Anne x V S w e lling t e sting of r o ck mat e rial Anne x W S trength t e sting of r o ck mat e rial Par t 6 C
onsolidation and permeability t
ydraulic cells and with pore pressure
measurement Par t 7 S hear strength t e sts (t o tal stress) Par t 8 S hear strength t e sts (ef fectiv e stress) Par t 9 In situ te s ts None BS 8004:1 9 86 F oundations Note
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 and standards
Eurocodes: BS EN 1990:2002
Basis of structural design
BS EN 1991-1-1:2002 Actions on structures
European standards for the Execution of special
geotechnical works Other structural Eurocodes eg BS EN 1993-5:2007 ISO/CEN Standards for identification and classification
Test standard for technical specifications
for ground properties
Geotechnical design Eurocodes: BS EN 1997-1:2004 BS EN 1997-2:2007
Site characterisation and determination
of ground property design values
Ground investigation and testing
The processes for obtaining ground parameters to use in design with EC7-1 are specified in several parts of the suite of European codes and standards, as indicated in Tables 1.1 and 1.2. It is evident that change over to the BS EN suite will entail the acquisition of many more standards and a period of adjustment to those documents that will replace BS 5930 and parts of BS 1377.
Table 1.1 lists the standards and technical specifications (TS) that are being produced by CEN and ISO, which will replace UK codes, standards and practice, covering the same subject matter. Most of the standards and TS documents are to be finalised or, in some cases, drafted, so it is not yet possible to fully identify conflicts with UK
The main areas in which EC7-2 differs from current BS codes and standards requirements are listed in Table 2.1, from which it can be seen that a fundamental change involves the introduction of some compulsory activities (the word “shall” is used). This change has consequences for the procurement of site investigation, for the clear specification of who does what and for how information is disseminated to all appropriate parties.
In fact, there are only two major changes to existing British site investigation practice and it can be expected that much if not all of the good practice guidance contained in, for example, BS 5930 will continue to apply in the future (see Section 6.4). The first major change concerns the provision of a geotechnical investigation report discussed later. The second concerns the effect of the requirement in EC7-1 for much greater consideration of settlement and deformation. This will entail much more attention being paid to the determination of the deformational properties of the ground, a difficult subject.
1 EC7-2 makes compulsory the provision of a ground investigation report to all relevant parties.
2 EC7-2 is more prescriptive than BS 5930 in its planning and execution requirements for ground investigation.
The emphasis in EC7-1 on better prediction of settlement and deformation raises the importance of ground deformation properties. While the option is available in EC7-1 to use a reduced strength value, akin to the strength mobilisation factor used in BS 8002, the need for better knowledge of the deformation properties of the ground from additional and specific testing should presage a profound change in UK geotechnical practice.
4 Some parts of BS 1377 and BS 5930 have been and will continue to be replaced by new BS EN documentation.
5 Some departures from BS 5930 terminology apply for soil and rock descriptions.
6 Design values of ground properties may be assessed directly as an alternative to applying partial factors to characteristic values.
7 Procurement processes may need to be clearer about who does what, quality assurance, professional indemnity implications and communication between interested parties.
Table 2.1 Some of the changes introduced by EC7-2
EC7-2 novel feature Impact on practice as embodied in BS codes and standards
Use of “shall” in Principle clauses, rather than “should”11 (examples from Section 2 Planning of ground
if the main ground investigations do not supply the necessary information, complementary investigations shall be undertaken. Clients may come to appreciate that, if they fund more
comprehensive, initial investigations, they can avoid the expense of further investigation at a later stage
it is stated that investigations shall be planned and data shall be adequate to manage risks12
the document states that a visual inspection shall be undertaken before planning the investigation programme and used in conjunction with a desk study
it also says that quality assurance systems shall be in place for all aspects of the work13
the necessary number of specimens to be tested shall be determined. Recommended numbers are contained in informative annexes but the status/validity of these will need to be discussed in the NA for EC7-2. It is unclear what the implications might be if the recommendations are ignored and things go wrong
a table of applicability of various field tests is also presented.
Section 3 on soil and rock sampling introduces categories of sampling method based on BS EN ISO 22475-114.
this implies that only certain sampling methods can be used to obtain samples of a certain quality class
the quality class relates to use in specific laboratory tests in order to give the test results required for the selection of characteristic values15.
Section 4 Field tests in soils and
rocks specifies that CEN standards
shall be used when specifying tests. Conversion of test results into “derived” values is introduced
existing BS 1377 and BS 5930 sections specifying test methods will become redundant where a corresponding standard exists. If a technical specification is listed for a particular test then either this or the BS can be used. See Table 1.2 for the tests affected.
Section 5 Laboratory tests on soils
general statements occur with “shall” throughout the section. Much is simply good practice but if things go wrong then decisions taken relating to clauses saying “shall” will need to be justified checks shall be made that the laboratory equipment used is
adequate, fit for purpose, is calibrated and within the calibration requirements
there is a requirement that all test methods and procedures shall be reported
a quality assurance system shall be in place in the laboratory16
all descriptions shall be to BS EN ISO 14688-1:2002 and BS EN ISO 14688-2:2004
for the laboratory tests listed in Table 1.2 there will be no withdrawal of corresponding BS documents as the CEN documents are all only technical specifications (TS) and EC7-2 allows the NA to adopt National Standards in preference. This will be the case in the UK (only the TS for the fall cone will be adopted) (see NA to EC7-2 in 2009). Section 6 Ground investigation report
deals 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 and geological features and a geotechnical evaluation of the information all methods shall be documented in accordance with the relevant
it shall include all relevant information on how the derived values were arrived at.
EC7-2 identifies an explicit hierarchy of investigations that is also found in BS 5930: geotechnical investigations17, which comprise the gathering of all relevant
information about the site18and a ground investigation
ground investigations, which comprise field investigations, laboratory testing and desk studies of geotechnical and geological information
field investigations, which comprise direct investigations (drilling, sampling and trial pits) and indirect investigations (in situ tests, such as the CPT).
The code further distinguishes between investigations for the purposes of design and for
In a clear departure from most current practice, EC7-2 makes compulsory the provision of some form of ground investigation report (GIR) as part of the geotechnical
design report. The code specifies that the GIR should include:
a presentation of all available geotechnical information including geological features and relevant data
a factual account of all field and laboratory investigations
a geotechnical evaluation of the information, stating the assumptions made in the interpretation of the test results
a statement of methods adopted (citing the relevant standards)
all relevant information on how a direct assessment of design values or “derived values” (see below) were determined, including any correlations used
any known limitations in the results.
The size of the GIR will depend on the complexity and value of the project, varying from a single page for a simple footing to volumes of pages for a major infrastructure project. While listing the general information required to reach a decision on the values of geotechnical parameters for a suitable design, it could be argued that EC7 places too much emphasis on the manipulation of test results and not enough on desk studies and other means to determine information on such matters as:
site geology, geomorphology and overall stability
Man’s influence on the site and the sensitivity of existing structures local experience and relevant published knowledge.
In addition to gathering all pertinent facts already known about a site, determining the ground properties for design using the Eurocode suite could be seen as a logical sequence of:
carrying out tests and interpreting the test results determining derived values
collating all geotechnical and other relevant information about the site
selecting characteristic values for factoring into design values, taking account of the requirements of the project19.
The testing and interpretation elements of this sequence are illustrated in Figure 2.1, which has been taken from EC7-2 and is further explained in Figure 2.2. In both, the term derived value is used – EC7-2 defines this as the value of a geotechnical
The processes for obtaining derived values are essentially the same as current good practice – EC7 may be seen simply as attempting to codify these processes.
It is this derived value that is used in EC7-1 for selecting a characteristic value from which to determine a design value. The selection is made in EC7-1 because, in the final analysis, it should be the designer’s responsibility. This may generate difficulty for the ground investigation contractor in some instances. For example, if the contractor is asked to provide characteristic rather than derived values of ground parameters, it may become difficult or risky unless there is sufficient information about the design
situations and the limit states pertaining to the project.
Ground identification and classification
For soil and rock descriptions (ISO-EN 14688-1, 14688-2 and 14689-1) some differences from BS 5930 terminology are apparent. Examples that have required a revision of our current terminology are shown in Table 2.2.
Table 2.2 Some terminological changes
(Fuller details can be found in Powell and Norbury, 2007 and Baldwin, Gosling and Brownlie, 2007).
Some of the new field testing standards are likely to cover not only equipment
specification, sizing and operation but also to introduce requirements for what might be termed “fitness for purpose”. In these, the test specification is related to its application and the required accuracy for the ground conditions, and for intended use of the results (this will become clearer as the documents are completed).
As the various documents become available, detailed comparisons with BS documents will have to be undertaken to identify any potential conflicts.
Determining the design values of geotechnical
One of the biggest changes for UK practice is the formalised process in the Eurocode for determining the design values of ground properties using partial factors and characteristic values.
EC7-1 states that design values of geotechnical parameters (Xd) shall either be
derived20from characteristic values using the following equation:
Xd= Xk/ γM (Equation 2.2, BS EN 1997-1)
or shall be assessed directly (Clause 22.214.171.124(1)P).
where Xkis the characteristic value and γMis the partial material factor.
Old terminology New terminology
Slightly organic Low-organic
There are changes in the boundaries between the classes
Highly organic High-organic
Very soft, soft, firm, stiff and very stiff (terms used in BS 5930 to describe shear strength)
Now used to describe the consistency of silts and clays. Shear strength descriptors become: vvery low, low, medium, high, very high and extremely high, (though maintaining the same strength ranges used in BS 5930)
Although the Eurocode clearly permits the direct assessment of a design value, it places priority on the use of factored characteristic values. There are situations in which it is more appropriate to assess, for example, a strength where the critical state strength value will be used in the design. This may also apply to the design values of deformation properties since these are rarely measured and are commonly deduced from correlations with strength.
In selecting the characteristic value, account should be taken of a number of matters that are listed in EC7-1, which then defines “characteristic value” as the characteristic
value of a soil or rock parameter shall be selected as a cautious estimate of the value affecting the occurrence of the limit state.
Design values of parameters may be required for both ultimate and serviceability limit state considerations. It is important to appreciate that, while the partial factor used to obtain the design value will have different values for ULS and SLS, so also may the characteristic value itself differ in calculations for these limit states.
The meaning and selection of a characteristic value have been debated for many years21. It is important to realise that, despite the formalisation in the Eurocode, the
selected value(s) is for the judgement of the designer, having considered all relevant
information, including prior knowledge of the particular site and all ground testing and assessment data. As the values of the partial material factor γMin Equation 2.2 are fixed in the National Annex (see Section 6.3), the designer has control of the design value through the selection of the characteristic value22.
Much has also been said23about the merits or otherwise of using statistics in the determination of characteristic values and it is important to appreciate that the Eurocode does not require their use.
Note: It is very important that the chosen correlation is appropriate for the prevailing geological condition.
Figure 2.2 General procedure for determining characteristic values from measured values
Selecting characteristic values
A list of the issues to be considered in determining characteristic values is shown in Figure 2.2. An example of how a characteristic profile of ground strength values might be obtained for a particular site is given in Appendix A1. It is important to appreciate that the selections made in Appendix A1 are quite subjective, so a averse or risk-taking designer might make a rather different selection which could be seen as a retention of the status quo in UK practice.
Characteristic values of ground stiffness and weight density24
The basis of structural design Eurocode, BS EN 1990, states that “The structural
stiffness parameters (eg moduli of elasticity … ) … should be represented by a mean
In problems involving ground structure interaction the stiffness of the ground is often a very important parameter26. In these cases, the use of a mean value for ground stiffness is questionable27. As EC7-1 does not define a characteristic value of ground stiffness, it is suggested that its definition should follow that for strength ie a “cautious estimate” and not a mean value.
EC7-1’s definition of characteristic strength value might be assumed also to apply to the weight density of soil and rock. However, the uncertainty about weight density is usually sufficiently low that there is no need to make a distinction between mean and cautious values.
Other attempts to deal with uncertainty in ground parameters
It is useful to compare the acquisition of conservative ground parameters in EC7-1 with approaches in other geotechnical design codes and guidance for ensuring the necessary caution in values for use in design. Historically, CIRIA R104 (Padfield and Mair, 1984) suggested that design may be based on moderately conservative values of parameters. Moderately conservative is defined as “a cautious estimate of the value relevant to the occurrence of the limit state” which compares closely with the EC7-1 definition. CIRIA C580 (Gaba et al, 2003) discusses these definitions.
The new principles of geotechnical
design in Eurocode 7
There are several key features of EC7-1 that make it different from the current BS geotechnical codes, these are:
unlike current codes, EC7 states that Principles shall be honoured. BS codes state only that things should be done28
EC7-1 embodies a design calculation methodology that makes sub-structure design fully compatible with superstructure design using the other structural Eurocodes29 EC7-1 explicitly identifies design limit states30
EC7-1, in ultimate limit state design calculations, makes use of partial factors applied to characteristic values of parameters, to account more directly for uncertainty in the values of parameters used in the calculations31and to achieve compatibility with structural codes that also conform to the basis of structural design laid down in BS EN 199032
the formal adoption of four alternative methods for achieving a geotechnical design (see page 18)
EC7-1 makes compulsory the provision to the client of a geotechnical design report, while EC7-2 requires the provision of a ground investigation report, to form part of the geotechnical design report.
Together, these provide a single set of guiding principles for all geotechnical designs that is absent in the current, diverse set of BS design codes.
1 Clear separation of ultimate (failure) condition from serviceability (settlement and comfort) condition.
2 Need to be aware of the important distinction between “permanent” and “variable” actions, since different values of partial factors apply to each. Similarly for “favourable” and “unfavourable” actions.
3 Use of “characteristic” to define values of ground properties for use with partial factors to form design values.
4 Application of separate partial factors to several aspects of uncertainty, rather than a single lumped factor of safety applied to cover all uncertainty.
5 Partial factor values have been largely selected to avoid failure and are not necessarily sufficient to ensure acceptable movement. A check on movements will often be required.
Good communication between geotechnical and structural engineers is required for clarity on permanent and variable structural loads, and on tolerable movements of foundations. Communication between concerned parties will need to improve.
A simple alternative to performing settlement calculations for an SLS check is permitted in specific circumstances. This involves adopting a reduced value of strength in the ULS calculation in order to limit mobilised strains.
EC7-1 explicitly identifies one or a combination of the following four design methods to be used to ensure that the performance of a geotechnical structure will avoid exceeding the specified limit states:
using prescriptive measures33(see Section 3.3)
using tests on models or full scale tests34(see Section 3.4) using the Observational Method35(see Section 3.5) using calculations (see Section 3.7).
Although they are found in our current national geotechnical codes, these alternatives are not as clearly recognisable as in EC7-1. Formal use of the Observational Method post-dates BS codes, although many of its features, such as monitoring of performance in the field, have been used for many decades.
To help establish design requirements, EC7-1 recommends the classification of designs into Geotechnical categories (GC): GCs 1, 2 or 3 are proposed according to the complexity of the structure, of the ground conditions and of the loading and according to the level of risk that is acceptable for the purposes of the structure36. GCs may also be used to help establish the extent of site investigation required and the amount of effort to be put into checking that a design is satisfactory. No such classification system exists in BS geotechnical codes, although a similar system has been introduced for classifying retaining wall problems in CIRIA C580 (Gaba et al, 2003).
As GCs are “recommended” their use is not compulsory in EC7-1.
EC7-1 is as much about the “checking”37of a design as about carrying out design. It does not provide detail of how design calculations are to be performed but rather presents a framework for ensuring that the resistance offered by a design to the destabilising actions on it is adequate to prevent a limiting condition being exceeded. The same could be said for some of the current BS codes, for example, BS 8004 does not describe in detail how to design footings or piles against bearing capacity failure or settlement. Conversely, BS 8002 and BS 8081 provide far greater detail than is found in EC7-1.
In this chapter, the first three of the general design methods are briefly discussed before concentrating on design by calculation (Section 3.7), since it is in this regard that the principles of EC7-1 are most different from BS codes.
Design by prescriptive measures
Prescriptive measures involve conventional and generally conservative rules in the design, and attention to specification and control of materials, workmanship, protection and maintenance procedures. Partial factors are not intended to be used with
prescriptive measures38. Prescriptive measures usually involve the application of charts and tables that have been established from comparable experience39and they implicitly contain their own safety factor. Very often, the concept of “allowable stress on the soil” is used in these charts or tables40.
Prescriptive measures can be applied in cases where calculation models are not available or not appropriate41. Examples of the application of prescriptive measures concern durability, such as sacrificial thickness to accommodate corrosion loss, and local rules of good practice, such as prescribed depth of footing to avoid seasonal volume change in clay soil42.