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DESIGN CHANGE APPROACH OF BS EN 1997 IN THE UK
A new European suite of geotechnical design, testing and construction documents will in due course largely replace British codes and standards. This purpose of this paper is to examine the changes in design and constructions once the new Eurocode is fully implemented in the UK. In this context, this paper highlights the important features of the new Eurocode system and seeks to show how they may affect practice. Some previous studies have been the source of this examination and evaluation. For example, the report published by the Institution of Structural Engineers (2004) has highlighted the challenges facing engineers in adapting to the Eurocodes. The purpose of this report is to assist the engineers to understand the most important differences that geotechnical engineers are encountering while implementing new suit of geotechnical Eurocode and advocates the preparation of guidance to ease their passage into practice.
The main changes to geotechnical practice introduced in the Eurocodes are concentrated in Eurocode 7 Geotechnical design. 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.
Figure 1 illustrates the system of new European documents while Tables 11 and 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 calculations
* 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 checking 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.
The application of safety in limit state design calculations
Current British codes contain a range of different ways of introducing safety into a geotechnical design, from the “working state” method using a global safety factor, generally adopted in BS 8004, to the partial factor method adopted in BS 8081. BS 8002 adopts yet another approach, that of the “(strength) mobilisation factor” whereby movement is limited by reducing in the calculation the value of strength (and thus strain) mobilised in the ground.
Most users of BS 8004 will have performed a design calculation that, by virtue of the (large) global factor of safety employed will have ensured not only that a ULS is avoided but also that settlements are acceptably small. In contrast, EC7-1, in ULS calculations, uses partial factor values that are intended to ensure that geotechnical failure is avoided, and may not be large enough to keep settlements to an acceptable level, particularly for softer, more compressible ground such as normally-consolidated or lightly over-consolidated clay or granular soil (BS 8004 Clause 126.96.36.199.2.1, p 9).
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.
Institution Of Structural Engineers (2004) National strategy for implementation of the structural Eurocodes: Design guidance Report prepared for The Office of the Deputy Prime Minister, IStructE, London
Powell, J (2003) A simple guide to in-situ ground testing - in 7 parts. Part 2: Cone penetration testing 144065, BRE Bookshop, London
Powell, J J M and Norbury, D R (2007) “An update on implementation of EC7” In: Ground Engineering, June 2007, pp 14-17