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Preparing for geotechnical Eurocodes

The British Geotechnical Association held a meeting Geotechnical Eurocodes – preparing for impact at the Institution of Civil Engineers on 10 October 2007.

Presented by Dimitrios Selemetas (Cementation Foundations Skanska), and reported by Brian Simpson (Arup), Andrew Bond (Geocentrix), John Powell (BRE) and David Norbury (David Norbury).

The changeover to Eurocode 7 (EC7) will begin in 2008 and must be completed by 2010. As this deadline approaches, the question of the likely impact of EC7 on our existing design practice has never been more critical.

With these thoughts in mind, four distinguished members of the BGA, who have been heavily involved in Eurocode development for the UK, provided an overview of how the introduction of the Eurocodes will impact on our natural reluctance to changing existing design practice. Copies of the four presentations can be downloaded from the website where Andrew Bond (Geocentrix) is keeping a blog with all the latest news regarding the implementation of Eurocodes in the UK.

Brian Simpson (Arup) described the key aspects of Eurocode 7 (EC7) Part 1 setting the basis of geotechnical design with reference to the design approach for ultimate limit state (ULS) and serviceability limit state (SLS) calculations. Simpson started his presentation by stressing that the new Eurocode System is a co-ordinated system, that is, there is cross-reference between the new Eurocodes, the new standards for field and laboratory testing as well as the Euronorms (Table 1) relating to execution of special geotechnical works.

Of the three different "design approaches" to account for the interactions between geotechnical actions and resistances, Simpson concluded that "design approach 1" is the most consistent approach for ULS design, where the design must comply with all combinations of partial factors in all respects, both geotechnical and structural.

Simpson highlighted that in accordance with the agreement between the European Committee for Standardization (CEN) and national standards organisations, all conflicting national standards must be withdrawn by March 2010. However, British Standard documents will continue to be part of how the UK operates, that is, how we deliver the EC7 approach.

John Powell (BRE) gave a brief overview of EC7 Part 2 concerning ground investigation (GI) and testing and highlighted some distinctive differences with BS5930. EC7-2 is supported by new testing standards (31) and technical specifications (16) that discuss the methods to be used to obtain the basic and derived parameters from which the design values are acquired (Figure 1).

Powell stressed that the Eurocode is not a code as we would tend to use the word but rather a standard in which we commonly encounter the statement "you shall". In addition, there is a list of things to do in contrast to the BS-type description of how to do things, which will inevitably affect our GI practice.

Following along these lines, David Norbury (David Norbury) gave a thorough review of the attachments to Part 2 of EC7, which comprise a series of standards on drilling and sampling, field and laboratory testing and soil and rock description. Norbury stressed the time has come to deploy the new procedures into practice, inform and train staff and, above all, to notify clients.

Finally, Bond gave a brief overview of the basis for pile foundation design followed by a set of examples of pile design using ground parameters and the results of field tests. The main message from Bond was that the more data the designer has, the less conservative the design can be (under EC7), but overall the results are not dissimilar to what our normal methods tend towards.

Following this stimulating presentation the audience was invited to raise questions to the speakers. John Harrison (Imperial College London) opened the debate by suggesting that if the Eurocodes were being introduced to a discipline where no other codes or standards existed, then engineers would have no difficulty adopting them. He suggested that in 20 years' time engineers will have adapted perfectly to them. The question was, how should the industry best start to work with the Eurocodes now, to avoid mishap caused by using them in conjunction with existing codes and standards?

Simpson replied that geotechnical engineers are familiar in using several ways to calculate results so it would be necessary in the early stages to carry out some cross-checking that will build their confidence in the use of the code. Bond added that a draft of EC7 was published in 1994 with the intention to gain experience in practicing the new code. However, Bond concluded that a good design based on previous codes would still be a good design with EC7.

Mark Crosby (Capita Symonds) asked the speakers for their views on the impact of EC7 on the new CDM regulations in which the competency of designers relies on past experience, especially in the field of geotechnical engineering. Simpson replied that the new Eurocodes are not to change the CDM regulations; engineers can still rely on their experience in the way they carry out geotechnical analysis.

Michael De Freitas (Imperial College London) pointed out that in the past it had been difficult to get familiar with some of the changes in the stratigraphical names that the BGS introduced for UK ground conditions.

Following this line of argument De Freitas questioned how easily engineers would come to terms with the new European materials and descriptions, and stated that some guidelines would be needed. Norbury replied that engineers can rest for guidance in BS5930, which is expected to be revised to incorporate the new standards. On the same note Powell agreed that it would be best to keep BS5930 as the basis for guidance and update accordingly to reflect the changes by the new Standards.

Norbury also reminded the audience that comparative studies between BS5930 and the new standards have been carried out in the past and referred to the articles Prepare for EC7 by Powell and Norbury (GE June 2007) and Soil and rock descriptions by Baldwin, Gosling and Brownlie (GE July 2007).

Andrew Symonds from a piling contractor speculated that with the new EC7 we might have missed the opportunity to use cone penetration test (CPT) results directly in the design. Bond replied that the method of using CPT results in pile design works well in uniform granular deposits, which is not very common in the UK. Bond added that EC7 effectively provides a common framework of reliability where it gives you the level of reliability based on a method of analysis of your choice.

Ko Ron (Highways Agency) thanked Bond for providing a link to his presentation (and those of the other speakers) on his website. He then enquired about the legal status in the case where a designer chooses not to follow specific clauses in the national annex or a technical specification on the grounds of previous experience, or with reference to a valued textbook and the structure he designed collapses.

Simpson replied that if a document is valued in this country – for example, Foundation Design and Construction by M J Tomlinson, which is "like a code of practice", according to Simpson – then this document can be used in court. However, from a legal point of view, it all depends on the judge.

Having said that, Simpson continued, the use of phrases such as "you shall" in the Eurocode is intended to identify those paragraphs that are mandatory, allowing no alternative. This is an improvement compared with the traditional "you should" found in the British Standards that leaves it open to interpretation.

To clarify the difference between "you shall" and "you should", Bond referred to a training course regarding use of British Standards in which a lawyer was asked whether there was a difference between the meanings of the two terms. After replying positively, the lawyer was then asked again to explain the difference. His response was: "I do not know". Powell added that in some cases the codes states that "you shall" but continues to state "if relevant". However, who is to decide what is relevant and what is not, he said. On a positive note, Norbury pointed out that should the designer choose not to follow the code, he should justify his decision.

Lee Parry (Atkins) enquired whether EC7 provides any flexibility for slope stability analyses in selecting appropriate material properties and factors of safety. Simpson confirmed EC7 does provide flexibility in that it allows you to use the observational method and, where the risk of failure is lower, to use lower factors of safety. Powell and Bond added that there is a lot of flexibility in selecting characteristic values. Following on from this, Peter Gilbert (Atkins) stressed it is not easy to find guidance in the Eurocodes on the use of engineering judgment for the selection of characteristic value. Bond agreed the Eurocodes do not specify how to select characteristic values, which is essentially the biggest variable in the design.

Barry Clarke (Newcastle University) put forward the issue of educating engineering graduates to EC7. In particular, Clarke asked the speakers whether they would expect the graduates to have knowledge of EC7 and asked their view on how the industry would support university education towards this aim. Bond replied that universities should teach graduates about reliability and variability of soil parameters that form the fundamental principles of EC7.

Bond argued that graduates should have knowledge of the principles rather than the details of the code. Simpson also stressed that there is a need for graduates to be able to think along the lines of Eurocodes and referred to the Crossrail project – the design of which is based on Eurocodes. With reference to the way that universities should prepare towards educating graduates, Simpson referred to training courses that CEN has scheduled for next year in which trainers are to receive training on Eurocodes.

In conclusion, Simpson gave a positive message on the implementation and use of the Eurocodes, by saying that the more we adopt the new codes the more we would find the benefits, arguing that it is a good engineering practice to comply with a unified set of documents. In the meantime, there is a lot of work still to be done to preserve the knowledge enclosed in the British Standards.

Table 1: Euronorms – executions of special geotechnical works

EN 1536: 1999 Bored piles
EN 1537: 1999 Ground anchors
EN 1537: 1999 Ground anchors - Corrigendum
EN 1538: 2000 Diaphragm walls
EN 12063: 1999 Sheet-pile walls
EN 12699: 2000 Displacement piles
EN 12715: 2000 Grouting
EN 12716: 2001 Jet grouting
EN 14199: 2005 Micropiles
EN 14475: 2006 Reinforced fill
EN 14475: 2006 Reinforced fill - Corrigendum
EN 14679: 2005 Deep mixing
EN 14679: 2006 Deep mixing - Corrigendum
EN 14731: 2005 Ground treatment by deep vibration
EN 15237: 2007 Vertical drainage

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