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Counting to Seven

Eurocode 7 - Geotechnical Design is coming.Richard Driscoll explains the progress of the new code and its integration with, and replacement of, British Standards.

The UK civil engineering community should be well aware of the imminent approach of a suite of European geotechnical design codes and standards that will, in due course, replace much of the collection of national design documents.

Engineers are encouraged to read the emerging European drafts, particularly those concerning ground investigation and testing, before changes are no longer feasible.

As well as the two Eurocode design documents produced by CEN committee TC250/SC7, two other CEN committees, TC341 and TC288, are producing test standards, technical specifications and execution standards (Table 1). The content and a brief description of BSEN1997-1 can be found in reference 1 which also presents a list of all the structural Eurocodes.

BSEN1997-2- Eurocode 7 - Part 2 The conversion of the pre-standards ENV1997-2 and ENV1997-3, Geotechnical design assisted by:

laboratory testing (part 2) and field testing (part 3), has been under way for over a year. It was agreed to combine the two documents into a single Part 2, Ground investigation and testing.

Part 2 covers planning of ground investigations; soil and rock sampling and groundwater measurements; field tests and laboratory tests on soil and rocks;

ground investigation reports; and informative annexes - showing this Eurocode covers much of the material provided in BS5930 and BS1377. Figure 1 shows the relationships between the documents.

Part 2 is now more closely aligned to BS5930, the test specifications that were previously in Part 3 being removed to the specific testing standards in Table 1.

The implications for both BS1377 - Part 9 and BS5930 are that any sections dealing with insitu test specification now covered by CEN documents will need to be removed.

For laboratory tests, if any of the drafted technical specifications become full testing standards, then the relevant parts of BS 1377 will have to be removed.

Part 2 considers in some detail the planning, execution and reporting of ground investigations. It is more prescriptive than UK industry is used to, as codes such as BS5930 give more guidance and there is little of the 'shall' that appears in Part 2.

In Part 2 the 'test results' (results directly obtained from the test) are taken forward to 'derived values' of strength, stiffness or test specific parameters to be used in design. It is these derived values that are used to determine the 'characteristic' values that are used in Part 1 for the design itself.

Many 'informative' annexes in Part 2 give examples of the use of parameters/derived values for design, especially for insitu tests in design procedures. In some countries these informative annexes could be made normative by the National Annex.

Guidance material A number of guidance documents are being prepared to help engineers adapt to the Eurocodes.

First, as one of a set of guides for all the Structural Eurocodes, Thomas Telford will publish A designer's Guide to Eurocode 7 Part 1, timed to appear when the BSEN is published.

This guide will cover all aspects of EN1997-1. Around the end of 2004, the Office of the Deputy Prime Minister, Buildings Division, will publish a simple, practical design guide, covering BSEN997-1 and BSEN1997-2 (see news this month).

It will focus on UK application of the Eurocode, specifically the adoption of Design Approach 1 (see reference 2 for a description of the three design approaches in EN1997-1). It will also explain in greater detail how the Eurocodes and standards will be implemented in UK building and related geotechnical works.

A growing amount of webbased guidance is being compiled.

GeoTechNet, the European Geotechnical Network project, is assembling a list of references to the technical background to the limit state, partial factor design methodology of the Eurocode and is collecting a set of design examples to illustrate its application, especially the three design approaches and the differences they make to calculation results.

This material is restricted to GeoTechNet members but general information and some worked example material will be available to all (www. geotechnet. org).

Implementation Each country is required, through its national standards body (eg BSI), to implement the EN Eurocodes by publishing them without alteration. The only specific local changes that are permitted must be implemented in a National Annex.

As levels of safety implicit in Eurocodes are a matter for individual member states to stipulate, a CEN TC250 document states:

'The National Annex may contain. . . information on those parameters which are left open. . . to national choice, the NDPs [nationally determined parameters]. . . ie:

lvalues and/or classes where alternatives are given in the EN Eurocode [values for the partial g factors; see reference 2] lvalues to be used where a symbol only is given in the EN Eurocode lcountry specific data [eg depth of foundations to avoid frost heave or clay shrinkage] lthe procedures to be used where alternative procedures are given in the EN Eurocode [choices in Design Approach are given, see reference 2] ldecisions on the application of informative annexes [eg method for calculating settlement] reference to non-contradictory, complementary information to assist the user in applying the Eurocode'.

The British National Annex for BSEN1997-1 is nearing completion and will address the above aspects, including reference to material deemed necessary for the safe application of the Eurocode and to assist in its use.

And, since national codes are generally advisory and very rarely contradict the principles of the Eurocode, it is expected that engineers may continue to make reference to much of the practical advice they contain.

Where there is a specific point of conflict of principle with the Eurocode, for example where a BS refers to a global factor of safety whereas the Eurocode requires the use of partial factors, the BS will be amended.

Since the TC341 committee does not have the time or the resources to produce a full set of testing standards for the range of tests included in BS1377 (specifically the laboratory tests) and has elected to produce the 'Technical specifications' in Table 1, which are 'informative' and not 'normative' (ie they are not mandatory, only 'advisory'), the British NA will refer to those parts of BS1377 not covered by TC341;

large parts of BS5930 are also expected to be retained since they do not conflict with BSEN1997-2 and provide additional guidance on the planning and scope of site investigations.

For the execution standards written by TC288, there is obvious correspondence with some BS codes, for example, BS 8004:1986 (Foundations) and BS 8081:1989 (Ground anchorages), bearing in mind these BSENs are intended to deal with construction-related matters and not design.

BSI examined the earlier of these standards for conflict with the British Standards and found only minor matters such as some concrete specifications. Amendments to the British Standards will be issued so they may be retained for the practical guidance the equivalent European documents can lack.

Timing of the roll-out of the geotechnical Eurocode parts is expected to be roughly as follows:

lBSI publishes BS EN1997-1 mid 2004 (also BS EN1997-2 later in 2004 or in early 2005).

lA first draft of the National Annex for BSEN1997-1 will be completed by mid 2004 to be issued for public comment when Part 1 is published. The NA will be amended and finally, and separately, published in 2005. A NA for Part 2 is expected in a similar time-frame.

lTC341 test standards and technical specifications are published by BSI during 2004 and 2005.

lTC288 standards are completed and published by the end of 2005.

lThe CEN/ISO documents are published by the end of 2005.

As the complete design of say, a building foundation, will require not only BSEN1997 for the geotechnical aspects but also the Loading Code (EN1991), the Concrete Code (EN1992) and codes for masonry and timber structures, the Eurocodes will be implemented as 'packages', in this case a 'buildings package'.

Only after a period of 'co-existence' of the package with the equivalent national codes will BSI be required to amend or withdraw any conflicting national documents; the period of co-existence should not exceed three years.

European geotechnical codes and standards therefore may not fully replace the national documents until 2008 or beyond.

lFor more information contact Hilary Skinner, email: skinnerh@ bre. co. uk; tel: 01923 664183.

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