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Search for common ground Eurocode 7, the new European code on geotechnical design, is taking shape. Andrew Bond of the Geotechnical Consulting Group reviews some of its controversial aspects and summa

Europe is undergoing a revolution. First, with the introduction of the euro in January 1999; then with the publication of the first part of Eurocode 7, some time in 2001. Few geotechnical engineers outside those intimately involved with the drafting of Eurocode 7 are aware of the implications the new design standard will have on their existing practice.

In 1976, the European Commission decided to sponsor development of a set of codes of practice for building structures. In 1980, under an agreement with the Commission of the European Communities (CEC), the International Society of Soil Mechanics and Foundation Engineering surveyed member states' codes of practice for foundations. This led to the publication in 1987 of a draft model for Eurocode 7.

The CEC sponsored further work on this draft until 1989/90, when responsibility for it was transferred to the Comit Europen de Normalisation (CEN). CEN members include the national standards bodies of Austria, Belgium, the Czech Republic, Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Italy, Luxembourg, Netherlands, Norway, Portugal, Spain, Sweden, Switzerland, and the UK.

CEN established a technical committee, TC250, to oversee the development of all the Eurocodes (Figure 1). Subcommittee SC7 of TC250 (see box right) is responsible for Eurocode 7, Geotechnical design.

October 1994 saw the publication of the European pre-standard ENV1997- 1, Eurocode 7: Geotechnical design - Part 1: General rules. This pre-standard was intended for experimental use in member countries for three years, after which a vote of all CEN members (see below) would decide whether to convert the ENV into a full European standard or 'Euronorm' (EN).

Key features

The features of ENV1997-1 that distinguish it from previous geotechnical codes of practice include:

Division of clauses into principles (P) and application rules (AR).

Adoption of a limit state design philosophy based on partial factors of safety and three design cases (A, B, and C).

Distinction between permanent, variable, and accidental actions (both favourable and unfavourable) in ultimate limit states.

Definitions of design and characteristic values (for actions as well as material properties).

Numerous checklists of things to be considered in a design.

Principles are general statements and definitions for which there is no alternative; whereas application rules are generally recognised rules which follow the principles and satisfy their requirements. In the pre- standard, principles are denoted by the letter P following their clause number. For example, 2.1(6)P states that 'For each geotechnical design situation, it shall be verified that no relevant limit state is exceeded'; and 2.1(7) states 'This design requirement may be achieved by use of calculations . . . , adoption of prescriptive measures . . . , experimental models . . . , [or] an observational method.' Thus 2.1(7) is the application rule that satisfies the principle given in 2.1(6)P.

Design Cases A, B, and C are the most controversial element of Eurocode 7. Each case was intended to guard against a particular type of failure:

Case A, loss of static equilibrium.

Case B, failure of the structure or structural element.

Case C, failure in the ground.

However, the ambiguous wording of the pre-standard and the difficulty of matching this system to practice in some member countries has led to heated debate about the efficacy of the safety system. Major revision of the safety system seems inevitable before agreement is reached for the final form of Eurocode 7 (ie EN1997-1).

The word action is one of several terms that will be unfamiliar to new readers of the code, but it will soon take its place in the geotechnical engineer's vocabulary. Actions include forces applied to a structure (direct actions) and imposed deformations or accelerations caused (for example) by soil movements, temperature changes, or uneven settlements (indirect actions).

Another of the pre-standard's hotly-debated clauses is the definition of the characteristic value of a ground property as a 'cautious estimate of the value affecting the occurrence of the limit state' (see 2.4.3(5)P). The meaning of this clause has been interpreted in different ways by different engineers (eg 'sometimes a mean, sometimes a reasonable minimum', 'the conventionally selected value', 'a moderately conservative value') and will continue to defy precise definition for years to come. Figure 2 shows where this term fits in the hierarchy of values used in Eurocode 7.

The application rule that accompanies Principle 2.4.3(5)P goes on to state that 'If statistical methods are used, the characteristic value should be derived such that the calculated probability of a

worse value governing the limit state is not greater than 5%.' Unfortunately, geotechnical engineers are rarely blessed with sufficient test results in each soil stratum to justify the use of statistics.

Even if there were a sufficient number of test results, it is far from clear that (a) they would be normally distributed or (b) they actually represent the properties of the ground in situ. Nevertheless, the 5% rule gives an indication of degree of conservatism assumed by the code.

Towards the Euronorm

The remainder of this article describes the progress that has been made over the past 12 months towards the introduction of Eurocode 7 as a full European standard in 2001. The official publication date for the EN is sometime in 2000, but, owing to delays already encountered, the actual publication date is more likely to be mid-2001.

The 12th meeting of TC250/SC7 took place in Stockholm in June last year. At that meeting, the results of two key votes of all CEN members were announced:

To proceed with the conversion of Eurocode 7 Part 1, General rules, from an experimental pre-standard (ENV) into a full Euronorm (EN).

To publish Parts 2 and 3 of Eurocode 7, Design assisted by laboratory testing and Design assisted by field testing, as experimental pre-standards.

Like the pre-standard Part 1, Parts 2 and 3 will be issued for experimental use in member countries for three years, after which a vote of all CEN members will decide whether to convert them into full European standards.

A working group (SC7/WG1) was established at the 11th meeting of SC7 in Vienna in January 1997 to prepare the conversion of Part 1 into a Euronorm.

The key issue at the working group's five meetings so far has been the extent to which the partial safety factor system employed in the pre-standard needs revision. Strong arguments have been made for the replacement of Cases A, B, and C by a single design case, with partial factors applied either to the actions and/or material properties themselves, or to the effects of those actions (eg resulting earth pressures). Equally strong arguments have been made for the retention of Cases A, B, and C in their current form.

A breakthrough in the group's discussions was made at the meeting in Brussels in January this year, where it was agreed that the introduction of model factors (eg for a particular calculation method) and a new Case D (dealing with uncertainties in action effects and resistances, rather than in actions and material properties) might bridge the gap between the different national perspectives.

Working Group 1 has also been looking into adding a new section to Part 1 covering overall site stability. In the pre-standard, this subject is dealt with in the sections on spread foundations, pile foundations, retaining structures, and embankments and slopes. The embankments and slopes section may be split into one dealing with natural slopes and another dealing with man-made (but not reinforced) slopes.

The 13th meeting of SC7 took place in Athens in May this year. The main decisions taken were:

Parts 2 and 3 of Eurocode 7 will be combined into a single Part 2 dealing with everything involved in preparing a ground investigation report. Part 1 will specify what information is required from this report in order to achieve a design.

The introduction of model factors and a new Case D was approved, with Case D being subdivided to accommodate differing national requirements.

Partial factors on actions originating in the ground (eg from the self- weight of soil) may be lower than on actions transmitted through the ground (eg from the self-weight of structures).

Delegates from four countries (Professor Ulrich Smoltczyk from Germany, Christophe Bauduin from Belgium, Giovanni Bosco from Italy, and Richard Driscoll from the UK) were appointed to the project team that will make the editorial changes to ENV1997-1 to convert it into a full Euronorm.

The distinction between actions originating in the ground and those transmitted through it provides an opportunity to make Eurocode 7's safety system more economical. In the existing Case B, load factors

of 1.35 on permanent and 1.5 on variable actions are combined with material factors equal to 1.0; whereas in Case C, a material factor of 1.25 on the soil's coefficient of shearing resistence (tan ) is

combined with load factors of 1.0 on permanent and 1.3 on variable actions.

Thus, ENV1997-1 deals with uncertainties in the actions and material properties independently of each other. A combined case, with load factors of 1.35/1.50 and a material factor of 1.25 applied simultaneously, produces designs that are too conservative. However, this conclusion will need re-examining if load factors on actions arising from the self-weight of the ground are reduced below 1.35. Further work on this is needed.

The fifth meeting of SC7 Working Group 1 also took place in Athens, immediately after the SC7 meeting. The working group decided to prepare proposals for the next SC7 meeting that would involve:

Adding a new section on anchorages between the existing sections 8 (retaining structures) and 9 (embankments and slopes).

Moving some of the informative annexes given in ENV1997-2 and -3 into EN1997-1 (thus bringing all the general rules governing geotechnical design into one document).

Throughout the conversion period, members of SC7 have held discussions with the project team developing Eurocode 0, Basis of design (see Figure 1), to ensure that it caters for the specific requirements of geotechnical engineers.

Europhobe or Europhile?

Eurocode 7 presents a logical framework for the design of geotechnical structures and offers the prospect of a universal design approach based on sound engineering principles. Opponents of the code seem unwilling to abandon traditional methods and their (often vocal) criticisms tend to obscure its many good features, which include:

A clear distinction between principles and rules.

Explicit consideration of limit states.

Numerous checklists.

Encouragement of systematic thought in design.

The development of Eurocode 7 gives geotechnical engineers the opportunity to talk with structural engineers in a common technical language and to apply their knowledge and experience in many different countries, across Europe.

Acknowledgments

The author would like to thank the following for their comments on various drafts of this article: Richard Driscoll, Brian Simpson, Roger Frank, and Robert Mair.

Notes

1. This history is taken from the Commentary on Eurocode 7 (1998) written by Dr Brian Simpson (Arup Geotechnics) and Richard Driscoll (Building Research Establishment) for the UK's Department

of the Environment, Transport and the Regions.

(To be published shortly by CRC, available to Ground Engineering readers at a special discount rate. See advertisement right).

2. The English Language version of Parts 2 and 3 of Eurocode 7 should be available from national standards bodies from June 1998. French and German translations will follow.

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