Design requirements

3.2.1 Commitment to limit state design

Perhaps the most significant requirement of Eurocode 7 is the following commitment to limit state design:

For each geotechnical design situation it shall be verified that no relevant limit state ... is exceeded. [en 1997-1 §2.1(1)P]

For many geotechnical engineers across Europe, this represents a major change in design philosophy, away from the traditional allowable (a.k.a. permissible) stress design involving a single, lumped factor of safety.

Traditional geotechnical design, using lumped factors of safety, has proved satisfactory over many decades and much experience has been built on such methods. However, the use of a single factor to account for all uncertainties in the analysis - although convenient - does not provide a proper control of different levels of uncertainty in various parts of the calculation. A limit state approach forces designers to think more rigorously about possible modes of failure and those parts of the calculation process where there is most uncertainty. This should lead to more rational levels of reliability for the whole structure. The partial factors in Eurocode 7 have been chosen to give similar designs to those obtained using lumped factors - thereby ensuring that the wealth of previous experience is not lost by the introduction of a radically different design methodology.

Limit state philosophy has been used for many years in the design of structures made of steel, concrete, and timber. Where these structures met the ground was, in the past, a source of analytical difficulties. The Eurocodes present a unified approach to all structural materials and should lead to less confusion and fewer errors when considering soil-structure interaction.

Limit states should be verified by calculation, prescriptive measures, experimental models and load tests, an observational method, or a combination of these approaches. These are discussed later in this chapter. Not every limit state needs to be checked explicitly: when one clearly governs, the others may be verified by a control check.

3.2.2 Complexity of design

A welcome requirement of Eurocode 7 is the mandatory assessment of risk for all design situations:

... the complexity of each geotechnical design shall be identified together with the associated risks ... a distinction shall be made between light and simple structures and small earthworks ... with negligible risk [and] other geotechnical structures. [en 1997-1 §2.1(8)P]

The idea here is that when negligible risk is involved, the design may be based on past experience and qualitative geotechnical investigations. In all other cases, quantitative investigations are required.

There are many schemes for assessing risk that may be used in conjunction with a Eurocode 7 design. For example, the approach outlined in the UK Highways Agency's document HD22/023 requires designers to identify possible hazards for a project or operation within that project. The cause of each hazard is detailed and its probability and impact both assessed on scale of 1 to 4. These two numbers are multiplied together to provide a 'risk rating', which helps the designer decide whether measures are required to mitigate the risk. The exercise is repeated when the mitigating measures are in place to show that the risk has been reduced below acceptable levels. Should the risk still be too high, further measures would be needed or the project may need to be abandoned or redesigned. Such risk assessments clearly satisfy the requirements of Eurocode 7 to identify the complexity and risks of geotechnical design.

3.2.3 Geotechnical categories

To assist geotechnical engineers in classifying risk, Eurocode 7 introduces three Geotechnical Categories, their design requirements, and the design procedure they imply — as summarized below. The Geotechnical Categories are defined in a series of Application Rules, not Principles, and hence alternative methods of assessing geotechnical risk could be used.

Design requirements

Design procedure

Small and relatively simple structures ... with negligible risk

Negligible risk of instability or Routine design ground movements; ground and conditions are 'straightforward'; no construction excavation below water table (or (i.e. execution) such excavation is 'straightforward') methods

No examples given in EN 1997-1

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