A factored strength approach for the limit states design of geotechnical structures

This paper discusses the implications of an approach to the limit states design of geotechnical structures in which the soil strength parameters are factored, as is the one contained in the European construction code. It is shown that a factored strength approach has very different implications for the two customary types of geotechnical analyses; namely, the study of plastic mechanisms and interaction analyses. The relationship between partial safety factors and the reliability of a system is highlighted first, with the aid of approximate reliability calculations relative to two example problems. It is then demonstrated that the method of factoring the soil strength may be regarded as an effective way to assess the distance of a structure from the activation of a plastic mechanism, and to relate this distance to a conventional measure of its reliability. Using the same examples, it is argued that the procedure of factoring the soil strength parameters is not appropriate for the evaluation of the internal forces in structural elements: most of the soil-structure interaction analyses are nonlinear and, at least partly, empirically based. The insertion of unrealistic parameters, such as the factored strength properties, in a complex calculation may produce ambiguous and uncontrollable results.