Reliability-based design applied to retaining walls

Geotechnical engineers have long used the lumped factor of safety approach in the design of foundations and retaining walls. A more recent alternative is the limit state approach using partial factors, for example as suggested in Eurocode 7. Yet another approach is perhaps more flexible and rational: design based on a target reliability index that reflects the uncertainty of the parameters and their correlation structure. Among the various versions of reliability indices, the Hasofer-Lind index and first-order reliability method (FORM) are more consistent. This paper illustrates practical reliability-based design procedures for retaining walls based on the Hasofer-Lind index and FORM. Correlated normal and non-normal random variables are considered. The efficient spreadsheet-based probabilistic approach, based on the work of Low & Tang, achieves the same result as the Hasofer-Lind method and FORM, but uses an intuitive expanding dispersion ellipsoid perspective that greatly simplifies the computations and interpretations. The differences between a reliability-based design and one based on partial factors will be discussed. Sensitivity information as conveyed in a reliability analysis will be studied. The probabilities of failure inferred from reliability indices will be compared with Monte Carlo simulations. This paper deals only with certain aspects of reliability, namely methodology and concepts, and not reliability in its widest sense.