ABSTRACT
Slope stability is an old and familiar problem in geotechnical engineering. Since the early beginnings, impressive advances have beenmade and awealth of experience has been collected by the profession. The overall performance of the general methodology has been unquestionably successful. Nevertheless, the design of slopes remains a challenge, particularly with new and emerging problems. As with all geotechnical projects, a design is based on inputs, primarily loads and site characteristics. The latter includes the subsoil profile, soil moisture conditions and soil properties. Since all of the above are estimated from incomplete data and cannot be determined precisely for the lifetime of the structure, an uncertainty is associated with each component. To account for this, the traditional approach is to apply a safety factor, derived largely from experience. This approach has worked well, but difficulty is encountered where experience is inadequate or nonexistent. Then reliability-based design (RBD) offers the option of evaluating the uncertainties and estimating their effects on safety. The origin of RBD can be traced to partial safety factors. Taylor (1948) explained the need to account for different uncertainties about the cohesional and frictional components of soil strength, and Lumb (1970) used the standard deviation to represent the uncertainty and expressed the partial safety factors in terms of the standard deviations. Partial safety factors were incorporated into design practice by Danish engineers in the 1960s (Hansen, 1965).
