Reliability-based geotechnical design in an Australian context

The design philosophy adopted by the international geo-engineering community over the last few decades has largely evolved from working stress design (WSD), in which a single or lumped factor of safety (FoS) is adopted, to a load and resistance factor design (LRFD) approach. In LRFD, partial factors are applied to actions (i.e. loads), soil parameters and / or resistances. These partial factors vary in magnitude depending on the relative uncertainty of each parameter to which they are applied. The approach adopted in Australia has differed, with both WSD and LRFD approaches being widely used. In the authors’ experience, there is limited awareness in the Australian geo-engineering community of the relationship between the concept of reliability and the partial factors adopted in LRFD, and therefore of the potential benefits of undertaking reliability-based design (RBD). The outcome of this is that RBD, in which the uncertainty of the variables which may affect the design is individually assessed, is rarely undertaken. This paper discusses the concept of RBD and its place within the framework of Australian Standards and presents practical means of adopting RBD with accompanying examples from the literature. The intention of the paper is to encourage practitioners to consider uncertainty in geotechnical design more rigorously, whilst acknowledging the importance of maintaining engineering judgement in design.