Performance of reliability-based design formats in geotechnical applications

Geotechnical design codes and guidelines are all switching from traditional factor of safety design to modern load and resistance factor design (LRFD) or partial factor design (PFD), in the belief that the latter two bring more flexibility and reliability consistency across various design scenarios, thus produce safe and cost-effective design outcomes. This paper first reviews the LRFD and PFD developed for geotechnical applications. A total of seven methods to calibrate the load and resistance factors are also introduced. The ability of the LRFD and PFD to produce designs with consistent reliability is examined and compared to that of a traditional factor of safety method using two examples of the bearing capacity of strip footings and the global stability of soil nail walls. Results showed that the framework of LRFD offers no apparent advantages over working stress design (WSD) in achieving more consistent reliability for geotechnical structures; the dispersion in design probabilities of failure could be five to seven orders of magnitude difference. The variation will be reduced to three orders if using the PFD. Neither reducing the variability in soil shear strength parameters nor allocating partial resistance factors with respect to soil types would efficiently harmonize the reliability levels when dealing with multiple soil layer conditions. In addition, the uniformity of reliability levels is insensitive to calibrations with or without presetting the load factors. This study provides insights into the LRFD and PFD frameworks currently developed for geotechnical applications.