A robust approach for selecting LRFD characteristic values of uncertain soil parameters for design of drilled shaft in sand

Load and resistance factor design (LRFD) is becoming a design method of choice in practice. However, even with LRFD, the need to reduce the uncertainty in the predicted performance of the geotechnical system is still apparent. The partial factors for a given geotechnical design using a given solution model are often specified in codes. Thus, the main prerequisite of the design using LRFD code is to select appropriate characteristic values of uncertain input parameters. However, the choice of these characteristic values often involves significant subjectivity, and thus the resulting geotechnical design by different designers may be quite different. This paper examines the five methods often suggested for selection of characteristic values for design with LRFD. In addition to cost and reliability, the focus of the evaluation is on the design robustness (i.e., how to make a design with LRFD most robust). For illustration purposes, design of a drilled shaft in sand is adopted and the best method among the five methods examined for selecting the characteristic values of uncertain input parameters is determined.