Fractile-based Method for Selecting Characteristic Values for Geotechnical Design with LRFD

Load and resistance factor design (LRFD), also known as partial factor design, is becoming a design method of choice in geotechnical practice. The partial factors (i.e., resistance factor and load factor) to ensure safety are specified in the adopted code; however, engineers still need to select values for the uncertain soil parameters in a design. The selection of these parameter values, often called characteristic values, is quite challenging given soil variability and limited test data. This paper presents a fractile-based method for the selection of the characteristic values for the uncertain soil parameters in a design with LRFD. A framework that considers safety, cost, and robustness is established to determine an appropriate fractile for selecting the characteristic values. This framework is demonstrated with two examples, a drilled shaft in sand and a shallow spread foundation. The results show that robust designs can be achieved using LRFD with the characteristic values obtained through the proposed fractile-based method.