Service Limit State Design for Individual Drilled Shafts in Shales

Abstract

A new service limit state (SLS) design procedure for drilled shafts is described, which utilizes strength resistance factors calibrated to achieve a target probability of failure. The new procedure can be used for varying values of the target probability of failure, and allows the shaft dimensions, soil strength, variability of soil strength, loads, and allowable settlement to be adjusted freely in the design. The procedure eliminates the need to establish resistance factors for the SLS on a case-by-case basis, which is a current practical issue for load and resistance factor design (LRFD). Sensitivity analyses presented indicate that resistance factors for a broad set of SLS design conditions can be reasonably established considering four parameters: the target probability of failure, normalized load, normalized shaft dimension, and the variability of soil/rock strength. Calibrated resistance factors are presented as a function of these four parameters in the form of resistance factor charts, as well as in the form of analytical equations developed through regression of the rigorously calibrated resistance factors. The proposed resistance factors range from 0.1 to 0.4, primarily because of the significant variability and uncertainty associated with side resistance for drilled shafts in shale.