Probabilistic Model and LRFD Resistance Factors for the Tip Resistance of Drilled Shafts in Soft Sedimentary Rock Based on Axial Load Tests

This paper proposes an empirical relationship for the tip bearing capacity of drilled shafts. The proposed relationship is based on the fracture initiation pressure of the rock mass. A database of 190 drilled shaft and plate load tests in soft rock is compiled. The method of Terzaghi is used to estimate the fracture initiation pressure from the tip stress-displacement relationships. It is shown that the back-calculated fracture initiation pressure is in reasonable agreement with the original and modified Griffith fracture theories. The fracture initiation pressure is then related to the unconfined compressive strength of intact rock, rock mass modulus of deformation, the geological strength index, tip displacement, and drilled shaft tip diameter. The resulting probabilistic empirical relationship is calibrated using the method of maximum likelihood. First-order reliability method is used to calibrate the corresponding resistance factors for use in a load and resistance factor design framework.