Base resistance of drilled shafts in soft rock using in situ load tests: A limit state approach

This paper presents a probabilistic limit state framework for the evaluation of the base resistance of drilled shafts in soft rock. In situ load tests and the Griffith fracture theory are used to evaluate the mode of failure for the rock mass under the drilled shaft base. The base resistance or the limit state in the context of this paper is defined by the contact pressure at which load induced vertical to subvertical cracks form and the contact pressure-displacement relationship passes into a steep and fairly straight tangent. The parameters affecting the base resistance are evaluated using an in situ load test database. A statistical approach using the maximum likelihood method is utilized for the development of the design model for the base resistance. Reliability analysis is used to calibrate the corresponding resistance factors. The in situ load tests and the Griffith fracture theory suggest that the soft rock mass underlying the drilled shaft base primarily fails by the formation of vertical to subvertical cracks. Field observations indicate that the displacements in the underlying rock mass are largely in the vertical direction and that the base displacements required to mobilize the proposed base resistance are generally less than 30 mm. Load test data show that the base resistance is chiefly related to the unconfined compressive strength of soft rock. The calculated resistance factors are found to slightly decrease with increase in the span length of the structure and increase with the increase in the foundation redundancy.