Design of Socketed Drilled Shafts in Limestone

Increased usage of drilled shafts to carry high‐foundation loads has resulted in the common practice to socket the shafts into bedrock to transmit these loads. However, end‐bearing uncertainties have lead various designers to rely solely upon skin friction within the sockets to transmit the loads. This paper reviews several design techniques/correlations to estimate rock/shaft friction values from lab tests. These methods rely upon: (1) Unconfined compression (q1) and (2) split tensile (q1)+qu tests on recovered rock cores to estimate the rock shear transfer values. A database of 14 field‐load tests compares the measured versus estimated rock shear values for the various methods, and suggests that a two‐parameter model (qu+qt tests) provides the best estimate or socket friction. A small‐scale field “pull‐out” test on grout anchors is suggested as an alternative method for estimating rock/shaft friction values.