Prediction of end-bearing capacity of rocksocketed shafts considering rock quality designation (RQD)

Existing empirical methods for determining the end-bearing capacity, qmax, use empirical relations between qmax and the unconfined compressive strength of intact rock, sc. As rock-socketed shafts are supported by the rock mass, not just the intact rock, one should consider not only the intact rock properties, but also the influence of discontinuities on rock mass properties when determining q_max. Although semi-empirical and analytical methods have been developed that can consider the effect of discontinuities, they are more complicated than the empirical relations and require information about discontinuities that is often not available or difficult to obtain in engineering practice. In this paper, an empirical relation between q_max and the unconfined compressive strength of rock mass, σ_cm, is developed. The new empirical relation explicitly considers the effect of discontinuities represented by rock quality designation (RQD), which is the parameter normally obtained in engineering practice. The accuracy of the expression for estimating scm based on RQD is verified by comparing its estimation values with those from the existing empirical expressions based on rock mass classification. Two examples are presented to show the application of the newly developed empirical relation between q_max and σ_cm.