END BEARING CAPACITY OF DRILLED SHAFTS IN ROCK. DISCUSSION AND CLOSURE

A discussion of a paper with the aforementioned title by L. Zhang and H.H. Einstein, published in this journal (Volume 124, Number 7, July 1998), is presented. The discussers present key points in four areas that they believe to be basic flaws with the authors' exploration of the end bearing capacity problem from a nontraditional rock engineering perspective: bearing capacity equation, load test database, bearing capacity model and rock mass rating (RMR), and side resistance equations. The discussers conclude the following: the well-known bearing capacity factor (N sub c) should not be redefined, available bearing capacity theory is adequate for continuum failure modes, the end bearing capacity factor (N* sub c) and N sub c are not functions of the rock material unconfined compressive strength (q sub u), empirical models should be based on the more up-to-date geological strength index (GSI) and modified RMR, and the side resistance needs to be examined carefully because the values given by the authors appear to be too high. Discussion is followed by closure from the authors.