End Bearing Capacity of Drilled Shafts in Rock

TLDR

Existing end‑bearing capacity relations assume a constant Nc, whereas this study shows Nc varies with rock strength. The study develops a new empirical relation between intact rock UCS and drilled‑shaft end bearing capacity and an analytical relation incorporating discontinuities via the Hoek‑Brown criterion. Using a database of 39 load tests, the authors derived the empirical relation and formulated an analytical model based on the Hoek‑Brown criterion to account for discontinuities. The results show Nc decreases with increasing UCS, the empirical and analytical relations agree, and the combined simplified relation accurately predicts end bearing capacity in instrumented test shafts.

Abstract

In this paper a new empirical relation between the unconfined compressive strength of intact rock and the end bearing capacity of drilled shafts in rock is developed. In addition, an analytical relation between rock mass strength and end bearing capacity is developed to explicitly consider the effect of discontinuities. Specifically, a database of 39 load tests is used in this paper to derive the new relation between end bearing capacity and unconfined compressive strength of intact rock. The derived relation indicates that the end bearing capacity factor, Nc, which is the ratio of the end bearing capacity, qmax, and the unconfined compressive strength, σc, of intact rock, decreases with increasing σc. This is in contrast to many existing relations assuming constant Nc values. Since this new empirical relation is derived from the results of load tests, the effect of discontinuities is implicitly considered. To explicitly study the effect of discontinuities, an analytical relation based on the Hoek-Brown strength criterion that considers the effect of discontinuities is developed. The new analytical and empirical relations are in good agreement and are thus combined in a simplified form for predicting the end bearing capacity of drilled shafts socketed into rock masses. A comparison with two examples from instrumented test shafts indicates that the recommended relation produces satisfactory predictions.