Does shaft resistance of piles in rock scale with RQD?

Rock socketed piles are a common foundation solution to transfer heavy loads from structures to the underlying rock mass. Their design methods are traditionally based on conservative empirical rules, some of which incorporate the rock mass quality through a reduction factor, often related to RQD. This paper presents a comprehensive analysis of an international database consisting of 72 in-situ load tests, reviews methods for borehole roughness description and back analysis of borehole roughness height, and presents a validation study for the reviewed procedures for quantitative roughness description. Our analysis reveals that in contrast to intuition, shaft resistance of piles in rock masses with lower RQD values tends to be higher than in rock masses characterized by higher RQD values. We explain this surprising result by the effect of RQD on borehole wall roughness.