Combined Side and Base Resistance in Rock-Socketed Drilled Shafts – A State of the Art Consensus

Abstract

Both side and base resistance contribute to drilled shaft rock socket axial resistance in most cases. While there are geological settings in which mobilization of side and/or base resistance may not be reliable, those situations are the exception, not the rule. The advent of bi-directional axial load testing of rock-socketed drilled shafts has resulted in a significant body of evidence regarding the independent behavior of side and base resistance mechanisms. Accordingly, over the last two decades, a wealth of data from these full-scale load tests of rock-socketed drilled shafts demonstrate this behavior when good design, construction and quality assurance/quality control (QA/QC) procedures are followed. The authors actively attempted to identify bi-directional test results to the contrary, including a survey of the entire Deep Foundations Institute (DFI) membership, but concluded there is a notable lack of evidence that side and base resistance do not mobilize simultaneously when proper design, construction and QA/QC procedures are followed in most geologic settings. Despite this fact, there are agencies and individual engineers that continue to disregard either side or base resistance for reasons that contradict available evidence, often resulting in excessive rock socket lengths that increase costs and the risk of problems during construction. The purpose of this document is to present a summary of the recent evidence indicating mobilization of combined side and base resistances at small, compatible displacements that are within the settlement tolerance of most structures in well-designed, constructed and inspected rock sockets. The recommended best practice for design of rock-socketed drilled shafts is to account for both side and base resistance unless there is a valid documented reason to disregard one or the other on a project-specific basis.