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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 bidirectional  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 rocksocketed
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.

Keywords: rock socket, side resistance, base resistance, axial loading, displacement  compatibility

Paul Axtell, P. E., D. GE, John Turner, Ph. D., P. E., P. G., D. GE, Benjamin Turner, Ph. D., P. E., G. E., Scott Jacobs, P. E., Kirk McIntosh, P. E., D. GE, Bernardo J. Vega, & P. E., Bernhardt Hertlein, M. ASCE, FACI, and Jesus Gomez, Ph. D., P. E., D. GE

Guidance Document