Combined Side and Base Resistance in Rock-Socketed Drilled Shafts

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The paper reviews evidence on axial resistance in drilled shafts socketed into rock, focusing on whether side resistance and base resistance can be counted together. Drawing on full-scale bi-directional load tests, the authors conclude that both mechanisms usually mobilize at small, compatible displacements—typically within about 13 mm (½ inch), and generally within 25 mm (1 inch). Tests across varied rock types and shaft sizes show substantial base resistance alongside side resistance; significant post-peak loss of side resistance is uncommon.<br /><br />The authors argue that concerns about displacement incompatibility, strain-softening, uncertain sidewall roughness, or the difficulty of cleaning deep socket bases do not justify routinely ignoring either resistance component. With thorough site investigation, suitable design and construction practices, effective base cleaning, and inspection and verification, both resistances can generally be relied upon. Poor cleaning can produce a soft base and require excessive displacement to mobilize resistance.<br /><br />Caution is warranted in project-specific conditions such as karst, highly variable formations, slaking-prone clay-rich rocks, or construction methods that may impair side resistance. These risks should be assessed and addressed through investigation, design choices, and construction controls rather than generalized assumptions.<br /><br />The paper’s recommended practice is to include both side and base resistance in design unless there is a valid, documented, project-specific reason to disregard one. Routine exclusion can unnecessarily lengthen sockets, increasing cost and construction risk.

Keywords

drilled shafts

rock socket

axial resistance

side resistance

base resistance

bi-directional load tests

load mobilization

socket design

base cleaning

construction verification