Side Resistance of Drilled Shafts in Weak Fine-Grained Sedimentary Rock-Article

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The study evaluates how drilled shafts transfer axial load through side resistance when socketed in weak fine-grained sedimentary rocks, including shale, mudstone, and claystone. The authors compiled load-test results from more than 65 drilled shafts, providing 93 unit-side-resistance measurements. The rocks generally had unconfined compressive strengths (UCS) of 0.48–4.8 MPa.<br /><br />Comparisons with existing design methods showed that many predictive models, developed using data from both weak and strong rocks, do not reliably estimate resistance in weak rocks. In particular, several commonly used power-function models overpredicted resistance at lower UCS values and underpredicted it at higher values. A linear relationship provided a better fit.<br /><br />The proposed design method estimates unit side resistance as approximately 30% of the average UCS along the socket wall, with UCS limited to 1.4 MPa for this calculation. The authors recommend applying the relationship only within its supported range; site-specific testing is advised, and other methods should be considered at higher UCS values.<br /><br />The database indicates that shaft diameter has little influence on side resistance. Maximum resistance is generally mobilized at relatively small shaft movements—typically under 25 mm—and rough socket walls can increase resistance and help maintain it after peak load. Construction defects, such as water contamination during concrete placement, can substantially reduce resistance. Because post-peak resistance often declines little, the authors conclude that both side and tip resistance may be considered in design, subject to appropriate project-specific evaluation.

Keywords

drilled shafts

axial load

side resistance

socketed rock

weak sedimentary rocks

unconfined compressive strength

load test results

unit side resistance

design method

shale mudstone claystone