Combined Side and Base Resistance in Rock-Socketed Drilled Shafts

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The guideline concludes that both side resistance and base resistance should generally be included when designing rock-socketed drilled shafts. Full-scale bi-directional load tests across varied rock types and shaft sizes show that both components commonly develop at compatible, relatively small displacements—often 13 mm (½ inch) or less, and generally within typical structural settlement tolerances. Significant base resistance is therefore usually available without requiring excessive movement, while post-peak loss of side resistance is uncommon.<br /><br />The authors find little evidence that either resistance component should routinely be ignored when shafts are properly designed, constructed, and inspected. Omitting one without a project-specific justification can unnecessarily increase socket length, cost, and construction risk. It may also reduce attention to base cleaning and inspection, potentially harming construction quality.<br /><br />Reliable combined resistance depends on thorough subsurface investigation, appropriate shaft and concrete design, stable excavation, effective removal of cuttings and sediment, proper concrete placement, and inspection of the socket and base. Available tools can verify base cleanliness, including for wet construction.<br /><br />Caution may be warranted in particular conditions, such as karst with nearby cavities, highly variable rock formations, slaking claystone or shale, or construction practices that weaken the sidewall interface. These concerns should be assessed specifically rather than generalized to all rock sockets. Where conditions are uncertain, project-specific investigation, testing, conservative design values, or justified exclusion of a resistance component may be appropriate.<br /><br />The recommended best practice is to account for both side and base resistance unless a documented, project-specific reason supports disregarding either one.

Keywords

rock-socketed drilled shafts

side resistance

base resistance

bi-directional load tests

load transfer

socket design

rock formations

construction quality

base cleaning

project-specific investigation