Combining Side and Base Resistance in Soil Bearing Drilled Shafts-PPT Presentation

PPT Presentation

Pdf Summary

This presentation examines whether side and base resistance should be combined when designing drilled shafts bearing in soil. Although several design codes and guides permit combining the two, some practitioners exclude base resistance because of concerns about displacement compatibility, softening side resistance, construction quality—especially base cleanliness—and established practice.<br /><br />The research team reviewed more than 50 axial load tests, mostly bi-directional, selecting cases with well-characterized ground conditions. The results showed no evidence that side and base resistance cannot be mobilized simultaneously. While individual side-resistance behavior may soften with displacement, the overall top-down load–displacement response was consistently hardening. The authors note that bi-directional testing has limitations and that additional evaluation of top-down tests is needed.<br /><br />The presentation argues that ignoring base resistance can lead to unnecessarily long shafts, higher costs and risks, and misleading assumptions about stiffness. It also challenges the idea that omitting base resistance is a reliable way to avoid construction problems: inadequate base cleanliness may contribute to poor concrete quality throughout the shaft. Modern tools are available to clean and inspect shaft bases.<br /><br />The authors acknowledge that excluding base resistance may reduce cleanliness requirements or perceived remediation risks, and that serviceability concerns may influence design decisions. However, they conclude that combined side-and-base behavior best represents the actual response under top-down loading. Designers should generally use both resistance components, supported by appropriate investigation, construction controls, and testing where uncertainty exists. Neglecting either side or base resistance should be the exception rather than the rule.

Keywords

drilled shafts

side resistance

base resistance

soil bearing

axial load tests

bi-directional testing

top-down loading

displacement compatibility

base cleanliness

shaft design