Combined Side and Base Resistance in Rock-Socketed Drilled Shafts-PPT Presentation

PPT Presentation

Pdf Summary

The presentation argues that rock-socketed drilled shafts should generally be designed to use both side and base resistance, unless a documented, project-specific reason justifies excluding one. It is a consensus overview, not a design manual, load-test interpretation guide, or substitute for sound investigation and construction quality control.<br /><br />Some practitioners omit side or base resistance because of concerns about differing mobilization displacements, sidewall strain softening or degradation, uncertain socket roughness, difficulty achieving a clean base, and limited confidence in design or verification. However, bi-directional load tests across varied rock types and shaft diameters indicate that base resistance can begin mobilizing at very small displacement and that side and base resistance can mobilize at compatible displacements. In typical cases, measured displacements are within the tolerance of civil structures, even when strength limits are not reached.<br /><br />The presentation identifies conditions requiring caution, including highly variable rock, karst or soluble bedrock, and some clay-rich rocks. These should be identified and evaluated during site investigation.<br /><br />A bridge example illustrates the potential benefits. At the Stan Musial Veterans Memorial Bridge, a design using both resistances and informed by load testing reduced the number of shafts, socket lengths and volumes, and pile-cap and cofferdam dimensions compared with a side-resistance-only baseline. The presenters report approximately $10 million in savings and reduced construction risk.<br /><br />Codes vary in how explicitly they address combined resistance, and many agency guidelines are silent. The recommended practice is to account for both side and base resistance, supported by proper site investigation, design, construction, and verification.

Keywords

rock-socketed drilled shafts

side resistance

base resistance

load testing

mobilization displacement

rock socket

site investigation

construction quality control

bridge foundation design

karst bedrock