Optimizing Rock-Socketed Drilled Shaft Design Through Enhanced Subsurface Exploration and Load Testing-PPT Presentation

PPT Presentation

Pdf Summary

The presentation explains how enhanced subsurface exploration and load testing can improve the design and construction of rock-socketed drilled shafts. These shafts transfer load through friction along the rock socket and end bearing at its base; soil overburden friction is generally not counted. They are especially suitable for heavily loaded structures where competent bedrock is relatively shallow.<br /><br />Conventional investigations that stop at the top of rock provide limited information, often forcing engineers to use conservative presumptive capacities and specify larger shafts. A more comprehensive approach includes longer, more frequent rock cores, laboratory strength testing, and geophysical borehole logging. Optical and acoustic televiewers, gamma-ray logging, and caliper surveys help characterize fractures and other bedrock features alongside the core data. This improves confidence in rock quality and supports more specific socket and shaft recommendations.<br /><br />Bi-directional static load testing can validate those recommendations. In the case presented, testing demonstrated capacity at twice the required design load. The enhanced investigation and testing cost approximately $130,000, while the allowable end-bearing pressure increased from 80 to 180 ksf.<br /><br />A 3,000-kip example illustrates the potential savings: a non-socketed shaft was estimated at about $8,000, compared with about $5,200 for a smaller-diameter rock-socketed shaft—a saving of roughly $2,800 per shaft. At that rate, savings on 46 shafts would offset the investigation and testing cost; the project included about 143 shafts.<br /><br />The presentation recommends obtaining rock cores at least every 15,000 square feet, with cores extending at least 30 feet, performing geophysical logging in those holes, and allowing additional coring where needed. Load testing should be considered based on project needs and the availability of confirmed local performance data.

Keywords

rock-socketed drilled shafts

subsurface exploration

load testing

bi-directional static load test

bedrock characterization

rock cores

geophysical borehole logging

end-bearing pressure

shaft design

foundation engineering