Bi-Directional Load Test (BDLT) for the New Headworks Facility Project in San Francisco, California-Paper

Paper

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The New Headworks Facility in San Francisco was built on challenging ground: liquefiable artificial fill, thick compressible Bay Mud, shallow groundwater, and high seismicity. To support structures carrying substantial vertical and lateral loads, the project selected large-diameter drilled shafts, with axial resistance initially based on skin friction rather than end bearing.<br /><br />A bi-directional load test (BDLT) program was conducted to verify shaft capacity and refine the design. Two 36-inch-diameter, approximately 125-foot-long test shafts were instrumented and tested. Based on subsurface data and nearby load-test results, the load cells were positioned 82 feet deep to balance the resistance mobilized above and below them. Both shaft segments mobilized effectively, particularly in the second test, providing useful data on load transfer and skin friction.<br /><br />The tests showed that revised axial capacities were approximately two to three times the original estimates. Using the measured capacities, the design team reduced shaft lengths by 20 to 40 feet, averaging about 30 feet across roughly 700 production shafts. This cut total installed shaft length by approximately 21,000 feet and generated substantial project savings.<br /><br />The paper concludes that BDLT can be an effective cost-optimization tool for large infrastructure projects when supported by thorough subsurface investigation, careful load-cell placement, coordination with testing contractors, and sound interpretation of field data.

Keywords

San Francisco

Headworks Facility

bi-directional load test

BDLT

drilled shafts

skin friction

axial capacity

seismicity

Bay Mud

foundation design