Bi-Directional Axial Load Test on 3-m Diameter Drilled Shaft in Tucson Caliche-Video

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Video Summary

A bi-directional load test on a 3-meter-diameter drilled shaft for Tucson’s 22nd Street Bridge replacement evaluated whether the planned shafts could be shortened. The test shaft was 80 feet long, with the loading assembly near its base. It resisted nearly 18,000 kips; the test reached the jacks’ maximum stroke without exceeding their safe operating pressure. The test mobilized about 75 ksf of base resistance—above the AASHTO limit for untested shafts—and substantial side resistance, supporting shorter designs and confirming the current beta method.<br /><br />Construction encountered 16 feet of exceptionally strong caliche, with compressive strengths comparable to concrete. Excavation took five days and severely damaged tooling, highlighting the need to plan production methods accordingly. The shaft’s dry, curved base still achieved high resistance.<br /><br />Interpreting the test required lowering the estimated concrete modulus: standard formulas produced physically impossible load distributions because specialized drilled-shaft concrete differs from conventional concrete. The presentation emphasized early collaboration with the testing contractor and careful judgment in test planning and interpretation.

Keywords

bi-directional load test

drilled shaft

Tucson 22nd Street Bridge

caliche excavation

beta method

base resistance