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

An overview is presented of a unique axial bi-directional load test performed on a 3.0-m diameter (118inch) drilled shaft in January 2023 in Tucson, Arizona as part of a bridge replacement project. The shaft was excavated using crane-mounted grab tooling, resulting in a slightly curved base and corresponding uncertainty about the nominal base resistance relative to a conventional flat excavation base. The shaft was approximately 80 ft long and the load assembly was installed about 8 ft above the base. The load assembly location was selected to balance the estimated axial resistance above and below the jacks, as is customary. However, the presence of a 13-ft thick layer of strongly-cemented caliche with unexpectedly high compressive strength (2.8 to 5.1 ksi) produced higher than anticipated side resistance above the load assembly, resulting in a very stiff reaction against which significant load was transferred to the bottom segment. The maximum test load of approximately 9,000 kips resulted in just 1/8-inch upward displacement of the top segment and 5.0 inches of downward displacement of the bottom segment when the jacks reached their maximum stroke. Mobilized unit side resistances for the top segment ranged from about 2 to 9 ksf. Mobilized base resistance was on the order of 75 ksf. Since a primary objective of the load test was to confirm unit base resistance exceeding the AASHTO limit of 60 ksf for refusal blowcount sand, the test was deemed very successful and allows significant optimization of drilled shafts for the project. The test results also confirm that a curved base resulting from grab tooling can generate significant base resistance.