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

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The paper presents a bi-directional axial load test on an 80-ft-long, 3-m-diameter drilled shaft installed for a bridge replacement project in Tucson, Arizona. The test aimed to verify that the shaft could achieve unit base resistance above the AASHTO limit of 60 ksf and to assess the construction method, including temporary casing and grab-tool excavation.<br /><br />The shaft was excavated using crane-mounted chisel-and-grab equipment. A 12–15-ft-thick layer of strongly cemented caliche, later confirmed by core testing to have compressive strengths averaging about 4,000 psi, made excavation difficult and provided much greater side resistance than anticipated. The grab tooling left a curved, non-flat base.<br /><br />During testing, the jacks applied approximately 8,994 kips in each direction before reaching their maximum stroke. The upper shaft moved only 0.125 in upward, while the lower segment moved about 5 in downward. The unexpectedly high resistance in the caliche above the jacks limited mobilization of upper-shaft side resistance. Measured unit side resistance ranged from about 1.8 to 9.1 ksf in the upper segments; the lower segment mobilized about 15.9 ksf. Maximum unit base resistance was approximately 74.6 ksf.<br /><br />The test therefore exceeded the 60-ksf design threshold and showed that grab-tool excavation can produce substantial base resistance despite a curved excavation bottom. Results also supported the use of conservative design estimates for side resistance. The authors note that interpreting strain-gauge measurements required engineering judgment, particularly in selecting a representative concrete modulus.

Keywords

drilled shaft

bi-directional load test

axial load

unit base resistance

AASHTO limit

temporary casing

grab-tool excavation

caliche

side resistance

bridge replacement