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

PPT Presentation

Pdf Summary

The presentation describes a bi-directional axial load test on a 3-m-diameter, 80-ft-long drilled shaft for the 22nd Street Bridge replacement in Tucson, Arizona. The test was intended to verify whether shorter shafts could replace the owner’s preliminary 10-ft-diameter shafts, designed to extend 90–95 ft. Using a rational β-method, the project team estimated that 60–65-ft shafts could provide adequate resistance, potentially reducing total drilled-shaft length by nearly 500 ft.<br /><br />The test shaft was equipped with four 3,000-kip jacks. It reached a maximum load of 8,994 kips, with 0.125 in. of upward displacement and 5.0 in. downward displacement; testing ended when the jacks reached their maximum stroke. Results showed an equivalent top load of 9,300 kips at 0.2 in. displacement and demonstrated 75-ksf base resistance for a non-horizontal shaft bottom—well above factored axial demands.<br /><br />The test supported the reliability of the current β-method in high-blowcount granular soils and highlighted the substantial resistance provided by Tucson caliche. However, the caliche also created significant construction challenges. The analysis further showed that standard concrete-modulus assumptions could misrepresent load distribution; a 40% reduction to the ACI 363 modulus was used to interpret the test. Early involvement of a specialist load-testing engineer was identified as a key factor in the test’s success.

Keywords

bi-directional axial load test

drilled shaft

Tucson Arizona

bridge replacement

beta method

caliche

load testing

axial resistance

shaft design

construction challenges