Design and Construction Benefits of Performing a Bi-Directional Test on a Drilled Shaft in Sand, Gravel, and Cobble Deposits-Paper

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The Russell Street Bridge replacement in Missoula, Montana, used 24 drilled shafts in gravel, sand, cobbles, and occasional boulders. Before full construction, engineers performed a bi-directional static load test on a 5.9-foot-diameter, 75.5-foot-long production shaft to verify design assumptions and measure soil resistance. The test reached 10,707 kips.<br /><br />The test enabled the design team to use an LRFD resistance factor of 0.70, compared with 0.55 for shaft resistance and 0.50 for base resistance without load testing. For the eight river-pier shafts, this reduced required embedment by 16 feet per shaft, saving 128 feet of total construction length. The resulting construction-cost savings exceeded the testing cost by approximately $25,260 and shortened the schedule by an estimated four days. Lateral resistance requirements limited additional savings, and governed the lengths of the smaller abutment shafts.<br /><br />The test also helped avoid remediation. Crosshole sonic logging identified possible anomalies near the bases of three production shafts. Test results showed that shaft resistance alone, without relying on base resistance, was sufficient to carry the design loads, so coring or grouting was not required.<br /><br />Finally, measured shaft and base unit resistances were about twice the values predicted by the design software. These results contribute useful data for future designs in similar gravel-and-cobble deposits, while requiring careful consideration of differing site and construction conditions. Overall, the case demonstrates that load testing can reduce foundation costs and schedule, resolve construction concerns, and improve the evidence available for future design assumptions.

Keywords

Russell Street Bridge

Missoula Montana

drilled shafts

bi-directional static load test

foundation design

LRFD resistance factor

shaft resistance

base resistance

construction cost savings

gravel and cobble deposits