Load Testing of Large Diameter Shafts for the I-205 Abernethy Bridge

The Oregon Department of Transportation is performing seismic retrofit and widening of the existing I-205 Abernethy Bridge over the Willamette River. The Abernethy Bridge is approximately 1/2-mi-long and supported on 14 piers bearing on a combination of spread footings and driven piles due to the varying depth of basalt bedrock. Seismic analyses for the retrofit and widening indicated seismic-induced lateral deformations extend to depths greater than 100 ft at some pier locations and result in significant foundation demands. Full replacement of the substructure was required at the majority of the piers and the design included 6- to 12-ft-diameter drilled shafts extending to over 240 ft depth. ODOT implemented a bi-directional load test program to evaluate constructability issues and confirm resistance of permanently cased, large diameter shafts bearing on Basalt. Shaft instrumentation included the unique application of strain sensing fiber optic cable to provide a continuous measure of strain, and corresponding side resistance, along the shaft length. This paper presents results of the load test program, including interpretations of mobilized side and base resistance in basalt. The paper also provides an overview of the fiber optic strain results and discuss constructability considerations that potentially affected results.