Abstract:
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-mile-long and supported on 14 piers bearing on a combination of spread footings and deep foundations due to the varying depth of basalt bedrock. Project specific design criteria required the structure to remain operational following a deterministic, lower level Cascadia Subduction Zone event and to meet Life Safety/No Collapse standards for higher level, probabilistic 1000-year return period seismic ground motions. Seismic analyses for the retrofit and widening included 2D non-linear time history analysis of the bridge alignment to evaluate liquefaction, pore pressure development, and the deformation response of the bridge profile. The analyses indicated seismic-induced lateral deformations extend to depths greater than 100 feet at some pier locations and result in significant foundation demands. A combination of large diameter drilled shafts and ground improvement was selected to support the structure and provide resistance to seismic-induced lateral deformations. Full replacement of the substructure was required at the majority of the piers and the design included 6- to 12-foot-diameter drilled shafts extending to over 240 feet depth. The design also included jet grout and deep soil mixing ground improvement to depths up to 150 feet below ground surface. This case history presents an overview of the seismic design challenges and foundation solutions, highlighting unique aspects of the project design and construction.