The Fishing Wars Memorial Bridge in Tacoma, Washington won a 2020 Project of the Year Award from the American Public Works Association (APWA) in the $25-$75 million structures category. GeoEngineers Inc. provided geotechnical services for a partial replacement of this heavily trafficked bridge over the Puyallup River.
The bridge had been carrying traffic over the Puyallup River from Tacoma toward the suburb of Fife for nearly a century, and it needed to be replaced. In 2017 GeoEngineers began geotechnical services for the first phase of bridge replacement as a member of a design-build team led by Atkinson Construction and Jacobs Engineering. Together, the team successfully replaced the western-most portion of the bridge, extending from Portland Avenue to the Puyallup River.
As the geotechnical subconsultant on the project, GeoEngineers investigated soil conditions and gave recommendations for embankment construction, the drilled shaft deep foundation design and seismic slope stability. The geotechnical team, led by Dave Phelps, PE and Morgan McArthur, PE, investigated the site and found soft clays and liquefiable alluvial deposits. For drilled shaft foundations, this means very high seismic downdrag loads. To overcome the high seismic loads, GeoEngineers and the design-build team used large (8-9-foot diameter) deep drilled shafts that extended beyond the soft clay and liquefiable alluvial deposits and into deeper, very compact glacially consolidated soils.
The conceptual design for the project included a zone of ground improvement at the toe of the existing Portland Avenue embankment, which was intended to improve seismic stability of the slope. To reduce the cost and timeline of the project, the design-build team proposed removing the zone of ground improvement, and instead constructing a flexible structural earth wall (SEW) abutment at the toe of the slope. This design allows the SEW to move laterally during a seismic event, but not enough to impact the bridge or cause catastrophic failure.
To manage this risk, King Chin, PE and GeoEngineers’ Performance Based Design Group also performed seismic slope deformation analysis to model the extent of lateral deformation and the resulting lateral loads on the shaft foundations. Through collaboration with the design team, GeoEngineers was able to recommend an appropriate spacing between the SEW and the first bridge to allow the SEW structural earth wall to deform laterally under seismic stress.
The project began construction in 2018 and was completed in Summer 2019. The remaining portion of the bridge, extending from the west side of the Puyallup River into Fife, still needs to be replaced, and the City of Tacoma hopes to finish it once funding is secured.
