SR 520 Replacement Bridge Pontoon Construction Facility

(Grays Harbor and Aberdeen, WA)  Landau Associates completed geotechnical and hydrogeologic design studies for WSDOT at sites in Grays Harbor and Aberdeen to characterize groundwater conditions and soil properties at potential locations for WSDOT’s SR 520 floating bridge pontoon construction facility.  The facility was proposed to include a concrete casting basin approximately 1200 ft long, 420 ft wide, and extending 25 ft below the groundwater table. The focus of the geotechnical study was foundation support alternatives, construction dewatering, and the seismic impacts induced by earthquake loading (particularly liquefaction and lateral spreading) on the foundations and walls of the proposed structure and nearby riverbanks.

A foundation of 150 ft long driven steel piles supports the settlement-sensitive structure. Landau Associates considered a number of approaches for dewatering, and conducted groundwater modeling to predict the offsite impacts from dewatering programs.

Code-based and empirical seismic analyses suggested that it was not practically feasible to construct the proposed structure as planned, given the very poor soils and the site’s proximity to the seismically active Cascadia Subduction Zone. To better assess seismic risks, Landau Associates conducted seismic analysis based on advanced numerical modeling, which indicated much less deformation and less loads on the proposed structure than was indicated by code-based and empirical analysis methods – demonstrating that it was feasible to construct the proposed facility at the Aberdeen site.   Preliminary assessment by WSDOT concluded that numerical modeling analyses were responsible for more than $5 million in construction cost savings. 

Landau Associates also supported WSDOT in preparation of design-build specifications and provided technical consultation and review of design submittals.

Photo 1: WSDOT