The existing half-bridge carrying Pacific Avenue over a BNSF railroad tunnel had deteriorated and was posted for weight restrictions. This project replaced the half-bridge for Cowlitz County, requiring design for a bridge and roadway realignment over the tunnel. The new bridge is a 115-foot long, 40-foot-wide, single-span structure that replaces the former 80-foot long, three-span half-bridge.
Parametrix provided structural engineering, permitting, and early coordination with BNSF to identify requirements and restrictions that would influence final approval from the railroad. To meet BNSF requirements, the new bridge design reduced the number of piers and located them outside of the BNSF right-of-way. The piers were founded on auger cast piles to limit vibrations to the existing tunnel during construction. The superstructure was designed with WSDOT W53DG precast, prestressed, concrete girders, which provide sufficient inspection clearance above the tunnel portal, and minimize profile changes to the existing roadway. Wing walls support the approach roadway and protect the piers from slides on the steep hillside.
The stormwater design needed to treat runoff in a small space, so the team created a unique solution that used a compost-amended filter strip to maximize use of the available space. The team also worked with staff from Cowlitz County and WSDOT Local Programs to ensure that NEPA and SEPA requirements were met.