SR16 Burnham Borgen Interchange and Canterwood Boulevard NW
Location: Gig Harbor, Washington
This significant interchange improvement project included more than 3,000 feet of retaining walls, consisting mainly of soldier pile cantilever and MSE walls. A value engineering study was performed to find the most cost-effective wall types and details for several retaining walls up to 28 feet high. The original concept involved soldier pile tieback walls in a fill situation to mitigate poor soils. These walls were replaced with MSE walls with ground improvements, resulting in significant savings. DEA structural engineers designed two cantilever soldier pile retaining walls with precast concrete fascia panels that included architectural details.
The transportation improvements resulted in several environmental mitigation improvements, including the creation of 1.29 acres of added wetland area, stream restoration and relocation, installation of a new fish-passable box culvert in a location that had long been a barrier to migrating salmon, and the daylighting of another section of creek by completely removing another fish-blocking culvert. DEA managed the environmental documentation; obtained SEPA approval in May 2008; and obtained approvals from WSDOT, Department of Ecology, WDFW, and Department of Archaeology and Historic Preservation in order to go to construction during the 2008 fish window. The Nationwide Permit 14 (NWP 14) for this project issued by the USACE required the City of Gig Harbor to mitigate project-caused impacts by developing a wetland site within the same drainage basin. The city owned no properties in the vicinity of sufficient size to mitigate impacts to wetland adjacent to the project. In order to accommodate the accelerated permit schedule, DEA planners and biologists conducted a GIS alternatives analysis and site reconnaissance to find an appropriate mitigation site. DEA’s creativity led to the city negotiating with a local developer to acquire land near the project, keeping the project on schedule.