Photo courtesy of Parsons Parsons designed the main span of its bridge replacement over the Stillaguamish River to clear the river’s natural banks, reducing the bridge’s environmental impact.

State Route 532 is the only connection from Camano Island to the Washington mainland, carrying 30,000 vehicles a day. It is also the primary route to Interstate 5 for residents of Stanwood and surrounding communities in Snohomish County.

Population has grown dramatically in the area, and this access route had become overwhelmed and unsafe. So the Washington State Department of Transportation hired Parsons as the designer and lead partner in a multifaceted project to fix the problem.

The project’s cornerstone is a new 400-foot-long, three-span bridge that crosses the mouth of the Stillaguamish River and is designed to meet modern earthquake standards, require less maintenance and provide wide shoulders for disabled vehicles and emergency responders.

The 190-foot main span clears the river’s natural banks, reducing its environmental impacts. The bridge is designed to withstand a 1,000-year seismic event despite deep on-site alluvial fills that have a high potential for liquefaction during a significant earthquake.

Environmental concerns were critical from the beginning since the project area contained 64 different wetlands and 16 different jurisdictional ditches.

To protect water quality, Parsons designed the bridge’s wide center span so that piers could be driven primarily into dry soil rather than into the active river channel. The design also minimized the number of piles driven into the channel, protecting fish from the effects of impact pile hammer use.

Public understanding and support was also critical for this project since the improvements affected many businesses and homes, and created major traffic management problems during the construction process. Parsons combined one-on-one outreach to community members with a broad-based media outreach addressing key stakeholder concerns, and met personally with every business owner and homeowner directly impacted by construction.

The project was completed seven months ahead of schedule. The bridge opened to traffic in the summer of 2010.