Growing populations and increased commuter traffic led the Washington State Department of Transportation (WSDOT) to seek comprehensive safety improvements, including nine miles of highway widening, climbing lanes, seismic upgrades, and replacement of the General Mark W. Clark Memorial Bridge with the new Camano Gateway Bridge.

As a key design-build team member, Shannon & Wilson provided environmental compliance management and geotechnical engineering for work along the entire project corridor.  Shannon & Wilson's compliance management role included design collaboration, wetland delineations, water quality monitoring and reporting, fish exclusion and mitigation monitoring, and permitting compliance.  Our geotechnical work involved performing subsurface exploration and providing design recommendations and construction observation for geotechnical-related project elements.

The new bridge spans an environmentally-sensitive tributary of the Stillaguamish River, all within a floodplain with deep layers of loose, liquefiable soils.  We minimized environmental impact by constructing a 190-foot main span bridge with no in-river piers-improving river hydraulics, reducing environmental impacts, and limiting in-water work.  We isolated the bridge's lateral force resisting system from the mechanically stabilized earth (MSE) abutments by founding the bridge on deep drilled shafts that extend below the liquefiable soil.  By using reinforced soil slopes (RSSs), we eliminated expensive retaining walls and ground improvement, reduced impacts on wetlands, and improved aesthetics.