An Environmentally Smart Bridge Replacement and HOV Program

In the Puget Sound region of Washington state, they are particularly sensitive to cultural and environmental resources. HDR is the general engineering consultant leading multiple design-build and design-bid-build projects within the $4.56 billion SR 520 Bridge Replacement and High-Occupancy Vehicle Program. The goal is to build and maintain a sustainable, integrated multimodal transportation system that supports healthy communities and economic vitality while protecting the environment.

The 7-mile corridor extends from Interstate 5 in Seattle to I-405 in Bellevue and includes six-lane bridges and an adjoining highway, featuring two general-purpose lanes and one HOV lane in each direction. The new floating span is approximately 7,700-feet long. All components have been designed to withstand a 1,000-year earthquake event and up to 89 mph winds.

To address specific environmental challenges within the corridor, GEC staff designed new stormwater treatment and detention facilities to capture and treat water before it is discharged into the surrounding watershed. Something the previous bridge did not do. The new floating bridge manages runoff with high-efficiency sweeping and stormwater discharge into lagoons within the pontoons on the floating structure itself. This reduces concentrations of dissolved metals that have adverse effects on migrating fish. In addition, the newly completed, highway sections remove fish-migration barriers, improve Yarrow Creek and provide larger fish-passable culverts. Wetlands are being restored or replaced throughout the corridor.

While replacing critical transportation infrastructure with sustainability in mind, the program also plans for historical resources and those with connections to Native American tribes in the region. It links centers of economic development, creates safer and more accessible transit connections, increases access to public transportation, provides a 14-foot-wide bicycle and pedestrian path that runs the full length of the corridor, expands park-and-ride parking capacity, offers more recreation options and reconnects neighborhoods.

The corridor’s eastside segment opened in 2014, and the floating bridge — the longest in the world — opened in 2016, and the west approach structure opened in 2017. The 2015 legislative session authorized the remainder of the funds needed to complete the project, or an additional $1.56 billion. With these new funds, the construction for the entire corridor is scheduled to be completed in 2027.