SR 520 Floating Bridge and Landings Replacement

Washington State Department of Transportation/KPFF Special Projects Division

Seattle, Washington

SR520 Floating Bridge

Photo: WSDOT

Hart Crowser was the Geotechnical engineer of record for the western fixed approach to and all anchors for the proposed 1.5-mile-long floating bridge across Lake Washington. The $2.9 billion project replaced the existing SR 520 floating bridge in its entirety and widened the bridge and improved future capacity concepts.

The new hollow concrete floating bridge pontoons are anchored by three different types of underwater anchors: drilled shafts, gravity anchors, and fluke anchors. The new anchors were specially designed for the very difficult soil conditions and withstand loads that exceed the existing bridge anchors’ capacities by nearly five times due to the significant increase in pontoon size and increases in environmental loading requirements.

The project also features large-diameter deep foundation elements for the western approach that have been installed to soil depths of 100 feet in 40 feet of water into soils with significant artesian water pressures. Because of the difficult site soil conditions, the majority of the bridge anchors were installed in very soft lake sediments that have extraordinarily low strength. Our suggestion of using rock ballast to enhance the capacity of the anchors cut the anchor size roughly in half, saving the Washington State DOT considerable money and reducing the difficulty of building and transporting such large anchors.