Structural Design of Deep Water Pontoon Mooring Anchors

After nearly half a century of service, the world's longest floating bridge is now clogged by traffic, showing its age, and in need of replacement. The Washington State Department of Transportation (WSDOT) is building a new State Route (SR) 520 floating bridge capable of withstanding heavier windstorms and moving significantly more people and goods. Much like a large floating marine dock, the new bridge will consist of concrete pontoons moored to 58 deep water anchors by 8 miles of steel cables. The bathymetry and subsurface conditions of Lake Washington pose unique challenges for the moorage of floating pontoons. The water depths at the anchor locations vary from 30 feet to 210 feet, and subsurface conditions range from very dense to very soft and include soils with unique engineering properties. These conditions necessitated the use of three anchor types: drilled shaft, gravity, and fluke.