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Elliott Bay Seattle Seawall

Photo: COWI

​​An important piece of Seattle infrastructure - designed by COWI Marine's team of experts.

COWI Marine's Seattle office recently completed the final structural design and geotechnical design review for the new Elliott Bay Seawall. Located in downtown Seattle, the new seawall will protect the waterfront from storm damage and shoreline erosion. In this City of Seattle project, COWI Marine is a design subconsultant to Parsons and the General Contractor-Construction Manager consists of a Joint Venture between Mortenson/Manson.

The seawall is an important piece of infrastructure for Seattle. It protects the city’s waterfront from wind-driven storm waves and the erosive tidal forces of Puget Sound and Elliott Bay. By protecting Seattle’s waterfront, the seawall also preserves vital commercial and industrial interests, such as major regional utilities, the Alaskan Way Viaduct, and the Washington State Ferries Colman Dock terminal.

Constructed in the early 1930s, the existing seawall presents several risks which the new seawall will eliminate. The current seawall consists of a reinforced concrete wall with a 5-foot cantilevered sidewalk supported by a timber relieving platform and steel sheet piling along the water’s edge. This structure is worn from age and the corrosive marine environment. Portions of the timber platform have sustained damage due to marine borers. Additionally, the original design does not meet modern earthquake design standards and is susceptible to soil liquefaction.

When completed, the new seawall will feature a 15-foot cantilevered sidewalk with a light penetrating surface in the form of glass blocks to improve the fish habitat along its 3,700-foot length. To mitigate the potential for soil liquefaction, the seawall will be supported on a jet-grout-improved cellular soil mass. The new seawall is planned to be constructed within a dewatered cofferdam, and the structure itself will consist of a cast-in-place support slab with custom precast concrete face panels, precast concrete zee-shaped superstructure segments, and precast sidewalk panels.

Precast elements are designed to be connected to the support slab by closure walls and a shear block. This solution will expedite construction and minimize the impact on adjacent piers, which are valuable sources of tourism for the city. Construction on the seawall will commence in the fall of 2013 and is planned to be complete in 2015.

LAST UPDATED: 03.02.2017