The $305 million, four-year Seawall Replacement project replaced Seattle’s 70-year-old seawall structure and provided seismic protection for improvements along the waterfront. The project involved demolition of the existing structure, relocation of numerous utilities and obstructions, and placement of thousands of new jet grout columns along nearly one mile of Seattle’s historic waterfront district.
The new seawall is an improved soil mass (ISM) gravity structure that combines a soil-cement improvement and seawall superstructure into a single structural system. Soil-cement, constructed using jet grout to stabilize liquefiable soils vulnerable to seismic events, was designed and built in a cellular pattern to stabilize the ground and serve as the new seawall’s foundation. The seawall was designed in precast units and cast-in-place slab. Together, the ISM and seawall elements behave as a single structural system. Completion of the project required demolition of the existing seawall, excavation and grading, placement of over 5,700 jet grout columns 3 to 6 feet in diameter to depths of 40 to 90 feet, removal of contaminated soils, placement of a 6-foot-thick support slab, erection of precast seawall panels, erection of precast Z panels to support a cantilever supported pedestrian promenade, extensive buried-utility replacement, installation of light penetrating surface walkway panels, and roadway improvements.
McMillen Jacobs provided expertise to the City of Seattle in the following areas: project advisement and construction consulting support to analyze schedule; analysis of various issues affecting project time and cost; geotechnical and structural analysis of temporary support structures; evaluation of construction monitoring data and analysis of quantities of work installed; and general engineering consulting to address the many challenges associated with replacement of an aging structure. McMillen Jacobs was involved in the project for nearly three years and collaborated with the City’s staff through project closeout.