Ground Improvement Design and Construction for Seattle's Elliott Bay Seawall Replacement and Retrofit-Paper

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Seattle’s Elliott Bay Seawall protects a mile of downtown waterfront and critical infrastructure. Much of the original wall, built between 1916 and 1936, rested on timber piles and retained 25–50 feet of loose, non-engineered fill containing construction debris, sawmill waste, and other materials. After the 2001 Nisqually earthquake, assessments found the seawall and its infrastructure vulnerable even to a moderate seismic event. The City therefore replaced nearly 3,200 feet of seawall and retrofitted another 500 feet.<br /><br />The final design uses an improved soil mass (ISM) made from interconnected jet-grouted soil-cement columns. Extending from beneath the original seawall platform to dense glacial soils, the cellular structure comprises seaward and landward panels connected by a center panel. It supports the new seawall and provides seismic restraint. Nonlinear dynamic soil-structure analyses informed the design, with ISM widths of 26–60 feet, ground replacement of 50–64 percent, and specified column strengths varying by soil type.<br /><br />Jet grouting was selected to address variable ground, debris, thousands of existing timber piles, buried utilities, and tight urban construction conditions. Hayward Baker installed approximately 6,000 columns over three construction seasons. Some reached 93 feet deep; the project used about 175,000 cubic yards of grout-treated soil, making it one of the largest jet-grouting projects constructed.<br /><br />Key construction challenges included limited workspace, equipment selection, spoils handling, wood debris, and obstructions. Lessons learned emphasized the importance of early obstruction identification and contingency planning, selecting methods suited to complex subsurface conditions, and allowing adequate space for safe, productive construction.

Keywords

Elliott Bay Seawall

Seattle waterfront

seismic retrofit

jet grouting

improved soil mass

soil-cement columns

Nisqually earthquake

timber piles

ground improvement

downtown infrastructure