Ground Improvement Design and Construction for Seattle’s Elliott Bay Seawall Replacement and Retrofit-Article

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Seattle’s Elliott Bay Seawall was built over loose, liquefaction-prone fill and soft estuarine and beach deposits. The 2001 Nisqually earthquake highlighted the wall’s deterioration and seismic vulnerability, prompting replacement of about 975 meters and retrofit of another 152 meters.<br /><br />The new design uses an improved soil mass (ISM) made of roughly 6,000 overlapping jet-grouted soil-cement columns arranged in cellular panels. The ISM supports the seawall superstructure, restrains liquefiable soils, and provides seismic resistance. Jet grouting was selected because it could be installed amid thousands of existing timber piles, buried utilities, variable soils, and severe space constraints. Nonlinear, two-dimensional dynamic analyses informed the design, with performance criteria established for three earthquake levels. The ISM is 7.9–18.3 meters wide, with 50–64 percent of its area improved; required soil-cement strength varied by soil type.<br /><br />Construction took place over three seasons from 2014 to 2017. Key challenges included timber debris and piles, live utilities, restricted access, overhead obstructions, and managing large volumes of wet grout spoils. Crews used surveys and vacuum excavation to locate obstacles and utilities, protective casings and monitoring to reduce risks, and coring and strength testing for quality verification.<br /><br />The authors conclude that jet grouting was particularly suited to the site’s complex conditions. They emphasize identifying obstructions early, preparing contingency plans to avoid production delays, and carefully coordinating the space needed for equipment and other construction activities. They also note that more time for preliminary testing could have improved construction efficiencies.

Keywords

Elliott Bay Seawall

jet grouting

soil-cement columns

improved soil mass

seismic retrofit

liquefaction resistance

Nisqually earthquake

Seattle waterfront

timber piles

construction challenges