Deep Foundation Challenges for a Combined Sewer Overflow Treatment Station in Seattle-Video

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The presentation described geotechnical challenges and construction of a King County wastewater treatment project in Seattle’s Duwamish Delta. The site had deep, loose alluvial soils, shallow groundwater, liquefaction risk, contamination, nearby utilities and structures, and a strict schedule. The team used staged construction and exploration, site-specific seismic analysis, rigid inclusions beneath the process building, settlement monitoring, and limits on dewatering. Advanced analysis reduced predicted liquefiable soils from about 150 feet to 15 feet.<br /><br />The intake pumping station was built within a 98-foot-diameter permanent secant-pile shaft. Because dewatering was restricted, crews excavated underwater, then placed an 8,300-cubic-yard tremie slab in a 26-hour pour. The unreinforced secant piles relied on ring compression, making accurate alignment and overlap essential. After curing, the basin was dewatered, the walls cleaned and lined, and the interior structures installed.

Keywords

Duwamish Delta

geotechnical challenges

seismic analysis

secant-pile shaft

tremie slab

wastewater treatment project