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

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The Georgetown Wet Weather Treatment Station in Seattle was designed to treat up to 70 million gallons per day of combined sewer overflows and store 1.2 million gallons. Its construction faced difficult ground conditions: more than 200 feet of compressible deltaic soils, seismic liquefaction potential, shallow groundwater, industrial contamination, undocumented utilities, and nearby settlement-sensitive structures. Strict Consent Decree milestones also constrained the schedule.<br /><br />For the Process Building, designers used 35-foot-deep rigid inclusions beneath a load-distribution mat to control settlement while avoiding alternatives that were less suitable for the fine-grained soils or would generate contaminated spoil. The reinforced concrete water-holding structures were designed to tolerate some settlement. Water filling and settlement monitoring were used to allow anticipated movement before installing grade-sensitive features such as weirs, equipment, and pipe connections. Advanced seismic analyses substantially reduced estimated liquefaction-related settlement compared with preliminary estimates.<br /><br />The 100-foot-diameter, 92-foot-deep influent pumping station and equalization basin was built within a secant-pile ring. To avoid groundwater drawdown that could redirect contamination or cause off-site settlement, excavation proceeded without dewatering. A 32-foot-thick tremie concrete seal provided uplift resistance, after which the basin could be dewatered for interior construction.<br /><br />Extensive monitoring tracked settlement, shoring movement, groundwater, and vibration. Vactor-truck pre-clearance also helped locate undocumented utilities before critical work. The facilities were completed within the required schedule and budget. Monitoring indicated that some off-site subsidence was vibration-related rather than caused by dewatering; observed effects were manageable, and no building-damage claims were reported.

Keywords

Georgetown Wet Weather Treatment Station

Seattle

combined sewer overflows

rigid inclusions

seismic liquefaction

secant-pile ring

tremie concrete seal

groundwater monitoring

settlement control

industrial contamination