MDCSO-WWTS is a new 240 MGD capacity wet weather treatment station, which operates intermittently, and provides physical treatment steps (screening, solids removal and disinfection) during heavy storms, to prevent untreated overflows into the east waterway at the mouth of the Duwamish River. The scope includes the demolition of existing structures and foundations, shoring and excavation work, and soil management (including disposal of contaminated soils, ground improvements, and tremie slabs. The scope also includes ballasted sedimentation, UV disinfection, solids handling, and odor control systems. Site development challenges are expected to include deep liquifiable soils with potential for seismically induced settlement (necessitating deep foundations). Key constructability considerations include 125-foot depth secant pile shoring or a diaphragm slurry wall which extends beneath the buoyancy slab of the equalization basin.
