Seattle sewer lines installed in the early- to mid-20th century simply can’t handle the up to 70 million gallons of polluted stormwater that flows into the Duwamish River. And as Seattle continues to grow and impervious surfaces create additional runoff, the combined sewer-wastewater systems that fill much of the Seattle area exceed the capacity of existing wastewater treatment facilities.
A $262-million project near Seattle’s Georgetown neighborhood was put in place in order to change that reality.
Pacific Pile & Marines portion of the project primarily consisted of outfall construction which for 306 LF of 54-in OD HDPE outfall pipe into the Lower Duwamish Waterway. 106 LF of the outfall pipe is installed within a shored trench with the remaining 200 LF installed on existing grade or within a shallow burial. The final 50-ft of the outfall includes (8) 20-in duckbill valves for the diffusion of effluent into the waterway. Outfall calls for precast concrete anchors bolted onto the pipe to provide weight and stability. Outfall work calls for the trestle, excavation, and shoring. Work also calls for the removal and disposal of steel and creosote pilings and other debris along the shoreline, post-construction restoration of the shoreline armoring, and restoration of an existing upper-intertidal swale. 19 LF of 60-in ID, lined and coated, steel effluent conveyance pipe is also to be installed along with a drop structure. Drop structure is to be constructed of cast-in-place concrete within a shored and dewatered excavation area. Pipe spools will be cast into structure walls for connection to effluent conveyance and outfall pipes with being a flap gate check valve on the entrance to drop structure.