Abstract:
Designers and constructors of Seattle’ s $275M Georgetown Wet Weather Treatment Station overcame classical Puget Sound lowland geotechnical challenges to construct a process building to treat up to 70 MGD of combined sewer overflows, provide 1.2 MG of sewage storage, and convey the treated effluent 1, 600 feet to a new outfall in the tidewater Duwamish River. The site was underlain by over 200 feet of very soft to medium dense deltaic deposits with moderately high compressibility, low strength, and susceptibility to seismically-induced liquefaction. The industrial history of the area made contamination a consideration for deep foundation selection and restrictions on construction methods.
This case history will describe some of the key aspects of the project, including:
- Design for predicted sea level rise over the 75-year design life
- A process building structure able to tolerate more-than-commonly-accepted settlement by utilizing a robost reinforced concrete slab and relatively short rigid inclusions combied with load distribution mat
- Settlement monitoring and water preloading prior to installation of weirs, level-sensitive equipment, and yard piping connections to the process building
- A 96-foot diameter, 92-foot deep secant pile intake pumping station/equalization basin utilizing a 32-foot thick tremie seal to reduce construction dewatering risks
- Approximately 1, 800 feet of 96- to 60-inch diameter pipeline, a drop structure, and submarine outfall - partially beneath existing WSDOT 1990s era bridges, one of which replaced a 1950s wood pile-supported bridge.