Construction of a Deep Water Marine Outfall Using Design-Build Procurement

King County Washington Wastewater Treatment Division (WTD) is building a new wastewater treatment plant (36 MGD average wet weather flow (AWWF) membrane bioreactor or MBR) to serve the northeast portion of its service territory in the metropolitan area of Seattle, Washington. The project includes an associated 13-mile-long conveyance system (four tunnel segments) that will discharge effluent to Puget Sound, a marine estuary in western Washington. This presentation will cover the design considerations and construction of the marine outfall, which consisted of 417 feet of 84-inch-diameter polyurethane-lined and coated steel pipe and twin parallel 63-inch-diameter, mostly bottom-laid HDPE (5,018 feet and 4,768 feet). Each of these outfall pipes includes 250-foot-long diffuser sections, which are staggered to provide an effective 500-foot-long diffuser. The flanged steel pipe was constructed using cut and cover from the shoreline to an existing grade depth of -4 feet mean lower low water (MLLW). This section was sheet pile shored to limit the trench width in an eelgrass bed. The first 494 feet of the HDPE pipes were also installed using cut and cover with 237 feet of trench shored and 257 feet side-sloped. The remaining twin parallel HDPE pipes were bottom laid, installed using controlled submergence and have concrete or coated ductile iron collars to provide differing degrees of ballast depending on the slope conditions. The presentation will review pipe materials and construction methods used for the project and the benefits of each. Material selection was done to assure a performance specification of a 75-year design life. The project was a success for the WTD and design-build team finishing 22 months ahead of the required date for 8 million dollars (20 percent) less than engineer's estimate. The project won 10 regional, national and international peer awards including Engineering News Record's Best of the Best Projects for 2009 in the Heavy Civil/Public Works category.