Unique Challenges Drive Innovative Solution – Buoyant Interceptor Sewer in a Lake

When City of Lake Oswego officials and their design consultant realized that the interceptor sewer running through the middle of Oswego Lake was seismically unstable and undersized, they asked and were able to answer many difficult questions. “How do you replace a sewer in a lake? How do you support it? Will it work? Will it last? Can we maintain it? ” The result was an innovative pipeline, completed in May 2011 after a 10–year planning, design, and construction process. Total program cost for the program was 90 million to replace and rehabilitate undersized, deteriorating lines with a system designed to last at least 100 years.The innovative Lake Oswego Interceptor Sewer features a new 10,000—foot-long buoyant gravity pipeline consisting of 22- to 42-inch high density polyethylene (HDPE) pipe. The buoyant sewer is held to proper grade 11 to 18 feet beneath the lake surface by 428 ground anchors and tethers. A serpentine alignment provides thermal expansion loops to maintain grade despite wide-ranging water temperature. Stainless steel manholes are also buoyant and submerged, with access via removable caissons. The system also includes 7,000 feet of new pile-supported interceptor consisting of 22- to 42-inch HDPE pipe supported by 320 individual 18-inch piles. Rehabilitation of 6,000 feet of existing 18- to 36-inch-diameter sewers with cured-in-place pipe (CIPP) will extend the useful life of existing segments that are adequately sized and supported.Construction of the project was divided into two major phases: Lake Full (completed while the lake was full) and Lake Down (completed while the lake was drawn down 24 feet). Marine pipeline work was conducted during the lake full phase, and final connections and associated excavations were made during the lake down phase. General contractors and key subcontractors were prequalified to ensure that skilled and experienced teams would be conducting the work. The complexities of the design, as well as the related installation issues and inevitable problemsolving that occurred during construction, warranted significant involvement by the design team and an experienced construction manager. This resulted in strong communication between all parties to enable construction issues to be solved quickly and efficiently.Visit www.lakeinterceptor.com and view the professionally-produced “webisodes” to gain additional insight into this remarkable project.