Lake Oswego's Innovative Buoyant Interceptor Sewer System Takes Shape

"Why build a sewer in a lake? Why make it buoyant? Where has it been done before? Will it work? Will it last? Can we maintain it?" City of Lake Oswego officials and their engineering consultant asked and answered these and many other questions while designing and constructing the innovative pipeline, completed in May 2011 after a 10-year planning, design, and construction process. Total program cost for the six projects is $90 million to replace undersized, deteriorating lines with a system designed to last at least 100 years. Its centerpiece is a buoyant gravity HDPE sewer (10,000 feet 22-inch to 42-inch outside diameter, with 12,000 feet of smaller, air-filled HDPE buoyancy pipe, all DR 11 and 13.5), 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 which causes the pipeline to lengthen and shorten +/– 14 feet over its 2-mile length. Stainless steel manholes are also buoyant and submerged, with access via removable caissons. Several manholes on upland tributary trunk sewers were converted to debris sumps to trap sand/gravel/sediment where it can be easily removed rather than accumulating in the in-lake system. Pile-support systems (354 18-inch steel pipe piles supporting 7,000 feet of HDPE sewer) and CIPP rehabilitation (6,000 feet of 18-inch to 36-inch) are also used for significant reaches. While much of the work was constructed with the lake full, final connections, and associated excavations, were made with the lake drawn down 24 feet and with extensive bypass and lake drawdown pumping/piping systems. This paper reviews the drivers for the project, the alternative solutions considered, and the development, design, installation, and maintenance of the buoyant system, including some of the numerous challenges overcome. Visit loisnew.com and view the professionally-produced "webisodes" to gain additional insight into this remarkable project.