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East Side and West Side CSO Tunnels, Portland, Oregon


The West Side and East Side combined sewer overflow (CSO) tunnels dramatically reduce the volume of combined sewage and storm water flowing into the Willamette River. They are part of the City of Portland’s 20-year CSO control program, which will significantly improve water quality, encouraging recreational use of the river and creating suitable habitat for wildlife.

For the West Side CSO Tunnel, which was completed in 2005, WSP USA provided preliminary and final design and design services during construction for a 3.5-mile-long (5.5-kilometer-long) tunnel with an inside diameter of 14 feet (4 meters), as well as a pump station, shafts and near-surface pipelines that tie into the tunnel.

For the East Side CSO Tunnel, completed in late 2011, we provided preliminary and final design of a 6-mile-long (10-kilometer-long) tunnel with a diameter of 22 feet (7 meters), as well as associated shafts and pipelines. We designed an alignment that allowed the East Side CSO tunnel to pass under seven bridges crossing the Willamette and also traverse a channel with a bridge, a highway interchange, and railroad tracks. The tunnel was required to avoid foundations for the bridge and then snake through piles driven to support ramps for the interchange. Excavation for one of the pipelines was performed with microtunneling equipment. At more than 3,000 feet (930 meters) long, it is the longest microtunnel to date in the United States.