Willamette River Combined Sewer Overflows

LOCATION:  Portland, Oregon   |   OWNER:  City of Portland, Bureau of Environmental Services
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Part of a comprehensive program implemented by the City of Portland to reduce sewer overflows, the Willamette River Combined Sewer Outflow (CSO) Project is divided into the West Side and East Side CSOs. These two exceptionally large and technically challenging wastewater projects pass through central Portland. McMillen Jacobs Associates provided construction management services on both.

The West Side CSO Project has over 10,000 feet (3 km) of near-surface storm sewer lines, a deep tunnel, and six deep shafts, including the Swan Island Pump Station. The shaft for the Swan Island Pump Station, 135 feet (41 m) in diameter and 160 feet (49 m) deep, qualifies as one of the largest excavations to be carried out in soft ground in the U.S. The tunnel, more than 18,000 feet (5.5 km) in length and 14 feet (4.3 m) in diameter, was excavated in sand and gravel at a depth of 120 feet (37 m), with a water head exceeding 100 feet (30 m), and included a crossing under the Willamette River. This tunnel involved one of the first large-diameter slurry shielded tunnel boring machines (TBMs) used in the United States, and the lining consisted of a one-pass precast concrete segmental lining. Trenchless Technology Magazine proclaimed it “Project of the Year: New Installation” in 2005 for microtunnel efforts associated with the near-surface sewer lines. The West Side CSO wrapped up construction in 2005—under budget, ahead of schedule, and with a strong safety record.

The East Willamette CSO started construction in 2005. Its central feature is a tunnel that is 30,000 feet (9,100 m) in length and 22 feet (6.7 m) in diameter at depths ranging from 100 to 120 feet (30.5 to 36.6 m), and includes the construction of seven deep shafts. Along the tunnel route, a series of gravity conduits and drop structures will connect existing CSOs to the tunnel. Similar to the West Side CSO, the East Side CSO tunnels are being driven by slurry TBM and lined with a one-pass precast segmental lining. The East Side CSO was completed in 2011.