Portland, OR

Willamette River Combined Sewer Outflows

City of Portland, Bureau of Environmental Services


As part of a comprehensive program implemented by the City of Portland to reduce sewer overflows, the Willamette River Combined Sewer Outflow (CSO) Project was divided into the West Side and East Side CSOs. These two exceptionally large and technically challenging wastewater projects passed through central Portland, a dense urban area.

Fast Facts
  • Project reduced current sewage and stormwater overflows into the Willamette River by 94%
  • Tunnel alignment was designed to connect 13 existing combined sewer outfalls to the tunnel
  • The Swan Island Pump Station qualified as one of the largest excavations to be carried out in soft ground in the United States

Scope of Work

Delve Underground augmented the City’s staff by providing tunnel and shaft construction expertise and overall construction management services on the West Side and East Side CSO tunnel projects. In addition, Delve Underground provided design services on the Swan Island Pump Station shaft, the Portsmouth Force Main, and Balch Consolidation Conduit contracts, all key elements of the CSO program. 

Challenges & Innovations

The West Side CSO Project tunnel involved one of the first large-diameter slurry-shielded tunnel boring machines used in the United States, and the lining consisted of a one-pass precast concrete segmental lining. The East Willamette CSO included the construction of seven deep shafts with a series of gravity conduits and drop structures that connected existing CSOs to the tunnel. A progressive contract approach was implemented whereby the contractor was compensated on a cost-reimbursable fixed fee basis.