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The city of Portland needed to improve the quality of the Willamette River, a major tributary of the Columbia River. The West Side Tunnel aims to collect and store combined sewer overflows (CSOs) before they reach the river, and instead transports the overflow to a pump station, where it is treated before returning to the river.

Gallons Per Day Capacity
Reduction in combined sewer overflows discharging untreated water to the Willamette River
Awards for Excellence and Safety


The project involved the construction of a new main sewer collection, storage, and conveyance system. The system included a deep storage and conveyance tunnel, a belowground pumping station, five shafts approximately 130 feet deep, and a network of micro-tunneled and open cut pipelines and diversion structures to connect existing sewers to the tunnel.

As part of a series of other CSOs projects, the combined projects reduced the city’s sewer overflows in the Willamette River by 94%.

Technical Highlights


  • Installation of an 18,180-foot tunnel, with an excavated diameter of 16.5 feet, and a pre-cast concrete bolted, gasketed segmental lining
  • Two 16.5-foot diameter, full face slurry mixshield tunnel boring machines (TBM) were used for excavation, marking the first use of this technology in the tunneling industry
  • No pre-excavation grouting from within the tunnel during mining with either TBM

Pumping Station

  • A 160-foot-deep and 137-foot-diameter pumping shaft
  • 120 million gallons per-day capacity
  • Excavation support consisted of a four-foot thick slurry wall extending to a depth of 200 feet
  • Internal wall consisted of cast-in-place concrete tied to the external slurry wall to form a final composite wall
  • Some of the deepest jet grouting columns in the world, at six feet in diameter each, extended 320 feet deep to tie into the top of a mudstone formation to form a groundwater cut-off wall


  • A micro-tunnel TBM with multiple shields and diameter varying from 84 inches to 108 inches was used for excavation and to install concrete PCP pipe lining
  • Overall pipeline length of approximately 15,000 feet


  • Accepted into the Voluntary Protection Program (VPP) from OR-OSHA for an exemplary safety record
  • 2006 Contractor of the Year from the Oregon Association of Minority Entrepreneurs