West Side cso tunnel, shafts, pump station, and pipelines project
Joint Venture Members
Salini Impregilo S.p.A.
S.A. Healy Company
The project scope was to improve the sewage system of the City of Portland (Oregon) by controlling combined sewer overflows (CSO) that discharged into the Willamette River. It involved construction of a new main sewer collection, storage, and conveyance system for the west city area. It 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.
The project was constructed in an urban area.
The CSO Tunnel length was 18,180 feet, with an excavated diameter of 16.5 feet, with a pre-cast concrete bolted, gasketed segmental lining. Excavation work was carried out by two 16.5 foot diameter full-face slurry mix-shield Herrenknecht TBMs, the first use of slurry mix-shield TBMs in the U.S. No pre-excavation grouting from within the tunnel was required during mining with either TBM.
The work was conducted in saturated alluvial soils, variable from sandy silts to cemented gravels with cobbles and boulders below the water table. Over half of the tunnel alignment was in the Troutdale Formation, a dense abrasive formation of cemented gravel.
The Nicolai Shaft was the main mining shaft and was 120 feet deep with a 52 foot 10 inch internal diameter. Four other shafts had internal diameters ranging between 39 feet and 48 feet and depths between 112 feet and 141 feet. Each shaft had an external jet grouting zone which served as he break-in and break-out zones for the TBMs.
The pumping station shaft was 137 feet internal diameter with an excavated depth of 160 feet. The excavation support consisted of a four-foot thick slurry wall extending to a depth of 200 feet. The internal wall was cast-in-place concrete tied to the external slurry wall to form a final composite wall.
Below the slurry wall, jet grouting columns extended to a depth of 320 feet to tie into the top of a mudstone formation, to form a groundwater cutoff wall. The internal pump station excavation was performed in the dry through the use of three continually pumping dewatering wells inside the excavation.
The project included a pipeline network which connected the existing sewage system to the newly constructed shafts. A Herrenknecht micro-tunnel TBM with diameter varying from 84 inches to 108 inches was used to excavate the secondary tunnels and lay the concrete pipelines. Including open trench work, the overall length of the pipelines was approximately 15,000 feet.
A partnership was maintained with the Owner throughout the four-year duration of the project. There were no Contractor claims or disputes and a Dispute Review Board was never utilized.
Successful operation of pressurized face TBMs in variable ground conditions with minimal ground settlement.
Successful excavation, concrete placement and groundwater control in massive below-ground pump station construction.
Impregilo/S.A. Healy JV achieved Voluntary Protection Program (VPP) from OR-OSHA for an exemplary safety record.
Impregilo/S.A. Healy JV exceeded MBE/ WBE contract goals by 50% and received the 2006 Contractor of the Year from the Oregon Association of Minority Entrepreneurs.
The project was successfully completed on time and, adjusting for escalation, none of the Owner’s contingency budget was utilized.