Swan Island CSO Pump Station

LOCATION:  Oregon   |   OWNER:  City of Portland Bureau of Environmental Services
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Cost: $115 million

The Swan Island Pump Station (SIPS) connects the east and west sections of the Willamette River Combined Sewer Overflow (CSO) Project. This facility pumps combined sewage at a rate of 220 million gallons per day. Building it required the construction of a 135-foot diameter by 160-foot-deep shaft, making this one of the largest and deepest excavations in soft ground ever carried out in the United States. The excavation took place adjacent to the Willamette River, and required groundwater cutoff by jet grouting to a depth of 320 feet.

McMillen Jacobs Associates performed the preliminary and final design of the SIPS shaft. We assisted with the selection of the shaft site and configuration, performed geotechnical assessments, and provided design alternatives and cost estimates. During construction, McMillen Jacobs Associates provided structural engineering, design, and construction management services.

The pump station shaft’s final configuration employed slurry wall technology. Due to structural concerns associated with wall verticality, McMillen Jacobs Associates elected to use different design and construction approaches on each half of the 180-foot deep shaft. The upper half, designed to be self-supporting, relied on hoop compression and required no lining or supplementary support during excavation. This permitted rapid excavation of the shaft’s first half. From 80 to 160 feet, however, the slurry wall elements would not work in hoop compression due to the increased hoop forces caused by panel verticality. Therefore, McMillen Jacobs Associates designed the shaft’s lower half as a lined system, with the cast-in-place lining installed in “lifts” as the excavation advanced. McMillen Jacobs Associates designed the lower wall elements to temporarily span vertically during excavation and placement of each lining lift.

Ultimately, the slurry wall/liner composite system also works in hoop compression. This cost-effective solution eliminated the structural problems and risks associated with slurry wall verticality for such a deep shaft.