West Side Willamette River Combined Sewer Overflow
Already 100% operational, the West Side Combined Sewer Overflow (CSO) encompasses: a 3.5-mile-long, 14-foot-interior-diameter tunnel; 2 total miles of microtunneled interceptor pipelines; six large shafts; and 13 smaller shafts. The tunnel alignment runs alongside and then crosses under the Willamette River. Ground conditions throughout the alignment consist of soft and dense gravelly sand. Excavation occurred in a single pass well below the water table, at a depth of 120 feet, in the presence of water head exceeding 100 feet. This excavation marked the first use of large-diameter slurry mix-shield tunnel boring machines on a competitively-bid contract in the United States.
The West Side CSO big tunnel ends its course at the 220-million-gallons-per-day Swan Island Pump Station. Building this pump station necessitated first excavating a giant shaft adjacent to the Willamette River. (Groundwater cutoff included jet grouting to a depth of 320 feet.) The Swan Island Pump Station shaft dimensions – 135 feet in diameter and 160 feet deep – make this excavation one of the largest and deepest ever executed in soft ground in the United States.
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