Spokane, Wash., is completing a network of buried concrete combined sewer overflow (CSO) structures to store excess water. The structures are included in Spokane’s Cleaner River Faster initiative that reduces the amount of untreated sewage from combined sanitary and storm sewers that discharge into the Spokane River. The 380,000-gal 60-in. pipe CSO Basin 41 Control Facility was constructed using 60- and 72-in.-diameter Class IV reinforced concrete pipe (RCP) in 12-ft lay lengths. The system accommodates 120-in.-diameter precast concrete manholes in the storage structure.
Spokane’s sewer system was installed in the late 1800s, with sanitary sewage and storm water removed via a singular pipeline system that drained into the Spokane River or Hangman Creek. A wastewater treatment plant was installed in 1958, and treated sewage was discharged into the river. When flow exceeded the treatment plant’s capacity, untreated runoff was discharged into the river.
In the 1980s, Spokane constructed separate storm water systems and eliminated combined sewers in most of its northern area. The remaining combined sewers are in areas that are not easily separated. Constructing underground concrete tanks to hold the combined sewage during a storm and then draining it to the Riverside Park Water Reclamation Facility when the surge subsides is the preferred solution where separation is most difficult.
One such storage structure is Basin 41. The contractor, N & N Excavation LLC, preferred 12-ft lengths with single O-ring gaskets to limit the number of joints. The pipe meets the requirements of the city, Washington State Department of Transportation (WSDOT), and ASTM C-76 (Standard Specification for Reinforced Concrete Culvert, Storm Drain, and Sewer Pipe).
Prior to shipment, a D-load test was required, and WSDOT had to stamp the pipe approved for shipment according to the project specifications. After installation, the pipeline was hydrostatically tested for water tightness. Due to the tight timeframe of the project, H² Precast produced the 60-in.-diameter pipe and Cascade Concrete Products produced the 72-in.-diameter pipe.