Narragansett Bay Commission CSO Program, Phases I and II

The Narragansett Bay Commission’s CSO abatement program significantly reduces untreated sewer and rainwater overflows entering the bay. The system stores up to 64 million gallons (242 ML) of combined sewage, transferring it to a deep pump station at Fields Point treatment facilities.

Phase I of the project involved the construction of a 30-foot-diameter (9 M) TBM-driven Main Spine Tunnel with a cast-in-place concrete final lining. The 3‑mile (4.8 km) tunnel was situated at a depth of 250 feet (76.2 m), passing through meta-sedimentary rock formations. The project also included the construction of seven deep shafts, diversion structures, and consolidation conduits. A 300-foot-deep (91 m), 120-foot x 68-foot x 68-foot (36.5 x 20.7 x 20.7 m) rock cavern was excavated using the sequential mining method (SEM) to house a pump station. A total of 4,000 feet (1,219 m) of drill-and-blast adits were excavated to connect the Main Spine Tunnel with the drop shafts.

Phase II included the construction of nearly 3 miles (4.8 km) of interceptors, a deep-rock shaft, and a connecting rock tunnel. The project entailed the installation of 16,000 feet (4,877 m) of pipelines with diameters ranging from 42 to 72 inches (1,070 – 1,830 mm), utilizing a combination of open-cut, microtunneling, and pipejacking methods. A 240-foot-deep (73 m) shaft was constructed using ground freezing techniques, and a 2,800-foot-long (853 m) drill-and-blast rock tunnel was created to connect the Phase II facilities to the existing Phase I main spine tunnel.