Providence, RI

Narragansett Bay Commission CSO Program, Phases I and II

Narragansett Bay Commission


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.

Fast Facts
  • 3-mile (4.8 km) TBM-driven tunnel
  • Deep shafts, diversion structures, consolidation conduits
  • 16,000 feet (4,877 m) of pipelines
  • 4,000 feet (1,219 m) of drill-and-blast adits
  • Deep rock cavern housing a pump station

Scope of Work

Delve Underground was selected as part of a joint venture via two separate procurements (Phases I and II) to provide construction management services. The scope included prebid estimating and constructability reviews, construction management, resident engineering, and inspection.

Challenges & Innovations

Construction included such challenges as contaminated soils and water, significant water inflows into the tunnel, and postage-stamp-sized worksites in a dense urban setting. The tunnel had an initial lining of precast segments followed by a final lining of cast-in-place concrete. Ground freezing, secant piles, and jet grouting were all used for ground improvement and support of excavation.