Cost: Phase I: $274 million; Phase II: $150 million

Narragansett Bay Commission’s comprehensive combined sewer overflow (CSO) abatement program is dramatically reducing discharge of sewer and rainwater overflows that would otherwise enter the bay and its tributaries untreated. The Phase I system stores up to 64 million gallons (242 million liters) of combined sewage and transfers it to a deep pump station located at the Fields Point treatment facilities. McMillen Jacobs Associates was selected via two separate procurements (Phases I and II) to provide prebid estimating and constructability reviews, construction management, resident engineering, and inspection as part of a joint venture.

The Phase I centerpiece is a 30-foot (9 m) excavated-diameter TBM-driven tunnel extending three miles (4.8 km) at a depth of 250 feet (76.2 m) through meta-sedimentary rock formations. This “main spine” tunnel was supported by expanded precast concrete segments, and has a final lining consisting of cast-in-place concrete. The project also required multiple deep shafts, diversion structures, and consolidation conduits located within downtown Providence and along its historic waterfront; a pump station within a SEM (sequentially excavated) 120’ × 68’ × 68’ (36.5 × 20.7 × 20.7 m) cavern; and about 4,000 feet (1.2 km) of tunneled adits constructed with drill-and-blast methods and lined with cast-in place concrete. Some of the other construction methods used included ground freezing, jet grouting, raise boring, secant piles, and microtunneling. In 2009, the project was awarded the Project of the Year Award from the Underground Construction Association.

With the successful delivery of Phase I in 2008, the McMillen Jacobs Associates team was also selected for Phase II. We provided construction management and resident engineering services on this phase which was recently completed and brought on line. This phase includes nearly 3 miles of interceptors and a deep-rock shaft and connecting adit tunnel. The project includes 16,000 feet of pipeline with diameters ranging from 42 to 72 inches, installed using a combination of open cut, microtunneling and pipejacking methods beneath rivers and in densely populated areas; 240-foot deep shaft construction using ground freezing; and a 2,800-foot-long, drill and blast rock tunnel tying the Phase II facilities to the live Phase I main spine tunnel.