Narragansett Pawtucket CSO project shortlist 19 Dec 2019

Three phases of the Narragansett Bay CSO program

From 10 groups that prequalified in October three have been shortlisted to prepare fixed-price design-build proposals for the Phase III Pawtucket tunnel of the Narragansett Bay Commission CSO project in Rhode Island. Following a call on 23 December, proposals will be due in May or June 2020 from the three shortlisted teams:

• Cardi-Obayashi-McMillen Jacobs
• CBNA (Bouygues)-Barletta-AECOM
• Lane Construction Corporation-Brierley Associates Corporation

Stantec and its partner Pare Corporation, is the Program Manager team appointed for the Phase III project.

As the third and final phase of the program to control CSO discharges into Narragansett Bay the new tunnel in the city of Pawtucket will run 11,600ft (3.5km) adjacent to the Blackstone River to provide 58.6 million gallons of storage and direct combined flow to the Bucklin Point wastewater treatment facility in East Providence (Fig 1). The 30ft (9m) i.d. rock tunnel will be excavated by TBM at up to 180ft (55m) deep and has a conceptual design single-pass, gasketed, precast-concrete, segmental tunnel lining. The one pass segmental lining, as opposed to two pass systems used on previous phases of the program, is said to be selected as the best suited to controlling groundwater inflows, maintaining rock stability, ensuring long-term quality and reducing construction time.

Completed Phase 1 Providence storage tunnel

In addition to the tunnel the contract scope includes two working shafts for launching and reception of the TBM; four drop shafts of 6-8ft i.d. x 145-175ft deep (1.8-2.5m x 45-55m) and a pump station cavern of 62ft wide x 70ft high x 120ft long (20m x 22m x 36.5m).

The CSO control program for the city of Providence and the surrounding areas began in response to a 1992 consent agreement with the Rhode Island Department of Environmental Management (RIDEM). The main element of the US$360 million Phase 1 project, completed in 2008, was excavation of a deep rock storage tunnel and large pumping station cavern in Providence designed to store combined stormwater flows for subsequent pump out and treatment at the Field’s Point treatment plant. Phase II began construction in 2011 and consisted of a network of interceptor tunnels to connect additional outfalls to the Providence tunnel. The final component of the US$197 million Phase II programme was completed in 2015.

Details of the refined Phase III project

Before embarking on Phase III, the Narragansett Bay Commission (NBC), as the client of the large-scale infrastructure investment funded by ratepayers, elected to undertake a re-evaluation of technologies and methods for CSO control including evaluation of using green stormwater infrastructure alternatives to conventional grey water conveyance solutions. The NBC engaged a team of engineers, led by MWH (now part of Stantec) and Pare Corporation, and began the re-evaluation in October 2014 to update costs of the three tunnel alternatives that had been identified originally and address the regulatory, environmental, and economic issues involved with Phase III procurement. In January 2019, the Commission and the RIDEM signed a consent agreement that included an extended schedule to meet the Phase III CSO controls and mitigated issues of affordability for the 80,000 ratepayers.

The optmized Phase III plan is a four-phase implementation schedule that reduces the overall financial burden of the program to ratepayers and achieve the most significant water quality benefit after implementation of the Phase IIIA Pawtucket tunnel (Fig 2 and Table 1). Phase IIIB of the program includes design and construction of additional interceptors while Phase IIIC includes design and construction of the 10ft i.d. x 7,775ft (3m x 2.3km) deep rock stub tunnel under the Providence River to the east side. Phase IIID as the final phase of the program addresses the remaining outfalls in the Field’s Point service area and includes the West River and Branch Avenue interceptor tunnels (Fig 2).

Costs and goals of the four Phase III stages

With award of the fixed-price design-build contract for the Pawtucket Tunnel scheduled to be announced at the NBC Board meeting on 29 September 2020 and an anticipated notice to proceed in November or December 2020, the Phase IIIA project is programmed to reach substantial completion by December 2024.