Pawtucket is start of the third and final phase (in blue) of the CSO program

Excavation of the 11,600ft (3.5km) Pawtucket tunnel as the start of the final phase of the Narragansett Bay CSO control program in Rhode Island, USA, is awarded to the CB3A JV of CBNA (the Bouygues Travaux Publics subsidiary in USA) and Barletta Heavy Division. Notice to proceed has been granted under a design-build contract worth $450 million, with CBNA as the majority partner and AECOM the engaged design engineer and GZ Gall Zeidler sub-consultant for design of the larger diameter shafts and the SEM elements.

Competition for the 48 months contract was provided by the two shortlisted teams of Cardi-Obayashi-McMillen Jacobs, which decided to withdraw from the process in mid-2020 due to coronavirus pandemic disruptions, and the Lane Construction-Brierley Associates group that submitted a proposal valued at an estimated $565 million. A stipend was agreed by the Narragansett Bay Commission for the unsuccessful teams in recognition of the effort, resources and cost involved with preparing and presenting a proposal.

Located in the Rhode Island Delta, about 75km south of Boston, the Pawtucket project is the first part of Phase 3 of the Narragansett Bay Commission water management modernization program. The 3.5km x 9m (30ft) i.d. tunnel drive will follow the Seekonk River (Fig 1) and will be excavated by TBM. The TBM will be designed specifically to cope with the varied geology anticipated along the route. Stantec and its partner Pare Corporation, is Program Manager for Phase III of the project for the Commission.

Completed Phase 1 storage tunnel

The new CSO tunnel will collect, store and convey wastewater and stormwater to an existing water treatment plant. As well as the 3.5km TBM drive the construction contract includes excavation of three main shafts, one of which will serve as a pumping station, four collector drop shafts and connecting adits to the main conveyor. Carried out in the heart of an urban residential area of Pawtucket city, the project will be subject to measures designed to protect the environment and minimize the impact of the work on local residents.

Phases 1 and 2 of the CSO programme (Fig 1 pink and yellow), were completed in 2008 and 2015 respectively with a two-pass system of ribs-and-lagging primary support followed by cast insitu concrete used to line the early Phase 1 TBM tunnels. The new TBM project, to be excavated at up to 180ft (55m) deep, is designed with single-pass, gasketed, precast-concrete, segmental lining. GZ is providing independent design verification services for the segmental lining as well as completing the temporary and permanent works design of the larger shafts and the SEM pumping station cavern and the connector adits.

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.