Deep Impact: The Underground Tunnel Transforming Narragansett Bay
The Pawtucket Combined Sewer Overflow (CSO) tunnel will drastically reduce overflows, improving water quality and public health while advancing the economy in communities around Rhode Island’s Narragansett Bay.
Residents in and around Pawtucket, Rhode Island stand to benefit from cleaner waterways and improved public health, thanks to the new Pawtucket Combined Sewer Overflow (CSO) Tunnel.
Like many older cities, Pawtucket uses a combined sewer system where stormwater and sewage flow through the same pipes. Heavy rains can overwhelm these systems causing untreated wastewater to flow into Narragansett Bay, polluting the environment. The new CSO tunnel is designed to capture and store this excess stormwater and sewage until it can be safely treated and discharged, preventing these overflows and protecting the local ecosystem.
The tunnel is a major component of the final phase of the Narragansett Bay Commission (NBC)’s three-phase CSO Abatement Program, called Restored Waters Rhode Island. The program began in 1998 and aimed to lower annual CSO volumes and reduce annual shellfish bed closures. Phases 1 and 2, which focused on the Field Point Service Area in Providence, were completed in 2008 and 2015 respectively.
When operational in 2025, the Pawtucket tunnel is expected to reduce CSO volumes by 98 percent, significantly improving water quality and directly benefitting the health and well-being residents in the Central Falls and Pawtucket communities. Cleaner waterways will also advance residents’ economic opportunities through improved and more plentiful shellfish beds.
A complex cornerstone
The 2.2-mile-long, 150-foot-deep Pawtucket Tunnel is the centerpiece of the program’s third and final phase. About 30-feet in diameter, the tunnel can store 58.5 million gallons of combined sewer overflows considered about three months’ worth of storm and wastewater. In addition to the tunnel itself, several key components work together to capture, store and effectively manage the excess stormwater and sewage, including drop shafts that connect the surface level sewer system to the underground tunnel, a pump station to ensure proper treatment after the overflow event, smaller connecting tunnels, ventilation and monitoring, lining and waterproofing systems.
Developing the tunnel and these elements posed numerous challenges. The tunnel passes through varied geologic conditions, including complex sedimentary rock formations and a fault zone which pose risks to structural integrity and tunnel boring machine (TBM) performance. Potential groundwater inflow and high hydrostatic pressures — the weight of the overlying groundwater — added complexity to excavation and dewatering efforts. The tunnel also passes through densely populated urban areas, necessitating solutions that minimized surface disruptions, noise and vibrations. Existing infrastructure including roadways, utilities and buildings added constraints to alignment and construction techniques.
Strict environmental regulations and community concerns required careful planning and mitigation of ecological impacts. The involvement of multiple stakeholders, including government agencies, utility companies, and local communities, required extensive coordination and consensus-building. Balancing project timelines with public concerns was a constant challenge.
Innovations and solutions
As the designer on the design-build team, we brought our tunneling expertise to the project, crafting solutions to tackle these issues. We began with detailed geotechnical investigations to better understand the subsurface conditions. These studies guided critical decisions such as the tunnel alignment and the TBM’s selection and customization. The machine was specifically chosen to handle the area’s diverse geology while minimizing risks like settlement or groundwater inflow.
To ensure the tunnel’s long-term durability we used pre-cast concrete lining segments designed to withstand the corrosive environment presented by wastewater. Advanced hydraulic modeling helped optimize its functionality and capacity for effectively managing stormwater and sewage.
We relied on micro tunneling and trenchless technologies to minimize disruptions in densely populated areas — methods that allowed us to create connections with minimal surface impact and minimize the risks of adverse geology under a river crossing. The tunnel’s alignment was carefully planned to avoid conflicts with critical infrastructure and utilities, as well as to mitigate easement issues. Throughout construction real-time monitoring systems tracked progress, ground movements and groundwater conditions, ensuring safety and enabling quick adjustments as needed. A comprehensive risk management plan also helped efficiently and effectively address unexpected challenges.
System integration was another important aspect of the project. The Commission owns and operates key infrastructure — including the Field’s Point and Bucklin Point wastewater treatment facilities, over 112 miles of interceptors, numerous pump stations, tide gates and CSO facilities as well as a 62-million-gallon CSO storage tunnel. We applied our innovative engineering skills and precise execution to incorporate the tunnel project into the existing network.
Tunneling and beyond
The Pawtucket CSO Tunnel reflects engineering ingenuity and a deep commitment to community and environmental health. Designed to overcome complex geotechnical, environmental, and logistical challenges, the project highlights the innovative use of advanced tunneling technologies, including a customized TBM, pre-cast concrete linings and real-time monitoring systems. These efforts ensure the tunnel’s durability, efficiency and long-term performance as a critical piece of infrastructure.
Beyond its technical achievements, the tunnel is a significant step forward for the residents of Pawtucket and Central Falls. By drastically reducing pollution in Narragansett Bay, the project will provide cleaner waterways, improved public health, and a healthier local ecosystem. It also brings broader benefits, such as supporting economic growth through the revitalization of shellfishing and other water-dependent industries.
This project stands as an example of how thoughtful engineering and collaboration can address environmental challenges while enhancing communities, ensuring that the benefits of today’s investments are felt for generations to come.
