Combining Technology to Improve the Pawtucket CSO

Alex Britton, Project Manager, Keller North America, and Mark Careyva, PE, Project Executive, Keller North America

Urban areas across the US have constructed combined sewer systems that carry rainwater and sewage in the same pipes. While effective in many areas, these systems are increasingly taxed by urban development, climate change, and frequent heavy rains, leading to overflows into local water sources.

This issue is evident in Pawtucket, Rhode Island, where heavy rains cause the combined sewer system to overflow into nearby rivers and Narragansett Bay, contaminating them with untreated sewage and rainwater. Over the past thirty years, the Narragansett Bay Commission (NBC) has collaborated with stakeholders to design and construct an overflow system to hold combined water and wastewater until it can be treated and safely released into the Bay.

• Phase I, completed in 2008, included a tunnel, tunnel pump station, and seven drop shafts.
• Phase II, completed in 2014, involved two interceptors, two sewer separation projects, and a constructed wetlands facility.

The third and final phase of the Pawtucket CSO project involved constructing a 30-foot-diameter hard rock tunnel along the Seekonk and Blackstone Rivers at depths ranging from 115 to 155 feet. This phase required seven large-diameter shafts, ranging from 34 to 86 feet in diameter, supported by over 440 secant piles embedded into rock. Our scope also included three drop and three vent shafts, installed up to 100 feet deep with up to 85 feet of rock drilling for the vent shafts and 10.5 feet for the drop shafts.

Aerial of secant pile installation

Early Consulting and Better Design
The design team, CB3A (comprising CBNA and Barletta), engaged us early in the design process. This early involvement allowed us to optimize the techniques used for the shafts and mitigate potential challenges. Drawing from our experiences in South Hartford, CT, and other recent tunneling projects, we managed expectations and communicated achievable tolerances to accommodate tunnel-level connections.

Secant pile installation adjacent to Seekonk River

Initially, slurry walls were considered for constructing shafts due to their ability to mitigate water, but their complex mobilization led us to opt for secant piling after reviewing subsurface investigation results. Secant piling offered efficiencies over slurry walls, including reduced mobilization costs.

Optimization
Rock quality and strength were top concerns, along with limiting water infiltration between soil and rock layers. We determined that the rock socket could be reduced by up to 3 feet on each secant pile by performing a pre-excavation grouting program, focusing on the soil-to-rock transition to limit groundwater inflow.

View from bottom

Design to Overcome Logistics
With variable bedrock depths across the site, ranging from 15 to 80 feet below grade, we further reduced pile sizes. Secant piling allowed us to use different-sized tooling and shaft diameters, which was crucial given the site’s logistical constraints. For one outfall tunnel, we worked within an apartment complex parking lot, limited to ten parking spaces, requiring nimble operations.

Logistical constraints were also present at the drop and vent shaft locations, situated among residential and commercial buildings with limited trucking routes. The design team evaluated drilling diameters for drop/vent shafts based on achievable tolerances for carrier pipe placement, depth, final lining material, and minimum required grout annulus.

Building Shafts … and Trust
An integral part of this project was having an amazing onsite team from CB3A and Keller, who addressed concerns in real time, mitigating challenges and limiting downtime. Our teamwork involved staff from multiple regions, calling on various experts during early design involvement and onsite to maximize installation efficiency during production.

We completed our scope onsite, helping the CB3A team meet and exceed milestones with clear, open communication and the expertise to perform all project scopes under one contract.