Tunneling for Cleaner Waterways

Manchester’s Cemetery Brook Drain Tunnel Project Aims to Improve Water Quality in Merrimack River

The development of Manchester, New Hampshire, is inextricably linked to the Merrimack River. Afterall, it was the river’s waters that powered the textile mills that gave rise to the City’s development as the largest in the state and one of the largest in the region.

So, it is fitting that the City of Manchester’s largest public works project to date is dedicated to improving water quality in the river. The Cemetery Brook Drain Tunnel Project is in development as an approximately 11,700-ft long, large diameter stormwater conveyance tunnel to be constructed 30 to 80 ft beneath downtown Manchester.

It is the cornerstone of the Phase II Combined Sewer Overflow (CSO) program that will implement brook removal and sewer separation and address the capacity related backups and street flooding issues in the Cemetery Brook CSO basin. It is part of a Consent Decree between the city, the state and the U.S. EPA finalized in 2020.

“The Merrimack River is one of the economic engines that drives the City of Manchester,” said Fred McNeill, chief engineer for the City of Manchester. “Many of the City’s major industries and cultural destinations are on the banks of this majestic river.

The City has been actively addressing its CSO discharges into the Merrimack since the early 1990s. The City completed a 10-year, $58 million Phase I CSO Mitigation Program in 2009. The City then invested over $60 million in CSO mitigation measures between 2010 and 2020. The City is now executing a 20-year $300 million Phase II CSO Mitigation Program that includes the Cemetery Brook Drain Tunnel. This tunnel project will eliminate about 75% of the remaining CSO discharges into the Merrimack River.

“Over 600,000 people take their drinking water downstream from the City of Manchester. The elimination of CSO discharges helps ensure a safe and clean water source,” added Fred McNeill. The CSO ‘separation’ projects have been leveraged to contribute to urban revitalization by providing a complete infrastructure upgrade designed to last the next 50 years. For separation projects, the City coordinates with other local utilities to provide needed upgrades to subsurface water, gas, and telecommunications. As part of the separation projects the City then rebuilds the streets, addresses curbing, sidewalks, bike lanes, and ADA compliance. Lastly, landscape restorations incorporate green designs to address stormwater in a sustainable manner.

The City has engaged CDM Smith as its project engineer and is entering the procurement phase. The City has issued a request for proposals to solicit statements of qualifications for firms interested in constructing the Cemetery Brook Drain Tunnel Project. The RFQ represents Phase I of the two-phase best-value procurement selection process for the design-bid-build project. Mahmood Khwaja, vice president at CDM Smith, and project design manager, noted that “the Project team engaged the contracting community early on through informal and formal outreach efforts, resulting in a great response to our RFQ solicitation.”

The Cemetery Brook Drain Tunnel Project will be one of the most ambitious and challenging projects undertaken by the City of Manchester’s Department of Public Works. The design is being finalized, but currently the project team envisions a 12-ft inside diameter tunnel (14 to 14.5 ft OD) that is 11,700 ft long with seven drop shafts.

Although the majority of the alignment appears to be within good quality bedrock, the profile of the tunnel horizon along the alignment experiences several mixed face transition zones, as well as significant contiguous segments of soft ground below groundwater table. “CDM Smith has worked with the City of Manchester to develop a robust subsurface investigation program to best understand the subsurface conditions which include multiple difficult transitions between soft and hard ground,” added Mahmood Khwaja.

The subsurface strata are challenging ground conditions for tunneling, particularly through the soft ground and mixed face. The sections of the soft ground portion of the tunnel alignment are through ground that is considered “flowing”, where the ground does not have the inherent strength to maintain stable excavation without some type of pre-support. The preliminary interpretation of the subsurface condition suggests highly undulating bedrock profile along the tunnel alignment, particularly within the first 6,500 ft. The tunnel horizon transitions between soft ground, mixed face, and bedrock multiple times. For large segments of the tunnel alignment, open face tunnelling cannot be achieved. In fact, active face pressure needs to be applied when tunneling through the soft ground and mixed face portions.

A pressurized face TBM will be required when tunneling through the soft ground and mixed face conditions. In addition, the excavation will be completed as single pass, segmentally lined tunnel. The TBM will likely be operated in open mode for the tunnel constructed within the competent rock section. This means the face is not pressurized, and the cutterhead can be modified to have larger openings dressed with cutting tools that are specifically designed to excavate through rock. A “cross-over” or a “mixshield” machine capable of excavating soft and hard ground will likely be required for the project.

Summary

The Project team is preparing the final contract documents for an anticipated bid release in early Fall 2024. Dave Polcari, vice president and project director, CDM Smith, said: “CDM Smith is honored and proud to have worked with the City of Manchester to bring the Cemetery Brook Drain Tunnel from concept to final design. We truly value our role as a trusted partner in the development and implementation of the City’s CSO mitigation program from its inception. We look forward successful completion of this project and the overall program, and its benefits to the environment and the Manchester community.”

Timothy J. Clougherty, public works director for the City of Manchester, concluded: “The City of Manchester, Department of Public Works, Environmental Protection Division looks forward to embarking on this ambitious project. Construction of the tunnel continues the City’s goals of separating our storm and sewer systems and substantially improving the water quality of the Merrimack River. This next chapter will be the largest public works project in the long history of the City of Manchester and will benefit all Manchester area residents for decades.”

TBM would like to thank CDM Smith and the City of Manchester, New Hampshire, for providing information for this article. References include the 2023 RETC article “Understanding the Subsurface Conditions for the Cemetery Brook Drain Tunnel Project” by Mahmood Khwaja, David Polcari, and Michael S. Schultz, CDM Smith; and Frederick McNeill; Timothy Clougherty; and Benjamin Lundsted, City of Manchester; and the City of Manchester website.