Cemetery Brook Drain Tunnel — Manchester, NH

The Cemetery Brook Drain Tunnel (CBDT) represents the centerpiece of Manchester’s Phase II combined sewer overflow program. This project supports the city’s 30-year strategy to improve Merrimack River water quality and reduce flooding. Our team at Parsons serves as the program manager and construction manager. The project includes a 2-mile-long, 12-foot-diameter tunnel located 80 feet below ground. It also features seven drop shafts and a new river outfall. We remove Cemetery Brook from the combined system to convey storm flows safely to the river.

Project Alignment And Strategic Planning

Our tunnel alignment passes beneath dense residential neighborhoods, the Elliot Hospital district, and an active CSX rail corridor. This path connects an inland inlet shaft to a riverfront launch site. We assist the City in sequencing work under a 2020 consent decree. This broader program includes over $600 million in upgrades through 2040. We carefully manage tunnel construction, sewer separation, and treatment facility improvements.

Geological Challenges And Technical Solutions

Subsurface investigations revealed a challenging New England profile. The soil consists of fill, glacial deposits, and boulder-laden till overlying crystalline gneiss, granite, and schist. These conditions required a hard-rock tunnel-boring machine. We maintain disciplined wear management and handle geological transitions with care.

• Hard-Rock TBM: Chosen specifically for the crystalline bedrock geology.
• Tailored Design: Each of the seven drop shafts features a unique design to accommodate variable local geology and tight urban footprints.

Managing Urban Risks And Infrastructure

The project footprint remains tight in urban areas. For instance, drop shaft 6 reaches depths of 92 feet near a hospital. We perform this deep construction around live utilities that require protection until tunneling finishes. Ground improvement remains central to our risk management strategy.

• Low-Pressure Grouting: We use this to strengthen granular soils.
• Permeability Reduction: This process creates a uniform ground mass for safe TBM launch.
• Settlement Control: Our methods meet strict settlement tolerances for the adjacent railroad.

Upcoming Milestones And Operational Coordination

Factory acceptance testing of the hard-rock tunnel-boring machine is complete. We plan for the machine launch in late summer 2026. The TBM will advance from the riverfront under the railroad and urban corridor. Finally, the machine will break through into the last shaft for retrieval.

• Operational Scope: We coordinate multiple active work sites and manage the disposal of tunnel spoils.
• Risk Monitoring: We track ground behavior, utility integrity, and schedule progress.

Commitment To Community And Environmental Goals

We face a demanding set of tunneling challenges while following the consent decree schedule. Consequently, we support communication with regulators, railroad stakeholders, and the community. The CBDT project demonstrates the integration of hard-rock mining and deep urban shaft construction. We deliver this long-term backbone for flood resilience to the residents of Manchester.