The design-bid-build combined sewer overflow project, one of Manchester’s largest-ever public works projects, began in 2025, following nearly five years of design, when the city awarded the contract to Methuen Obayashi Joint Venture, a national civil engineering partnership with expertise in complex infrastructure and tunneling. The project is expected to finish by 2028.

Last month Khwaja traveled with the project’s design-construction team and Manchester Dept. of Public Work officials to Schwanau, Germany, where the TBM passed factory acceptance testing.

The U.S. Dept. of Labor signed a strategic partnership with Methuen Obayashi JV earlier this year to promote worker safety and health during construction. 

“The tunnel will have seven drop shafts that convey the stormwater from future separation projects and current stream flows to the tunnel,” says Robert Robinson, chief engineer for the Environmental Protection Division of the Manchester Public Works Dept.

The shafts will also serve as entry points for stormwater and provide access for the construction team, a city statement notes.

The project is “the cornerstone” of the Phase II CSO Separation Program that aims to reduce sewer overflows by 74%, Robinson says.

The team gathered at the Herrenknecht’s factory in Schwanau, Germany before the 281-ton tunnel boring machine passed factory acceptance testing.
Courtesy of Petra Enghauser, Enghauser Studio, Germany

Phase II follows the city’s investment of $100 million in Phase I to mitigate CSO activation. The Cemetery Brook tunnel and seven drop shafts will convey all the stormwater for Phase II CSO separation.

The tunnel will separate stormwater from wastewater and convey stormwater into the Merrimack River. The Cemetery Brook project and overall program that includes six additional projects through 2040 will improve drainage flows, reduce CSOs, and improve water quality in the Merrimack, Robinson says. The river is an important drinking water source for more than 600,000 residents” in five Massachusetts communities south of Manchester and in Greater Nashua, N.H., including many environmental justice communities. Manchester receives its water supply from the city’s 15-billion gallon Lake Massabesic Watershed.

The new system is also designed to reduce flooding by alleviating basement backups and street flooding that affects some 115,000 Manchester residents, a Parsons statement says. The project is designed to improve public health by reducing CSOs and increasing sewer system capacity during peak rainfall events.

The TBM’s interior segments will be carried along the main body of the TBM before being loaded onto the back of the TBM conveyor system and carried to the mechanical arm at the end of the shield where they are segmentally placed.
Courtesy Molly Foster

Complex Operation

Careful planning was necessary before tackling the most complex work involving boring through mixed face conditions where the soil and rock interface and through varying ground, says Molly Foster, senior engineer, structures, for Parsons Corp., the project’s construction manager. The contractor selected a slurry TBM and is completing ground improvements to significantly reduce the risk, she says.

“There have not been rock-related issues with the construction and the drilling and blasting for drop shaft two is underway,” Foster says.

Methuen Obayashi and their subcontractor, Austin Powder Co. are managing the drilling and blasting.

During tunnel design, CDM Smith faced technical challenges related to the difficult ground conditions with risk of encountering boulders.

“In areas where the tunnel traversed through transition zones from soft to hard ground five times and the hydraulic design required acute angle connection into the tunnel at one of the drop shaft locations,” the team designed several rings of the tunnel to be cut for an unusually long collar into which the adit (shorter, smaller tunnel connection) would be tied, Khwaja says.

Understanding the subsurface conditions required comprehensive ground investigation due to extreme variability in the ground, Khwaja notes. The investigation included more than 80 borings and an extensive geophysical survey, conducted over four phases totaling about two years.

Of the seven vortex drop shafts, five are off-line, and two are in-line, Khwaja adds. An additional two vent shafts connect directly to the tunnel crown of the tunnel. At the discharge end, the team designed a transition structure, an energy dissipation structure, and a discharge structure into the Merrimack River, says Khwaja.

The drop shafts have a vortex approach channel and a vortex drop. “The vortex approach channel transforms a standard linear flow into a rotational/swirling motion,” Khwaja says. “The vertical drop shaft acts as a vertical conduit to convey the swirling water to a lower level—the centrifugal force keeps the water ‘clinging’ to the outer walls, leaving a stable air core in the center, through which air can escape.”

Three of the off-line drop shafts have a deaeration chamber and an adit that connects into the main tunnel, the other two off-line drop shafts connect into the tunnel with an adit only.

Crews inject grout into the ground in a drop shaft to improve the quality and strength of the soil so the tunnel boring machine can operate more safely and efficiently.
Courtesy of New England Studio

Site Prep

Crews are currently excavating the launch pit where the TBM will be assembled and begin mining operations later this summer, says Jacob Blunden, project manager for Methuen Obayashi.

They are also installing tanks to hold the bentonite slurry, water and other solutions that support tunneling activities and utilities and infrastructure required for the TBM’s operation.

Working around busy city streets is tough for the team on a site with constrained access for vehicles. There’s only a narrow area between the heavily-traveled Queen City Bridge and the launch pit where the cranes must be positioned to lift and place the TBM components into the excavation.

With preparations underway for TBM assembly, the team is busy double checking everything including “truck routing, grade changes, rigging gear selection, swing radiuses, soil bearing capacity, electrical needs and worker safety requirements,” Blunden says.

Assembling the TBM will require a 700-ton all terrain-crane and specialized rigging, he says before noting the importance of “qualified, specialized workers needed to execute the work.”