TBM Launched for Potomac River Tunnel

DC Water announced on April 10 that it has started tunnel boring operations for the Potomac River Tunnel. With the first turn of Mary’s cutterhead underground, the machine pushed forward to begin tunneling, representing the transition from years of planning and preparation to active tunnel construction on the $819 million project.

“Launching Mary is a significant step forward for this project and for the future of the Potomac River,” said DC Water Chief Executive Officer and General Manager David L. Gadis. “This moment reflects years of planning, and the incredible coordination, engineering, and dedication required to bring a machine of this scale to life underground. It’s how we turn planning into progress.”

As Mary advances through the tunnel, the trailing gantries with supporting systems for power, ventilation, material transport, and worker access will be added. Once fully assembled, the 21-ft diameter, 1,200-ton, TBM will stretch to 700 ft in length.

Extensive preparations went into the launch, including constructing a 100-ft-deep overflow mining shaft and excavating a starter tunnel. The front portion of Mary was lowered and assembled inside the shaft and starter tunnel earlier this year.

During the launch the first concrete tunnel ring is installed and connected to a thrust frame, allowing the TBM’s hydraulic jacks to push the machine forward. As the cutterhead rotates, it excavates the ground while crews assist the TBM with simultaneously building the tunnel lining behind it using precast concrete segments.

Mary will bore approximately 2.4 miles north from West Potomac Park to Georgetown University, excavating through the District’s hard bedrock. Her sister machine, Emily, is scheduled to arrive from the Herrenknecht factory in Germany later this spring and will excavate south toward Joint Base Anacostia-Bolling, completing the full 5.5-mile tunnel system.

A joint venture of CBNA and Halmar International was awarded the construction contract in fall 2023.

The Potomac River Tunnel is part of DC Water’s Clean Rivers Program, a long-term initiative to reduce pollution in the Potomac River and Chesapeake Bay. Currently, about 650 million gallons of untreated combined stormwater and sewer overflow enter the river each year. Once completed, the 5.5-mile tunnel system will reduce overflow volume by 93% and cut the number of overflow events from 74 to just four in a typical year.

With Mary’s launch commencing, DC Water enters the next phase of construction as tunneling progresses beneath the city —advancing a project that will help protect local waterways, support the river’s diverse ecosystem, strengthen infrastructure, and support healthier communities for generations to come. Learn more at dcwater.com/prt.