DC Water lowers TBM “Mary” cutterhead for Potomac River Tunnel construction
(UI) — DC Water has lowered the cutterhead for tunnel boring machine (TBM) Mary, marking a key construction milestone for the Potomac River Tunnel project. The 21-ft-diameter cutterhead was set into a starter shaft approximately 100 ft below ground, where the TBM will be assembled prior to tunneling.
The 105-ton cutterhead will be connected to the TBM’s front shield, which was previously lowered into position. Additional components, including the middle shield, are scheduled to follow as part of the underground assembly process. Due to the machine’s size—more than 700 ft long when fully assembled—Mary is being built in stages within the starter tunnel rather than at the surface.
Once assembly is complete, Mary will excavate a 2.4-mile section of tunnel from West Potomac Park to the Georgetown University area. Excavation will proceed with simultaneous installation of precast concrete liner segments, while excavated material is conveyed to the surface. DC Water has also begun site mobilization at the Georgetown location to support construction of a drop shaft and diversion structure that will tie into the tunnel.
Mary is expected to begin tunneling in spring 2026. A second TBM, Emily, is scheduled to arrive later this year to excavate the 3.1-mile southern segment connecting to the Anacostia River Tunnel.
The Potomac River Tunnel is part of DC Water’s Clean Rivers Project and is designed to capture and store combined stormwater and wastewater during heavy rainfall events. When placed into service in 2030, the tunnel system is expected to reduce combined sewer overflows to the Potomac River by approximately 93%, significantly limiting untreated discharges during wet-weather conditions.
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