Something big is happening underground, as DC Water marks a major step forward for the Potomac River Tunnel Project. Today, crews successfully lowered the massive cutterhead for the tunnel boring machine (TBM) Mary. The 21-foot diameter cutterhead acts as a giant drill, to cut through rock and soil as Mary excavates the northern section of tunnel.

“Lowering Mary’s cutterhead is more than just a construction milestone—it marks progress toward a cleaner, healthier Potomac River for everyone,” said DC Water CEO and General Manager, David L. Gadis. “Soon, Mary will be fully assembled underground and ready to start her journey to help protect the river for generations.”

Mary’s massive size—over 700 feet long when assembled—and the confined space require her to be assembled underground from large components, including the cutterhead and shields, which are lowered individually and joined together inside a starter tunnel about 100 feet below ground.

The 105-ton cutterhead will be attached to the front shield, a large steel cylinder that encapsulates the tunnel boring machine’s body, which was lowered into the shaft on Monday. The middle shield is expected to be lowered on Friday. This process demands precise coordination and heavy lifting, as each component weighs hundreds of tons and must be positioned within inches of its final location.

Once assembled, Mary will start her 2.4-mile journey from West Potomac Park to the Georgetown University entrance site, sending excavated soil to the surface and constructing the tunnel’s concrete walls simultaneously. This week crews begin mobilizing for construction at the Georgtown University construction site as well, staging equipment and resources to build a drop shaft and diversion facility that will connect to the tunnel.

Mary is scheduled to begin digging northward in spring 2026. Later this summer, her sister TBM Emily will arrive from Germany to tackle the 3.1-mile southern stretch connecting to the Anacostia River Tunnel.

The Potomac River Tunnel is a key part of DC Water’s Clean Rivers Project, a long-term program to improve water quality and protect public health and the environment. The tunnel system will store combined stormwater and wastewater overflows during heavy rains, preventing millions of gallons of untreated sewage from reaching the Potomac. Once operational in 2030, it will reduce the volume of sewage and stormwater overflows into the river by 93% and lower the number of times this occurs from 74 to just four times in a typical year.

For more information about the Potomac River Tunnel Project, visit dcwater.com/prt.