Final Gantry Completes Mobile Support Train Behind Mary Stretching More Than 700 Feet

DC Water’s tunnel boring machine Mary may be the muscle behind the Potomac River Tunnel Project, but the gantries that trail behind her form the heart, brains, and guts of the operation. Now crews have lowered and connected the final gantry section, giving Mary the tools and equipment she needs to continue tunneling north.

This is a major turning point for the project because Mary can now move forward at full capacity. The cutterhead at the front of the machine does the digging, but it cannot operate for long without the gantries behind it.

The gantries provide the power, hydraulic, and logistics systems that push Mary forward. They also carry dirt and rock out of the tunnel, install the concrete tunnel liner, and provide ventilation and space for crews. With the gantries fully attached, Mary stretches 706 feet from end to end. That is more than twice the length of two football fields and nearly as long as the Golden Gate Bridge is tall.

“With Mary and the gantries assembled, we now have a moving factory underground to build one of the District’s largest and most important infrastructure projects,” said DC Water Interim General Manager Matthew T. Brown. “This feat shows the scale of the work and the important role it will play in improving water quality and protecting the Potomac River for the region.”

Mary is so large crews could not lower her and the gantries into the shaft in one piece. Instead, as Mary moved underground a crane lowered each 30-ton gantry into the shaft. Another machine then pulled each gantry into the tunnel, where crews connected it.

With the final gantry attached, Mary will excavate about 350 feet farther into the tunnel. Crews will then get the machine ready for the next stage of digging when Mary will shift from softer soil near the river to hard bedrock for the rest of her journey.

During this shift, Mary will stay in place inside the tunnel. The changeover will take more than two months. After that, the machine will keep moving north to dig the rest of the 2.4-mile tunnel from West Potomac Park to Georgetown University.

The Potomac River Tunnel is part of DC Water’s Clean Rivers Program, a long-term effort to reduce pollution in the Potomac River and Chesapeake Bay. Mary’s sister machine, Emily, will dig 2.1 miles south toward Joint Base Anacostia-Bolling. When the full 5.5-mile tunnel system is complete, it will reduce stormwater and sewage overflows to the Potomac River by 93 percent. It will also cut the number of overflow events from about 74 to four in a typical year.

Learn more about the Potomac River Tunnel Project at dcwater.com/prt.