Washington, D.C. -  Work on the Potomac River Tunnel, a key component of DC Water’s Clean Rivers Project to drastically reduce combined sewer overflows (CSOs) into the Potomac River, is progressing with tunnel boring preparations well underway and major TBM assembly operations now in motion.

TBM Mary Takes Shape Underground

At the West Potomac Park launch shaft, construction teams have lowered the 21-foot diameter cutterhead for the tunnel boring machine “Mary” into position — a critical step in preparing the machine for excavation. Mary will form the northern segment of the 5.5-mile long tunnel, driving from the launch point toward the Georgetown University area.

Because of the machine’s overall length (~700 ft when fully assembled), assembly is being completed gradually inside the starter tunnel rather than entirely at the surface. Once complete, Mary will begin tunnelling in spring 2026, installing precast concrete lining segments as it excavates through mixed geology including rock and soil.

A second TBM, named “Emily”, is scheduled to arrive in late 2026 to bore the southern portion of the tunnel toward the Anacostia River Tunnel connection. Emily has already completed factory acceptance testing (FAT) overseas, proving its readiness for deployment after transport and reassembly.

Why This Tunnel Matters

The Potomac River Tunnel is a central piece of DC Water’s Clean Rivers Program, an integrated system of deep storage tunnels, diversion structures and green solutions designed to keep untreated sewer and stormwater overflows out of the city’s waterways. Once in service, the tunnel system is expected to:

• reduce CSO volume to the Potomac River by approximately 93 % in average rainfall years;
• cut the number of overflow events from over 70 per year to just a handful; and
• convey excess flows to the Blue Plains Advanced Wastewater Treatment Plant for treatment rather than discharging directly into the river.

Project Delivery and Schedule

The CBNA Halmar Clean Rivers Joint Venture is constructing the tunnel under an $819 million design-build contract — the largest ever awarded by DC Water — managing everything from deep excavation and drop shaft construction to tunnelling and structural supports.

Work to support tunnelling, including site mobilization at West Potomac Park and adjacent Georgetown locations, continues alongside utility relocations and mitigation measures to control noise and traffic impacts. Mary’s excavation is targeted to begin in spring 2026, followed by Emily’s southern drive later in the year. For further information please visit https://www.dcwater.com/cleanrivers 07/26.