Two tunnel boring machines are at work on the same alignment under Washington, D.C., and they are built to do opposite things. One is cutting hard bedrock north toward Georgetown. The other is being assembled to push through saturated soft ground and mixed face south toward Joint Base Anacostia-Bolling. Same owner, same contract, same 18-foot finished tunnel — two fundamentally different machines, because the ground under the District changes character partway along the route.
That is the engineering story on the Potomac River Tunnel, and among the 14 tunnel boring machines now working across eight US cities it is the cleanest live case study in TBM selection available.
The Job
DC Water is building 5.5 miles of tunnel at 18 feet internal diameter — roughly 21 feet excavated before the precast segmental liner goes in — more than 100 feet below ground surface. The alignment runs beneath the Georgetown waterfront, West Potomac Park and the National Mall, then out along East Potomac Park past Hains Point, where it connects by gravity to the existing Anacostia River Tunnel and on toward the Blue Plains treatment plant.
The word doing the work in that sentence is gravity. This is not a pumped system. The tunnel has to fall continuously from Georgetown to the Anacostia connection with enough slope to keep solids moving at design flow and shallow enough not to bury the invert deeper than the shafts and drop structures can economically reach. On a 5.5-mile run, a few thousandths of a foot per foot decides how deep the Georgetown end sits and how tall every shaft has to be. Anyone sizing gravity sewer runs at any scale will recognize the tradeoff; the pipe slope calculator shows the same arithmetic on a job you can actually build in a week.
The performance target is specific and legally binding: a 93 percent reduction in combined sewer overflows to the Potomac in an average rainfall year. Today eight outfalls discharge roughly 74 overflow events annually, totaling about 654 million gallons. After the tunnel, DC Water projects about four events a year.
Why the Geology Flips
Washington sits on the Atlantic Seaboard Fall Line — the boundary where the hard crystalline rock of the Piedmont gives way to the unconsolidated sediments of the Coastal Plain. Georgetown and the northwest end of the alignment are on the rock side. Hains Point and the southern reach are Coastal Plain material: sands, silts and clays, saturated, under a river.
A tunnel that crosses that boundary crosses a machine-selection boundary with it. Hard-rock tunneling is a cutting and abrasion problem: disc cutters, cutterhead wear, muck handling, and the risk that a fault zone delivers water faster than you can grout it off. Soft-ground tunneling under a river is a face-stability problem: the machine has to hold pressure at the face against hydrostatic head continuously, because unsupported saturated sand does not stand up — it flows. Mixed face, where rock and soil appear in the same cutterhead at the same time, is worse than either, because the machine has to do both jobs simultaneously and the softer material erodes ahead of the harder.
DC Water's answer was to buy two machines configured for the two problems rather than one compromise machine for both. That decision costs more in capital and mobilization. It buys down the risk of a stuck cutterhead or a settlement event under federal parkland, where the tolerance for surface movement is close to zero.
Mary: North to Georgetown
TBM Mary launched on April 10, 2026 from a 100-foot-deep overflow mining shaft at West Potomac Park. She is about 21 feet in diameter, weighs 1,200 tons, and runs roughly 700 feet fully assembled with her trailing gear. She is mining approximately 2.4 miles north through the District's hard bedrock toward Georgetown University.
The shaft is worth pausing on. A 100-foot-deep mining shaft in West Potomac Park is not a hole in a vacant lot — it is a support-of-excavation problem on National Park Service land within sight of the Mall, and it also functions as the overflow structure once the tunnel is in service. The shaft has to be sized for TBM assembly and lowering, for muck removal for the duration of both drives, and for its permanent hydraulic function afterward. Those three requirements do not naturally agree on a diameter.
On the receiving end, crews began controlled blasting near the Canal Road NW entrance to Georgetown University the week of July 13, 2026, working the drop shaft that will bring flow down into the tunnel. Blasting inside a dense urban block means pre-condition surveys of neighboring structures, vibration monitoring at defined peak particle velocity limits, blast mats, and a public notification program — and it is the phase that generates the community complaints, not the mining itself, which is invisible from the surface.
Emily: South Into Soft Ground
TBM Emily arrived at the West Potomac Park construction site early on May 27, 2026, shipped disassembled from Germany through the Port of Baltimore after factory acceptance testing. She is also close to 21 feet wide, and she is built for the southern reach — softer soils beneath the river and mixed ground, with the ability to adjust her operating mode to hold the surrounding earth in place as conditions change along the drive. Trade coverage identifies both machines as Herrenknecht units, consistent with the rest of DC Water's Clean Rivers fleet.
Emily's drive runs roughly 3.1 miles south toward Joint Base Anacostia-Bolling. Tunneling under an active military installation adds a permitting and access layer that most utility jobs never encounter, on top of the ordinary requirement to keep surface settlement within millimeters under parkland and levees.
Reassembly is its own schedule item. A machine of this size arrives as a shield, cutterhead segments, drive unit, erector, screw or slurry circuit, and several hundred feet of backup gantries. Lowering, bolting up, commissioning and a shoved test drive typically consume months, not weeks, and that work has to share a single 100-foot shaft with Mary's ongoing muck-out.
The Contract and the Team
DC Water's board approved the $819 million design-build contract on October 6, 2023 — the largest award in the authority's history. The winning team is a joint venture of CBNA, the North American arm of Bouygues Construction, and Halmar International. DC Water has stated the JV was selected on the strength of the lowest price proposal combined with the second-highest technical score, which is a useful data point for anyone reading best-value procurement scoring on public work: technical rank alone did not decide it, and price alone did not either.
On the design side, Hatch leads design management and the detailed design of the underground works, including excavation support and concrete structures. COWI leads tunnel and adit design and provides geotechnical engineering.
Design started in 2019, mobilization at West Potomac Park came in 2024, and DC Water carries a construction duration of roughly six and a half years, with completion targeted for early 2030.
A Consent Decree Is a Bid Pipeline
The 2030 date is not an aspiration. It comes from a 2005 consent decree among DC Water, the EPA, the Department of Justice and the District of Columbia. The Potomac River Tunnel is the final CSO tunnel in the Clean Rivers Program, a roughly $2.99 billion capital effort driven entirely by that agreement.
This is the part of the market that contractors routinely underweight. Consent-decree work does not respond to interest rates, election cycles, or private development sentiment. A federal court order sets a date, the utility has to hit it, and the capital plan is effectively non-discretionary for the life of the decree. Chicago, Cleveland, Indianapolis and Atlanta all run comparable multi-decade programs. For a heavy civil or utility contractor, decree-driven water and sewer work is among the most predictable multi-year backlog available anywhere in construction — and it is close to invisible in mainstream coverage compared to highway and transit megaprojects.
The follow-on tiers are where most firms will actually participate: shaft and drop structure construction, adits and near-surface connections, precast segment supply, grouting, instrumentation and settlement monitoring, dewatering, odor control, and the eventual mechanical and electrical fit-out. Those packages surface as they are advertised on the public bid board, and segment and shaft work in particular tracks directly with concrete input pricing across a six-year program.
What Happens Next
Watch three things. Mary's advance rate through the Georgetown bedrock, which will tell you whether the hard-rock drive holds its share of the schedule. Emily's reassembly and launch out of the same West Potomac Park shaft, which is the near-term critical path. And the Georgetown drop shaft, where blasting is already underway and where a schedule slip would show up first.
The decree date does not move. Everything else on this job is negotiated against it.
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