The DC Clean Rivers Project has reached a significant milestone with the lowering of the front section of a 6.33m diameter tunnel boring machine (TBM), marking the start of assembly works deep beneath Washington, D.C.
TBM Assembly Underway 30m Below Ground
The CBNA–Halmar Clean Rivers JV is now assembling Mary, a 213m-long Herrenknecht multi-mode TBM, within a starter tunnel located approximately 30m below ground level.
The launch preparation represents a critical phase for the project, with the machine being assembled underground before commencing excavation of the 3.9km-long northern section of the Potomac River Tunnel.
Once complete, Mary will drive the tunnel between:
- West Potomac Park, and
- The Georgetown University entrance site
Multi-Mode TBM for Complex Ground Conditions
The selection of a multi-mode TBM reflects the challenging and variable ground conditions expected along the alignment. This type of machine allows operators to switch between different excavation modes, providing flexibility when transitioning between soft ground, mixed face, and potentially harder strata.
For tunnelling professionals, the deployment of a 6.33m machine in an urban, environmentally sensitive corridor highlights the continued reliance on adaptable TBM technology for complex infrastructure projects.
Tackling Sewer Overflows in the US Capital
The Potomac River Tunnel forms a key component of DC Water’s Clean Rivers Project, a major programme aimed at modernising Washington, D.C.’s ageing sewer system.
Currently, the city experiences combined sewer overflows (CSOs) during heavy rainfall events—occurring around 74 times per year—which discharge untreated wastewater into local waterways.
The new tunnel will:
- Reduce overflow events from 74 to just 4 per year
- Cut overflow volumes by 93%
- Provide substantial environmental protection for the Potomac River
Underground Infrastructure Driving Environmental Change
The Clean Rivers Project is part of a wider global trend where large-diameter sewer tunnels are being used to address legacy urban drainage challenges.
By capturing and storing excess stormwater during peak events, these tunnels help cities:
- Improve water quality
- Meet environmental regulations
- Build resilience against climate-driven rainfall patterns
With TBM Mary now being assembled below ground, the project moves closer to full-scale tunnelling operations. The successful delivery of the Potomac River Tunnel will be a major step forward in improving Washington, D.C.’s water infrastructure and environmental performance.
For the tunnelling sector, the project showcases the vital role of underground engineering in delivering sustainable urban solutions. 13/26.
