Beneath the streets of Washington, New York, Pittsburgh, Lynchburg, and dozens of other American cities, a long-overdue infrastructure program is gaining momentum. Combined sewer overflow (CSO) tunnels — large-diameter underground structures designed to capture and store untreated sewage and stormwater that would otherwise flow into rivers and bays during heavy rain events — are emerging as one of the most active segments of the U.S. heavy civil construction market. Driven by EPA consent decrees, federal water quality mandates, and aging infrastructure that was never designed for the rainfall intensities now common under a warming climate, this national tunnel wave is generating billions of dollars in construction activity across the country.

Washington, D.C.: TBM Mary Begins Her Journey

On April 10, 2026, DC Water officially launched Tunnel Boring Machine Mary, marking the start of active excavation for the $819 million Potomac River Tunnel Project — a key component of the District's long-term Clean Rivers Program. Mary is a 21-foot diameter, 1,200-ton, 700-foot-long machine that will bore approximately 2.4 miles north through hard bedrock from West Potomac Park to a drop shaft at Georgetown University. A second machine, TBM Emily, is scheduled to arrive from Germany in spring 2026 and will excavate 3.1 miles southward toward Joint Base Anacostia-Bolling, completing a 5.5-mile tunnel system.

Once complete, the tunnel system will reduce the volume of combined sewer overflow entering the Potomac River by 93%, cutting overflow events from approximately 74 per year to just four in a typical year. Currently, about 650 million gallons of untreated combined sewage and stormwater enter the river annually. The project is targeted for completion in 2030 and is being coordinated with a parallel National Park Service seawall rehabilitation effort in West Potomac Park, requiring close design integration between two independently managed programs.

The tunneling team completed controlled blasting operations at the West Potomac Park shaft in early 2026 before transitioning to TBM excavation. The front shield of Mary was lowered in sections beginning January 12, 2026, each piece weighing hundreds of tons and requiring assembly underground in a 100-foot-deep starter tunnel. TBM Mary has a 6.4-meter diameter, weighs 1,200 tons, and is 213 meters long, according to Tunnels and Tunnelling International.

New York City: First CSO Tunnel for Newtown Creek

In March 2026, AECOM announced that its joint venture with Parsons Corporation and EPC Consultants had been selected by the New York City Department of Environmental Protection to provide construction supervision for the city's first CSO storage tunnel. The project addresses Newtown Creek, a federally designated Superfund site spanning Brooklyn and Queens that has long been contaminated by industrial discharge and combined sewer overflow.

The project scope encompasses approximately 3.25 miles of large-diameter tunnel, underground storage and conveyance infrastructure, a tunnel dewatering pump station, and connections to the Newtown Creek Wastewater Resource Recovery Facility. Once complete, the system is designed to provide up to 50 million gallons of storage capacity, significantly reducing CSO discharges to the creek. The AECOM-Parsons joint venture will oversee what NYCDEP describes as part of a 16-year program to modernize the city's wastewater infrastructure and meet long-standing regulatory obligations.

A National Pattern: Consent Decrees and Capital Programs

Washington and New York are the most visible examples of a broader national pattern. Dozens of U.S. cities operate under EPA consent decrees requiring substantial reductions in CSO discharges, and most have turned to deep rock tunnels — sometimes called deep tunnel systems — as the primary solution. The scale of these programs ranges from hundreds of millions to several billion dollars per city.

• Pittsburgh, PA: The Allegheny County Sanitary Authority (ALCOSAN) is advancing a $2.8 billion system improvement program under an EPA consent decree, including tunnel segments along three rivers designed to capture overflows and transport them for treatment. Planned tunnel work includes segments along the Allegheny, Monongahela, and Ohio rivers.
• Lynchburg, VA: The city's Blackwater CSO Tunnel Project — the largest capital improvement in Lynchburg's history — is in its final stages of excavation as of May 2026, with approximately 700 feet of rock remaining. Excavation is expected to wrap up in July 2026, with full project completion targeted for July 2027.
• Cleveland, OH: The Northeast Ohio Regional Sewer District is advancing the Big Creek Storage Tunnel, a 20-foot diameter, 22,440-foot-long project with a total cost of approximately $260 million, designed to reduce CSO volumes by approximately 280 million gallons annually.
• Hartford, CT: The South Hartford Conveyance and Storage Tunnel, a 41-million-gallon underground sewer storage project, has been under construction with completion expected in 2026 after encountering scope and cost adjustments.

Why Tunnels, and Why Now?

The preference for large-diameter deep tunnels over surface-level alternatives reflects both technical and practical realities. In dense urban environments, above-ground or shallow conveyance infrastructure competes with existing streets, utilities, buildings, and transit lines for limited space. Deep tunnels can be routed beneath all of those obstacles, often through hard bedrock that provides structural integrity without the complex ground support required in shallower soil conditions. The tunnels serve a dual function: intercepting overflows from existing sewer infrastructure while providing long-term storage capacity that is slowly released to treatment facilities as capacity allows after a rain event.

Federal investment through the Clean Water State Revolving Fund has supported many of these programs. The EPA's State Revolving Fund programs have provided over $265 billion in financial assistance to more than 50,000 water quality infrastructure projects and 76,000 drinking water projects since their creation. The IIJA appropriated $50 billion for water infrastructure over five years, including significant allocations to the Clean Water and Drinking Water SRFs in FY2025 and FY2026.

Construction Complexity and Market Implications

CSO tunnel projects represent some of the most technically demanding and labor-intensive work in the heavy civil construction sector. TBM procurement and deployment, shaft sinking, precast concrete segment fabrication, ground freezing where required, dewatering systems, and final connection to existing infrastructure all require specialized capabilities. Projects of this scale typically involve design-build or construction manager-at-risk delivery models with specialized subcontractors for tunneling, geotechnical instrumentation, and permanent underground infrastructure.

The national CSO tunnel pipeline is likely to remain active through the end of the decade and beyond. Many cities have multi-decade consent decree compliance schedules with additional tunnel segments planned in later phases. The combination of regulatory pressure, federal funding availability, and aging combined sewer systems in over 700 U.S. cities means the underlying demand for this work is structural, not cyclical. For contractors, engineers, and owners with heavy civil underground capability, the CSO tunnel market represents one of the most durable project pipelines in American infrastructure construction.

Sources: DC Water, TBM Mary Launch (April 10, 2026); AECOM, Newtown Creek CSO Tunnel Contract (March 16, 2026); WSET, Lynchburg Blackwater Tunnel Update (May 4, 2026); EPA, Water Infrastructure Excellence and Innovation Recognition (April 8, 2026); DC Water, Clean Rivers Project News Spring 2026; Tunnels and Tunnelling International, Potomac River Tunnel Construction Begins (April 14, 2026)