DC Water has completed repairs on a 2.75m diameter, 140-year-old sewer tunnel in Washington DC.
Construction on the sewer tunnel on 22nd Street Northwest took nearly a year and was carried out in two phases. The second phase was completed one month ahead of schedule and under budget.
The tunnel section is part of the Northwest Boundary Trunk Sewer, built in the 1800s. Last year, an inspection revealed a 4m long by 0.3m wide void in the soil above the tunnel, which was in danger of collapse.
Significant structural issues were also identified, as well as issues upstream and downstream from the emergency repair. These included longitudinal cracks, missing cobblestones and bricks, and root penetration.
Construction, sometimes 24 hours a day, seven days a week, posed challenges for crews working two storeys below ground in difficult conditions. Throughout the repair works the sewer pipe remained active, with combined stormwater and sewer debris flowing through. Work had to stop when heavy rain inundated the system.
The work involved reinforcing the sewer tunnel and stabilising the soil around the initial void. This required injecting grout into the soil, framing the walls with rebar, and using high-pressure spray-applied shotcrete and geopolymer from 10cm to as much as 23cm thick. This included installing 139,000 linear feet of rebar; 25,450 rebar anchors; 28 tons of shotcrete; and 459 tons of geopolymer.
Emergency repairs began in July 2024 as part of the first phase of the project which involved stabilising the soil and reinforcing the tunnel around the initial void. Additional inspections identified more structural deficiencies, requiring a second phase to repair more than 366m feet further upstream and downstream.
DC Water operates approximately 3,220km of combined, separate, and stormwater sewers. Addressing aging water and sewer infrastructure throughout the district is a critical part of the 10-year, US$9.6bn Capital Improvement Programme. This includes replacing or rehabilitating similar large diameter sewer lines that have reached the end of their useful life.
