A joint venture of CBNA and Halmar supported by engineers Cowi and Hatch has started an $819m design-build contract for the 5.5-mile-long Potomac River Tunnel, intended to control sewage overflows caused by rain in Washington, DC.
It’s the largest-ever contract awarded by the utility DC Water.
The capital’s sewerage system frequently overflows when it rains.
In an average year, some 74 overflows cause around 650 million gallons of dirty water to spill into the river, DC Water says.
It hopes the 18-foot-diameter tunnel will cut those overflows by 93%.
The tunnel will run along the river 100 feet underneath the Georgetown waterfront, West Potomac Park, and the National Mall, carrying sewage and stormwater to DC Water’s treatment plant at Blue Plains.
Halmar said the consortium would use two tunnel boring machines customised for the varied soil conditions.
As well as the tunnel, the team will build nine shafts, adits connecting to the main tunnel, and near-surface structures linking the new tunnel to the existing sewerage system.
Work starts this year and is scheduled to finish in early 2030.
Hatch will be responsible for overall design management as well as the detailed design of underground works including support of excavation and concrete structures.
Cowi senior project director Steven Kramer said: “Our team began working in late 2023 and has already delivered several key early lead items which has enabled the design and build team to begin construction. Working with CBNA-Halmar through the tender process and now the actual project has been very exciting and gratifying for our engineers.”
