Prince George and Montgomery, MD
Bi-County East and West Water Tunnel
The Bi-County East and West Water Tunnel supplies potable water to Montgomery and Prince George’s Counties in Maryland. The tunnel, located in heavily weathered rock up to 120 ft below the I-495 Beltway around Washington, DC, was constructed by drilling through the rock and constructing a cast-in-place concrete liner. When the main riser of the new tunnel failed during initial acceptance tests, the owner retained SGH to investigate.
SGH investigated deficiencies in the tunnel that is 7 mi long and 10 ft in diameter. Highlights of our work on this project include the following:
- Reviewed designs proposed by others to repair the failed riser
- Assessed the adequacy of the tunnel liners and six other risers connected to the tunnel
- Analyzed available geotechnical data (with assistance from consultants) and interpreted new data collected after the failure
- Performed soil-structure interaction analyses of risers and thrust blocks and thrust block deflection analyses
- Discovered deficiencies in the design and construction of the liner and concluded that the liner might fail at certain locations where depth of embedment was low and the rock was weathered
- Developed a detailed test procedure to verify performance of the design liner and to provide information to design repairs
- Developed remedial repair concepts to address deficiencies
Ultimately, Washington Suburban Sanitary Commission, owner and operator of the main, elected to line the entire tunnel and risers to assure reliable performance of the tunnel.