Bi-county tunnel takes shape deep below Beltway

Light streams through the top of an access shift to a new water tunnel under construction near Connecticut Avenue and the Capital Beltway Wednesday. The tunnel will improve water service in Prince George's County and parts of Montgomery County. (WTOP/John Aaron)
John Mitchell, left, project manager with the Washington Suburban Sanitary Commission, explains the pipeline while standing inside it Wednesday. Once completed, the tunnel will stretch 5.3 miles from North Bethesda to Kensington, Md. (WTOP/John Aaron)

KENSINGTON – A lot is going on beneath some of the busiest stretches of highway in the Washington area – all to keep the tap water flowing.

“We’re standing about 165 feet below ground, at the intersection of Connecticut Avenue and the Beltway,” said John Mitchell of the Washington Suburban Sanitary Commission. Mitchell, the project manager for the Bi-County Water Tunnel, spoke inside an 8-foot-wide pipe, which will take water from North Bethesda to Kensington, Md., beneath Interstates 270 and 495.

The tunnel’s depth ranges from 90 to 200 feet, and workers must be lowered down a shaft via a crane-held platform to reach it.

Mitchell says the decision to construct a tunnel so far below the surface was the result of concerns about the impact on the nearby area, which would have included tree removal and the installation of sound barrier walls.

“When we factored all those things in, we found this was a reasonable alternative,” he says.

The tunnel, which is set to open in 2015, could handle 100 million gallons of water a day, about two-thirds of the system’s capacity. WSSC says the additional capacity is needed to keep pace with rapid growth in Prince George’s County, without affecting customers in Montgomery County.

“It’ll help with water pressures in southern parts of Montgomery County,” says Mitchell. Without the tunnel, residents in the Wheaton area may have experienced low water pressure.

Mitchell also says the pipe’s construction of cement-lined steel should keep it serving residents for a long time.

“We don’t plan on anybody getting down here for at least 100 years,” he says.

WSSC says the project will cost $146 million. Planning began in 2004, and construction began in 2009.

Once completed, the tunnel will stretch 5.3 miles.

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