Tunnel Work Is to Begin in Montgomery County on Last Major Link in Water System
Monday, July 27, 2009
Just outside the Capital Beltway in the heart of Montgomery County, workers will soon blast, drill and cut a massive 5.3-mile tunnel deep into the earth as part of a four-year project to install the final major link in the Maryland suburbs' water system.
When finished, the 10-foot Bi-County Water Tunnel between Interstate 270 in Rockville and the Church of Jesus Christ of Latter-day Saints' Washington D.C. Temple in Kensington will be big enough for a school bus to drive through. Running 90 to 200 feet below ground depending on topography, the tunnel will contain an 84-inch steel pipe to supplement a smaller main carrying water between the Potomac treatment plant and the growing number of toilets, fire hydrants and swimming pools primarily in Prince George's County.
Without the new pipe, some Wheaton residents could soon notice temporary drops in water pressure during prolonged droughts, according to officials with the Washington Suburban Sanitary Commission, which is overseeing the project. More than 100 million gallons of water will rush through the pipe each day, officials said.
The $168 million project will mark the first water tunnel built in the Maryland suburbs since the population boom of the late 1970s and early 1980s and will be funded with fees paid by developers for new infrastructure. Most major water pipes are buried eight feet underground. WSSC officials said that would have meant digging a huge trench, which would have destroyed more trees and caused more neighborhood disruptions.
Even so, many Montgomery residents and Beltway motorists will see and hear plenty. Early next month, bulldozers will begin clearing trees to build the tunnel's main construction shaft near Connecticut Avenue and the Beltway. In late September, some Kensington residents could hear what WSSC officials describe as "dull thunder," sounds of workers blasting the first large shaft, 35 feet in diameter, through bedrock. Some residents might feel vibrations from the blasts, although they won't be big enough to damage homes, WSSC officials said.
Early next year, crews will blast a second shaft through the tunnel's eastern end, at Stonybrook and Beach drives. In spring, some residents in the Old Farm and Tilden Woods neighborhoods near I-270 and Tuckerman Lane will hear explosions as workers build a third shaft at the tunnel's west end.
Meanwhile, Beltway drivers near Connecticut will see dump trucks hauling away more than 10,000 loads of dirt and rock. Much of the work will occur around-the-clock.
"This is going to be a critical component for the backbone of our water system," said John Mitchell, the WSSC's manager for the project's five-year design.
Mitchell said the steel pipe will be much more reliable than the large concrete mains that have burst in the past year. Those breaks caused widespread advisories to boil water and, in the case of a break along River Road in Bethesda, required motorists to be rescued from a cascade of water. The steel pipe will be encased in 18 inches of cement grout, which will limit rust, and have a corrosion-monitoring system to warn of any weakening, Mitchell said.
"This will be a steel pipe inside a tunnel inside solid rock," Mitchell said. "There won't be any type of catastrophic incident."
The pipe will not run under homes or businesses, WSSC officials said. The tunnel will be dug through land owned by the WSSC, the Maryland State Highway Administration and the Maryland-National Capital Park and Planning Commission. Although the main construction shaft at Connecticut will be near Rock Creek, it should not affect the creek, said Montgomery Parks Department spokeswoman Kelli Holsendolph.
About three-quarters of an acre of trees will be cleared and 1 1/4 acres of forest will be replanted after construction, she said.
Residents near the three shafts were most concerned that dump trucks would rumble past their homes, Mitchell said. He said the WSSC responded by agreeing to widen the Connecticut offramp from the Beltway's outer loop to give trucks direct access to the work site, avoiding Kensington neighborhoods. Trucks are not permitted to enter or leave the work area via the Beltway during the morning or evening rush. The work will generate three to 30 truck trips daily depending on the phase of the project, Mitchell said.
"Hopefully, they won't even know we're here," said Steve Pinault, the project's construction manager.
Because most of the work will be underground, noise is not expected to be a major problem, Mitchell said. The most will come from the Connecticut shaft, he said, but the loud hum of Beltway traffic should drown out much of it. Some residents near Tuckerman also will hear beeping from trucks' backup safety alarms, Mitchell said.
Tom Robertson, a Kensington resident who served on a citizens advisory group for the project, said the direct route from the Beltway exit ramp into the work site at Connecticut alleviated his community's concerns about truck traffic through the neighborhood.
"When you hear they're going to build a sizable tunnel, you immediately think they're going to do it in the worst way possible, but that doesn't seem to be the case," Robertson said. "I think they've dealt with the potential problems."
Gary Clark, an Old Farm resident and advisory group member, said his usually quiet neighborhood is bracing for the truck alarms. Although the WSSC agreed to route large trucks along Tuckerman, smaller vehicles, such as worker's pickups, will use his street.
"Obviously, we're not happy with it," Clark said. "But I guess they need to do this type of work. You have to continue to take care of infrastructure."
Pieces of the giant tunnel-boring machine, which is as tall as a bus and longer than two Metro rail cars, will be lowered underground by crane via the 140-foot vertical Connecticut shaft. After assembling the machine underground, workers will use its turning 10-foot circular "head" to chew through solid bedrock, Mitchell said. The head is covered with 17-inch "cutting heads," similar to pizza slicers, that spin and slice through about 100 feet of rock per day, Mitchell said.
The machine will first plow through 4,000 feet of rock to the eastern terminus at Beach and Stonybrook. There, the new pipe will connect to a 96-inch main.
Because it is too large to turn around in the tunnel, the machine will be taken apart and reassembled facing the opposite direction so it can go 4.5 miles west toward the Tuckerman shaft. There the pipe will tie into another existing 96-inch main. When the tunnel is finished, the machine will be dismantled and lifted out in parts through the Tuckerman shaft, officials said.
Fifty-foot sections of steel pipe from West Virginia will then be lowered into the ground by crane primarily via the Connecticut shaft and welded together.