Millions of tons of granite that have been sitting beneath Montgomery County for at least 540 million years — give or take a few dozen millennia — have been chomped up and hauled off, making way for a huge pipe that will gush with 100 million gallons of water each day.
It went on for almost five years as people above slept, or drove to work, or watered their begonias, oblivious to the fact that a 5.3-mile tunnel was being carved underneath.
“People never even knew we were here,” said John Mitchell, project manager for the Washington Suburban Sanitary Commission.
And when the water begins to flow next year, people won’t notice a bit of difference when they hop into the shower.
“They really won’t,” said Jerry Johnson, WSSC’s general manager. “The difference will be seen in the ability to expand and grow in other parts of the county. We have to be well ahead of [development] in putting our facilities in the ground and getting the service there.”
Tunneling is all the rage these days. D.C. Water is digging away beneath the District to create a monster sewage system. There’s one being dug for a highway in Seattle. New York tunneled for subway expansion. The Canadians tunneled under Niagara Falls to expand a hydroelectric-generation complex. The Russians built a wastewater tunnel under a river in St. Petersburg. The English and French tunneled under the English Channel.
In urban areas, better to tunnel than to open massive gashes in the ground like those that paralyzed parts of the District in the 1970s as the Metro system was built.
“The area has grown up, it’s very densely developed and there aren’t a whole lot of ways that you can put a pipe this size in the ground except to do a tunnel,” said Johnson, who added that the WSSC considered the open-cut approach. “The routing that would have to be done to accomplish that would have been just arduous because of the growth and development in the area.”
The WSSC provides water and sewer service to about 1.8 million people in Prince George’s and Montgomery counties. This project marks the first water tunnel built in the Maryland suburbs since the population boom of the late 1970s and early 1980s.
The tunnel, at a depth of 90 to 200 feet below the surface, depending on topography, runs between Interstate 270 in Rockville and the Church of Jesus Christ of Latter-day Saints’ Temple in Kensington.
Technology has helped more than a little bit.
The machine that chewed through the granite is the same type of tunnel-boring machine used on all the aforementioned projects. It’s a long, circular machine that rotates its cutting edge against the rock face, breaking away pieces larger than gravel but smaller than a grapefruit, delivering them to a 100-foot conveyor belt that carries them to a rail car in the rear.
Hauled to the surface through one of three big vertical shafts, the granite filled eight to 10 huge dump trucks each day.
“This is nobody’s kitchen counter top,” Mitchell said, cupping his hands to show the average size of each piece of granite.
They tend to name these tunnel boring machines. Seattle has “Big Bertha.” D.C. Water has “Lady Bird,” named for the former first lady. Montgomery County’s machine was “Miss Colleen,” named after the wife of a project manager on a job she did before arriving in Maryland.
Miss Colleen found her way from a start near Interstate 270 and Tuckerman Lane, to the end in Rock Creek Park near Stoneybrook and Beach drives, by using a laser that fixes on targets just behind the face of the machine and another just behind the machine. Then the machine is fed global coordinates and it takes another bite of rock.
The hole it created is 10 feet in diameter. A steel pipe that is seven feet in diameter and a half-inch thick has been inserted inside. Now, through small holes in the pipe every several feet, concrete is poured to fill the gap between tunnel wall and the pipe.
“We hammer on the wall of the pipe to be sure it’s fully sealed,” said Chris Davis, project superintendent for Southland Contracting. “It’s engineered to last 100 years, but it’s going to last much longer than that.”
Granite seems like pretty hardy stuff — rock-solid, you might say — but a million gallons of groundwater that has seeped through it is pumped from the tunnel every day.
When it opens early next year, the pipe will connect existing lines at either end, carrying water from north to south. It’s a transmission line, so there will be no feeder pipes from it to serve homes or businesses.
“The primary thing it will do is give us a great deal of redundancy, so that if we lose another major transmission line then we have the ability to work around other elements in the system,” Johnson said.
“Our only source of funds is the rate base,” he said. “Most of the cost of this project already has been factored in.”