Tunneling Through Tysons: 18 Feet Done, Many More to Go
Sunday, October 18, 2009
Cars crawl down Route 123 in the afternoon rush. Forty feet below them, giant machines and men wearing yellow hard hats begin their advance under Tysons Corner to bring Northern Virginia commuters their holy grail: a new subway.
At $85 million, the half-mile tunnel is the costliest and most complex engineering feat of the 23-mile Metro extension to Dulles International Airport. It will be built while 3,500 cars and trucks cross its path each hour, while the Courtyard Marriott serves breakfast and guests swim in its pool, while hands are shaken over aerospace deals at BAE Systems. It will carry on under two miles of tangled utility lines that convey to Tysons everything from electricity to some of the nation's most secret intelligence. As of Friday, after three months of digging and prep work, workers had hollowed out the half-mile tunnel's first 18 feet.
One wrong move and the foundation of an office garage could settle, a top-secret communique through the U.S. Army's microwave tower right above the tunnel's path by Clyde's restaurant could be lost.
"You've got gas lines, water lines, drainage lines, electrical duct banks, black wires and a lot more in a busy urban area, which makes for a very challenging tunneling environment," says Dominic Cerulli, the engineer for Bechtel in charge of building the tunnel. He guides visitors on the first tour of the project on a recent weekday. "I've been on jobs where you're tunneling out in the middle of a parking lot. Here you've got to keep businesses up and running."
When it opens in 2013, the first leg of the rail line will extend 11.5 miles from East Falls Church through Tysons to Wiehle Avenue in Reston. The tunnel, scheduled for completion in late 2011, will connect two of the four Metro stations in Tysons. Cerulli likes to say his project is the toughest part of the line. "But don't say I said that, because the guideway is also complicated," he jokes, referring to the elevated section, still 18 months off, that will carry the trains 55 feet above the Capital Beltway.
As the $3.2 billion project was submitted for federal money, many landowners and local officials had hoped for a four-mile tunnel beneath Tysons, but the cost threatened to sink it. "People are always surprised when they find out" a tunnel is still in the plan, says Marcia McAllister, spokeswoman for the rail project.
All summer, Cerulli's crews dug next to the intersection of Route 123 and International Drive so they could build the shorter tunnel's face. Just over the green construction mesh on the side of the road, the tunnel's main ingredients are scattered over a three-acre staging area behind Tysons II Galleria: a concrete-mixing plant, 60-foot steel tubes and girders piled like firewood. "These improve structural strength," Cerulli says, showing off the materials as he might a new car, never mind that an untrained eye might mistake them for junk. At the top of the hole in the earth, he points to a spot where his crew is about to lower a shaft so workers below the surface can breathe fresh air.
Next, it's down a narrow stairway to the face, two circles side by side like giant bicycle wheels that will open the inbound and outbound tubes, each 22 feet wide. Before mining began, workers had to secure the top of each arch. Only then could they begin a tightly choreographed movement of concrete and earth, digging through the soft Piedmont soil three feet at a clip before it could cave back in. About 100,000 cubic yards of sand, gravel and clay will be excavated before the tunnelers emerge in the Route 7 median and the tracks continue along the Dulles Toll Road.
The 2,400-foot tunnel will solve one of the most vexing problems for engineers who designed the first stretch of the new Silver Line: It must navigate Fairfax County's summit, a slope that rises 515 feet above sea level in the middle of Tysons. The climb would have been too steep for the trains, and tracks for the subway would have intruded on the already crowded visual landscape.
The solution was to send the trains underground.
From the look of the drawings taped to the walls in Cerulli's construction trailer, it's a complicated endeavor. The task involves precision at every step, anticipating what-ifs, staying on schedule -- "Saturday's my make-up day," he says -- and methodically ensuring the safety of workers once they start the dangerous work of crawling into small, unstable spaces.
At its peak, the Tysons tunnel will employ 70 laborers and a dozen engineers. The yellow hard hats are a tight and transient community, skilled workers from all over the world who go where the tunnels are. Cerulli, a 45-year-old civil engineer from Gainesville with "20 years pretty much focused on tunnels," handpicked the crew from his last job, the four-mile AeroTrain that will start transporting passengers to gates at Dulles Airport late this year. The work is dawn to dawn, 24 hours every weekday.