Metro digs deep to land at Dulles Airport

Wednesday, November 4, 2009

Workers in Northern Virginia are building a new subway all the way to Dulles International Airport. The trains will pass through Tysons Corner, a busy place where shopping malls, tall office buildings and a lot of traffic come together.

When engineers started to design the Silver Line a few years ago, they faced a tricky problem. Tysons is built on a gigantic hill that rises 515 feet (that's just 40 feet less than the Washington Monument) above sea level. The hill is too steep for the trains to climb without making passengers feel sick from the angle they would be forced to sit at.

What should the engineers do?

They decided that a tunnel could carry the subway underneath Tysons' stores, restaurants, hotels and offices. At first, a lot of people wanted a tunnel four miles long so the trip through Tysons could be all underground. But that was too expensive. So they decided to make the tunnel a half-mile long, starting where Route 123 and International Drive meet and coming up on Route 7. (When it's finished in 2016, the Silver Line will run 23 miles and cost more than $5 billion.)

Construction of the tunnel started last month. But before workers could blast through the soil, they had to dig a giant hole using cranes and backhoes to create the tunnel's face, or entrance. There will be two side-by-side tubes, their faces looking like giant bicycle wheels: One tube will carry trains into Washington; the other will be for trains heading to the airport. Workers are using expensive machines that can do just about anything. Listen to some of their funny names. The ITC 312 "knuckle boom" from Germany; the Putzmeister Sika PM 500 from Spain; the ETC18, a.k.a. "drill jumbo," from Sweden.

Once the machines dig through the soil, workers need to quickly harden it so the tunnel doesn't cave in. For that they spray a special material called "shotcrete."

The tunnel is being worked on 24 hours a day. If everything goes as planned, it will be completed in 2011.

Here are some fun things about the tunnel to think about if you drive through Tysons Corner with your family:

-- About 100,000 cubic yards of earth made of sand, gravel and clay will be excavated (dug up).

-- At its deepest, the tunnel will be 30 feet underground.

-- It will be 2,400 feet long.

-- It will cost $85 million to build.

-- At the peak of construction, 70 specially trained tunnelers and a dozen engineers will be on the job.

-- Workers moved two miles of utility lines that were in the path of the tunnel so they could excavate the soil.

-- About 3,500 cars and trucks an hour drive through a portion of Tysons where the tunnel will be.

-- Lisa Rein

© 2009 The Washington Post Company