Subway Update: What's Going on Under Tyson's Corner

Here's the August update from Dulles Metrorail, the folks, building the new subway line from East Falls Church to Reston.*

Construction of Phase 1 of the Dulles Corridor Metrorail Project has moved at a rapid pace since it began in March 2009. Roads have since been widened, concrete segments have been placed between piers, and cranes have taken over the skyline. However, the project has not only implemented aerial designs that can be seen while commuting, an abundance of fascinating work has been happening underground as well. 

The Dulles Corridor Metrorail Project includes two 2,400-foot tunnels that will carry trains moving in opposite directions beneath Tysons Corner. These tunnels connect the Tysons Central 123 Station and the Tysons Central 7 Station by traveling under International Drive to Route 7 and ending between SAIC and Marshalls. Coincidentally, these tunnels travel through the highest natural point in Fairfax County.
 
Two different techniques were used in creating the tunnels. First, the New Austrian Tunneling Method, also known as NATM, was used to excavate the centermost 1,700 feet of the tunnel. The remaining 700 feet, approximately 400 feet on one end and 300 feet on the other, were constructed by using a cut-and-cover process. These sections connect the NATM tunnels to the stations.
 
The cut-and-cover process is an excavation method that begins by digging a large trench in the ground. While this hole is open, construction workers place wooden boards, or lagging, on the outer walls to keep the ground from caving in. The lagging is supported by many steel beams that hold everything in place. When the mining is complete, the ground overhead is eventually restored, according to Rail Project officials.
 
The benefits of using this process were far more than alternate methods. Cut-and-cover tunneling is predominantly used for constructing shallow tunnels, which made it an ideal choice for assembling these portions of the Tysons tunnel, where depths range from seven to 30 feet below the surface. Also, the cut-and-cover process is one of the more cost-efficient tunneling methods, according to rail project officials.
 
Crews have many risks and difficulties in creating this tunnel. Cut-and-cover tunneling requires a large amount of room to excavate, which in turn makes it complicated to build in this area. So far, the project has constructed this portion of the tunnels safely and without incident.

Background:

Dulles Metrorail Project Overview: The Metropolitan Washington Airports Authority (MWAA) is constructing a 23-mile extension of the existing Metrorail system, which will be operated by the Washington Metropolitan Area Transit Authority from East Falls Church to Washington Dulles International Airport west to Ashburn.

The extension will serve Tysons Corner, Virginia's largest employment center, and the Reston Herndon area, the state's second largest employment concentration. And, it will provide a one-seat ride from Dulles International Airport to downtown Washington.

Phase 1 and 2: The project will include 11 new stations. It will be built in two phases. Phase 1 will run from East Falls Church to Wiehle Avenue on the eastern edge of Reston. It will include four stations in Tysons Corner-Tysons east, Tysons Central 123, Tysons Central 7 and Tysons West. Construction will begin in March of 2009 and it will be completed in 2013.

Phase 2 will run from Wiehle Avenue to Ashburn in eastern Loudoun County. A construction date has not been set for the extension that will serve Reston Town Center, Herndon, Dulles Airport, Route 606 and Ashburn.

MWAA and Dulles Transit Partners in March 2008 signed an updated $1.6 billion fixed-price contract to build the project, keeping the costs of the project to $2.6 billion.

*McLean Patch continues to hope that one day (before we are in a wheelchair) we can board the subway in McLean, go to Dulles Airport and fly to Paris.