Shani Wallis, TunnelTalk
- It is to be an underground station at Dulles International Airport on the Washington DC Metro extension into Virginia. The subsurface alignment was selected last week by the Metropolitan Washington Airports Authority (MWAA) over an aerial route alternative, but is not the deep mined option as originally designed.
Underground alignment selected over the elevated option for the Metrorail station at Dulles Airport
- A less expensive cut-and-cover station stop at the airport replaces a deep NATM mined tunnel and TBM running tunnels to save $330 million on the original $3.83 billion estimate for the 12-mile Phase 2 Metrorail extension to Dulles and on to Loudoun County in Virginia. Savings are accrued by reducing the length and depth of the tunnels, allowing air conditioning to be delivered from an existing airport facility and relocating a necessary electrical substation to above ground.
Cut-and-cover station and tunnels (green) selected over elevated option (pink)
- The 12-mile Phase 2 extension runs mostly in the median of the airport's express toll highway and includes five additional stations between Wiehle Avenue Station, at the end of Phase 1, and Route 772 in Virginia's Loudoun County. The airport station sits between the north parking garage and the main terminal and beneath the existing cut-and-cover pedestrian tunnel between the two. The elevated option was aligned in front of the multi-storey parking garage and connected also to the existing pedestrian walkway tunnel, which was designed and built initially to accommodate the planned underground metro station connection.
- With the underground airport station option confirmed at the MWAA's Board meeting last week, the next steps will be to move forward in completing additional engineering work and the federal and local approval processes.
- In the meantime, work on the 12-mile route and five stations of the $2.7 billion Phase 1 section of the extension is now more than 30% complete. Advanced as a PPP agreement with the Dulles Transit Partnership, a JV between Bechtel and the Washington Group, the project extends the metro network from the East Falls Church Station on the existing Metro's Orange Line, through the financial and commercial area of Tysons Corner and on to the Wiehle Avenue Station (Fig 1).
Metrorail extension with route through Tysons Corner (inset)
- Despite vigorous efforts to have the route aligned underground, the four-mile detour through Tysons Corner is on elevated guideway with three aerial stations and a short 2,400ft (750m) section of underground work. Instead of building the full section through proposed TBM-bored tunnels and TBM-bored stations of the Barcelona method, the short underground section comprises 1,700ft (518m) of twin tube NATM mined tunneling between a total 700ft of cut-and-cover work at the portals and a partially underground station (Fig 1 insert).
- Excavation of the NATM running tunnels was completed in October and November 2010 and work is progressing to complete the Phase 1 works towards an opening of services on first section of the extension in 2013.
Excavation of the Tysons Corner NATM tunnel
- Planning of the 24-mile Dulles Metrorail project by the Commonwealth of Virginia and the MWAA began in 2002. In 2007, MWAA took over responsibility for realizing the project and began management of the Phase 1 works. MWAA also took over operation of the Dulles Toll Road from the Virginia Department of Transportation. Under the 50-year agreement, part of the toll revenue will be applied to funding the Metrorail project.
- On the security side of the airport, MWAA continues its association with tunneling to improve operations and eliminate the mobile-lounge vehicles that carried passengers from the main terminal to mid-field concourses. First, the walkback pedestrian link to Terminal B was built in the early 2000s as a shallow depth NATM excavation.
- NATM was then also used to build sections of the AeroTrain people mover system that carries passengers to gates in concourse buildings A, B and C (Fig 2). The Atkinson/Clark/Shea JV completed the east AeroTrain tunnels from the main terminal through to concourse A and C stations and to the site of future terminal stations.
Walkback tunnel and stage one of AeroTrain
- These tunnels included 1,730m cut-and-cover work, 1,320m of TBM tunneling, and 251m of NATM tunneling with open-cut stations. Clark/Shea JV completed the west tunnels that comprised 573m of NATM work, 679m of cut-and-cover, and an open cut station.
- Services on the first phase of the AeroTrain opened in January 2010. Further tunneling will be contracted to complete the full circle service when new terminals are built and opened.
- As well as managing Dulles International Airport and Washington DC's local Reagan National Airport, MWAA has moved into the field of managing toll road facilities and manage the building and eventual operation of the Metrorail extension. The budget for operating the airports for 2011 is $950 million while the budget for developing the Metrorail and managing the toll road for 2011 is more than $1 billion.
- Tysons Corner in the air on Dulles link - TunnelTalk, August 2008
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