With more than 27 million passengers a year, the Dulles International Airport is one of the busiest hubs in the country. The Metropolitan Washington Airports Authority, project owner, created a scheme to maximize transportation efficiency at the airport via an extensive subway system. The new rail line, called the Airport Train System (ATS), was designed to eliminate the previous system of rubber-tired surface vehicles, which added to airport congestion. The USD $1.2 billion project includes a fleet of 29 rail cars capable of traveling up to 68 km/hr (42 mph) between stations.
While excavated using a variety of methods, a 560 m section needed to be TBM-driven due to the location of an active concourse directly overhead. The Atkinson/Clark/Shea JV, contractor for the twin tunnels, awarded a complete contract to Robbins in 2004for two 6.4 m (21.1 ft) diameter TBMs, back-up systems, cutters, and spare parts.
The tunnel passed through mudstone, sandstone and siltstone geology from 32 to 48 MPa (4,700 to 7,000 psi) UCS. Conditions in the tunnel required immediate grouting at the tail shield to limit settlement.
The owner specified Single Shield TBMs due to the short tunnel lengths, tunnel lining requirements, and immediate grouting requirements. Robbins refurbished the Single Shield TBMs, which were originally built in 1985 for the Taipei Metro Subway. Each machine received a new cutterhead, back-up system, thrust controls, and segment erector.
The machines were fitted with 15″ diameter disc cutters to bore in relatively soft rock. The back-up system was designed as an open gantry system for single-track muck cars.
Each of the TBMs bored two tunnel drives, 460 m and 100 m (1,500 ft and 335 ft) in length. The TBMs bored the first 460 m (1,500 ft) section and were then walked through the 180 m (600 ft) long Concourse B station, excavated by cut and cover, to their second heading. While the TBMs bored, precast concrete segments were erected within the TBM tail shield to form the tunnel lining. Both TBMs had to maneuver through sharp turns with radii of 125 m (410 ft) as they approached the Main Terminal.
The machines broke through in September 2006. Many other methods including the New Austrian Tunneling (NATM) method and digger shields were used on the project as well. Phase 1 is slated for completion in 2009 and will provide rail service from both Concourse B and Concourse C to the main Terminal.