Shown here is construction of the Dulles Corridor Metrorail Extension near the tie-in for the new line with Metro’s existing Orange Line at Interstate I-66 and the Dulles Connector Road.
The Metropolitan Washington Airports Authority (MWAA) is currently constructing a 23-mi. (37 km) extension of an existing Metrorail system (the Metro Orange Line). It will be operated by the Washington Metropolitan Area Transit Authority from East Falls Church to Washington Dulles International Airport west to Ashburn.
Areas served by the extension will include Tysons Corner, which is reportedly Virginia’s largest employment center, and the Reston Herndon area, which includes the state’s second largest employment concentration. It also will supply transportation from Dulles International Airport to downtown Washington.
According to Marcia McAllister, communications manager of the Dulles Corridor Metrorail Project, its purpose is to “provide high-quality, high-capacity transit service in the Dulles Corridor. New Metrorail service in the corridor will result in travel time savings between the corridor and downtown D.C., expand the reach of the existing regional rail system, offer a viable alternative to automobile travel, and support future transit-oriented development along the corridor.”
Most of the extension will be constructed in the median of the Dulles International Airport Access Highway and Dulles Connector Road, but the alignment also serves Tysons Corner and Dulles Airport.
The project includes 11 new Metrorail stations, a new rail yard on Dulles Airport property and improvements to an existing rail yard at the West Falls Church Station.
According to the Metrorail Web site, the alignment was selected because it offers the highest ridership potential with the fewest impacts on residential areas and the natural environment.
McAllister noted that the Airports Authority and Dulles Transit Partners, the design build contractor, signed an updated $1.6 billion fixed-price contract in March 2008 to build the project.
The Metrorail will be built in two phases.
Phase 1 will run from East Falls Church to Wiehle Avenue on the eastern edge of Reston, and covers 11.7 mi. (18.8 km). It will include four stations in Tysons Corner. Construction began in March 2009 and will be completed in 2013.
Phase 2 will run from Wiehle Avenue to Ashburn in eastern Loudoun County, and will cover 11.5 mi. (18.5 km). A construction date has not yet been set for this extension, which will serve Reston Town Center, Herndon, Dulles Airport, Route 606, and Ashburn. Preliminary Engineering is currently under way for this phase.
Phase 1 involves a total of 6 mi. (9.6 km) of aerial track — three miles inbound and three miles outbound. As of July 3, a total of 114,950 cu. yds. (87,885 cu m) of concrete had been installed.
In the six miles, there will be five at-grade new bridges and segmental bridge construction, which requires concrete segments that are being made at a site at the Dulles Airport. As of July 3, 1,178 of the 2,769 segments needed to be fabricated.
Three miles each of inbound and outbound guideway will be aerial, with an average above-ground height of 35.7 ft. (10.8 m). The tallest point of the guideway is reportedly 55 ft. (16.7 m), which is above the southbound entrance from Route 123 to I-495. The guideway is constructed by connecting more than 2,700 concrete segments — each weighing 25 to 40 tons (22.6 to 36.3 t) — using trusses.
The precast segments are being fabricated by Rizzani de Eccher, which is headquartered in Italy.
“The aerial team consists of leadership from Dulles Transit Partners (design/build contractor) and Rizzani’s U.S. division,” a Metrorail spokesperson said. “This team brings a combined experience of 50-plus years working on worldwide aerial and bridge projects.”
McAllister noted that the inbound and outbound tunnels that are being built in the heart of Tysons Corner will connect below the highest natural point in Fairfax County, at the intersection of Route 123 and 7. They will connect two rail stations, Tysons Central 123 and Tysons Central 7, and are expected to be complete by the end of the year.
She reported that the tunnels are being built using the New Austrian Tunneling Method (NATM), in which tunnels are excavated and initial lining construction is completed in short, sequential portions. This also is known as the Sequential Excavation Method (SEM) or Sprayed Concrete Lining (SCL) method.
“This cost-effective method of tunneling is ideal for the existing ground conditions, while allowing for adaptations to any changes in ground conditions,” McAllister said.
The length of each tube, including NATM and cut-and-cover tunnels, is approximately 2,400 ft. (731.5 m), with 700 ft. (213.3 m) of cut-and-cover and 1,700 ft. (518.1 m) of NATM method. The tunnel is about 1/46th of the entire alignment of Phase 1.
McAllister noted that this method successfully limited surface settlements and deformations to sensitive utilities and structures, and also allowed Route 123 and International Drive to remain open to traffic throughout excavation.
Tunnel depth varies between 7 and 30 ft. (2.1 and 9.1 m) and approximately 100,000 cu. yds. (76,455 cu m) of earth will be excavated in all. The excavated material is then transported to Dulles Airport property.
State-of-the-art equipment required for excavation and support includes a Boomer E2C 18 Drill Jumbo with an automated pipe arch canopy system. The jumbo is manufactured by Atlas Copco in Sweden. It was used to construct an arched roof over the tunnel to allow miners to remove rock and soil to create the tunnel opening. The jumbo was customized to drill and case holes with pipe, then grout the support pipes in place.
Additional equipment included tunnel excavators, a Shotcrete batch plant, and Shotcrete robots for application. State-of-the-art equipment and the Total Station Method were used to meticulously monitor in-tunnel deformations and surface settlements.
At peak, 70 craft personnel were on the site.
According to Leslie N. Pereira of Dulles Transit Partners, that particular group has used 750 material and equipment suppliers to date. A total of 188 different DBE have been used since the beginning of the project, and 170 first-tier subcontracts for services or construction work have been issued to date.
For the on-site construction work, there were 250 different subcontractors since the beginning of the project, and 157 were working on-site in July. CEG