Seattle Light-Rail Construction Begins

By Jeff L. Brown
Provided with the permission of ASCE - www.pubs.asce.org

Just a few months after opening its first light-rail line-a 1.6 ini (2.6 km) segment in Tacoma, Washington-Sound Transit, the mass transit agency for this region of the state, has begun construction on a longer, north-south system in Seattle that could ultimately link communities all the way from the city's northern suburbs to Sea-Tac International Airport. The partially underground line will include one of the deepest stations ever to be excavated in soil in North America.

After months of uncertainty, the Federal Transit Administration approved a $500-million grant for the $2.4-billion project last fill, clearing the way for construction to begin on the long-anticipated Central Link line. The initial, 14 mi (23 km) segment will connect downtown Seattle to the southern suburb of Tukwila, ending roughly I mi (2 km) north of the airport. Eventually, Sound Transit hopes to extend the line south to the airport and north to the University District and the northern suburbs, for a total of 24 mi (39 kin).

The alignment begins in downtown Seattle in a 1.3 mi (2.1 km) long tunnel that is currently being used by buses and will be retrofitted to accommodate light-rail. The retrofit is a "surgical procedure" calculated to minimize the amount of demolition required, says Joe Gildner, the deputy director of technical services for Sound Transit. Just northeast of the tunnel, an 800 ft (240 m) long cut-and-cover box will be excavated to serve as a turnback facility for the trains. The tunnel retrofit is being designed by a consortium consisting of Parsons Brinckerhoff, Inc., of New York City; Earth Tech, of Long Beach, California; and uRs, of San Francisco. Seattle-based KPFF Consulting Engineers and CH2M HILL, of Englewood, Colorado, are designing the cut-and-cover portion. LTK Engineering Services, headquartered in Ambler, Pennsylvania, is performing the systems engineering for this and all other segments of the project. In the industrial area just south of the downtown tunnel, the light-rail line will share an existing at-grade bus corridor, then veer eastward on an approximately 1,800 ft (550 m) long elevated section over an existing rail corridor. The elevated guideway will consist of 120 ft (37 m) spans of cast-in-place concrete, says Gildner. Dallas-based Huitt-Zollars, Inc., is the civil engineering consultant for this section, which also includes a 156,000 sq ft (14,500 m2) maintenance facility designed by LTK. This short elevated section carries the light-rail line into the Beacon Hill neighborhood, where the project team faces the challenging task of providing service to a community situated on top of a major ridge. The line will enter a 4,300 ft (1,300 m) long twin-tube tunnel to serve a station approximately 165 ft (50 m) below the surface. The ground consists of glacial soils, not rock, so crews will use either one or two earth-pressure-balanced machines to bore the tunnel. The tunnel will have a finished diameter of approximately 18 ft (5 m) and will be segmentally lined with precast concrete.

In addition to the rail tunnels, two large shafts will be excavated-a main access shaft and a shaft for emergency egress and ventilation. Slurry wall techniques will be used to stabilize the walls of the shafts as they are excavated. Auxiliary pedestrian and ventilation tunnels will be constructed using the sequential excavation method-a technique rarely employed at such a depth, Gildner says. The largest of the station excavations will be more than 40 ft (12 m) in diameter. A joint venture of Hatch Mott MacDonald, of Millburn, New Jersey, and Jacobs Engineering Group, Inc., of Pasadena, California, is designing the tunnel, with the Seattle office of the Dr. G. Sauer Company as a subconsultant. After emerging from the tunnel, the light-rail line is again elevated for approximately 2,100 ft (640 m) before descending to an at-grade alignment that passes southward through Rainier Valley. Here the traffic lanes of a major four-lane road will be shifted to either side to make way for light-rail tracks in the center. The corridor, at present typically 90 ft (30 m) wide, will be widened to as much as 141 ft (43 m) at station locations. The civil engineering design of this portion of the project was led by CH2M HILL.

The southernmost 4.5 mi (7.2 km) of the route is again elevated. This portion of the guideway, designed by Hatch Mott MacDonald, will be constructed of precast-concrete segments. When the Central Link opens, in 2009, it will carry passengers to an interim terminus at 154th Street, where they will have the option of boarding buses to proceed to the airport. Sound Transit's goal is to extend light-rail to the airport by 2011. Preliminary engineering for the extension is expected to begin in 2005.