The largest contract of the latest phase of Seattle’s light rail transit system, construction of the Beacon Hill tunnel and station, involves a highly complex deep mining process and some of the most sophisticated tunneling technology in use today. The tunnel is part of the Central Puget Sound Regional Transit Authority’s (Sound Transit’s) Link Light Rail, a $2.4 billion, 10-year project to link downtown Seattle and Tukwila with the Sea-Tac Airport.
The new light rail system is a critical element of the region’s long-term transportation network, intended to provide a high-capacity, congestion-free transportation route that serves some of the area’s largest population and employment centers. The program began in 1996 when voters in the Central Puget Sound region of Washington State approved local financing for a regional public transit system.
An initial 1.6-mi. (2.5 km), five-station Tacoma segment opened on Aug. 22, 2003, providing residents and visitors with a new way to get to Tacoma’s theater district, downtown office area, Union Station and the University of Washington.
In November 2003, Sound Transit broke ground on the initial segment of the Central Link, a 14-mi. (22.5 km) segment that begins at Westlake Station in the downtown Seattle transit tunnel and ends at the Tukwila/International Boulevard station, just north of the airport.
In addition to downtown Seattle and Tukwila, this segment will serve Seattle’s industrial area (SODO), Beacon Hill and Rainier Valley. The initial 14-mi. line, including the Beacon Hill tunnel and station, connects the cities of Seattle and Tukwila with Sea-Tac. Revenue service is expected to commence in 2009.
Project team members for this contract include Sound Transit, owner; Parsons Brinckerhoff, construction manager; Obayashi Corp., general contractor; Shannon & Wilson Inc., geotechnical consultant; Hatch Mott MacDonald/Jacobs, designer.
Construction of the Beacon Hill tunnel and station segment begins just south of downtown Seattle and runs through Beacon Hill from west to east, then turns south along Martin Luther King Jr. Way where it ties into an adjacent contract. The primary components of the segment, known as the C710 contract, include:
• Two 21-ft.-diameter (6.4 m) tunnels, each approximately 4,300 ft. (1,310 m) long.
• A 165-ft.-deep (50 m) mined station with a 52-ft. (15 m) diameter main shaft, a 30-ft. (9.1 m) diameter ancillary ventilation shaft, two 400-ft.-long (121 m) platform tunnels, two 64-ft.-long (19.5 m) concourse cross “adits,” two smaller platform cross “adits,” two transverse ventilation “adits,” and one longitudinal ventilation “adit.” (Adit is another term used for a tunnel.)
• 2,100 ft. (640 m) of cast-in-place concrete guide-way on the east side of Beacon Hill.
• A 400-ft.-long elevated station (to be known as Mount Baker Station) that ties into the elevated guide-way.
According to Rick Capka, resident engineer of PB, the project’s construction manager, the nature of the site and its configuration have presented numerous construction challenges.
“The station, which will feature two 400-foot-long platforms, is being mined in a densely populated urban setting through some of the most difficult soft ground conditions in the U.S.,” he explained. “This includes highly variable glacial deposits with water-bearing sands and silts about 50 feet below multiple perched water tables.”
A major construction challenge resulted from unexpected sand and water encountered at the Beacon Hill Station site. As a result, to avoid the unfavorable ground conditions, Sound Transit ultimately decided to shift the location of the station tunnels. Implementing this change required quick collaboration on the part of the entire project team — owner, designer and the contractor.
The station shift involved relocating the two platform tunnels 88-ft. (26 m) to the west and shifting the east transverse ventilation adit to line up with the ancillary shaft. These moves placed the tunnels on more favorable ground, and ultimately avoided a significant amount of ground pre-treatment known as jet grouting, in which columns of grout are injected into the ground in order to help stabilize unfavorable soils that would otherwise cause difficulty during excavation.
Managing the Process
The Beacon Hill Station tunnels were excavated using a technique known as the sequential excavation method (SEM). SEM entails excavating small sections of tunnel in a prescribed sequence, and then supporting the excavated ground with a combination of steel arch supports or lattice girders, welded wire mesh, and shotcrete. As the ground is excavated, a system of instruments is monitored in real-time to measure ground movement, and the ground conditions in the face of the excavation are recorded, enabling engineers to make quick decisions regarding required support in the tunnel.
In addition to the use of steel arches and shotcrete to support the excavation of the tunnels, the contract provides a variety of SEM “toolbox items,” such as a barrel vault canopy, grouted pipe spiling and rebar spiling for the contractor to use as needed.
Ground improvements from the surface, including jet grouting and dewatering wells, reduced the presence of sand and water in the face of the tunnel excavations.
After 23 months, the contractor completed the sequential excavation of the station tunnels in May 2007. The excavated volume of the station tunnels is approximately 50,000 cu. yds. (38,200 cu m), with a variety of geometries and cross sections, ranging from 235 sq. ft. (21.8 sq m) to 1,670 sq. ft. (155 sq m).
Another key component of the tunneling is a 300-ft. (91 m) long, 21-ft. (6.4 m) diameter, 345 ton (313 t) Mitsubishi Heavy Industries earth pressure balance (EPB) tunnel boring machine (TBM) to carve out the twin tunnels.
Including the trailing equipment, the tunneling equipment weighs 650 tons (589 t) and is 300 ft. (91 m) in length.
“The initial launch of the TBM on the southbound tunnel from the site’s west portal was difficult,” Capka noted.
The contractor had to cautiously advance the first 50 ft. (15 m) of the tunnel through landslide debris and loose to stiff clays and silt across the full face of the TBM.
However, the real challenge during the first tunnel drive occurred approximately 300 ft. (91 m) prior to the Beacon Hill Station. The TBM encountered a “mixed face” condition, in which a layer of sand was overlaying a layer of till and till-like deposits. Attempting to drive through the path of least resistance, the TBM climbed into the sand layer jeopardizing the tunnel alignment. With attention focused on the problem and strong collaboration between the contractor, owner and construction manager, the contractor got the TBM back to the correct alignment before reaching the station.
The first 350 ft. (106 m) of the TBM’s drive was completed in six weeks by crews working 10-hour day shifts. Day shift crews completed the remaining 1,345-ft. (409 m) drive in 12.5 weeks, reaching the Beacon Hill Station in July 2006. After completion of the SEM mining of the station’s platform tunnel, the TBM was moved across the station and relaunched on the east side, and where it mined the remaining 2,610 ft. (796 m) in 20 weeks, ultimately completing the first tunnel in May 2007. At the project’s peak, the contractor’s crews totaled 250 employees.
The initial segment of the light rail system is projected to open for revenue service in mid-2009. As it progresses, design is under way to extend the light rail north to serve the Capitol Hill, University District, Roosevelt and Northgate neighborhoods. There also is a planned 3.15-mi. (5.1 km) link that will run from downtown Seattle to the University of Washington with stations at the Capitol Hill and on the campus on the University of Washington, near Husky Stadium. CEG
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