East Link tunnel excavation nears completion in downtown Bellevue
Sound Transit’s East Link light rail tunnel is more than 90 percent complete, with crews expected to break through to the site of the future Bellevue Downtown Station in mid-July. The agency today released video of the tunnel and the final excavation work in progress to complete it.
Crews are on track to complete the tunnel within 15 months – five months faster than forecast in the baseline construction schedule.
Tunnel construction is taking place using a time-tested process known as the Sequential Excavation Method (SEM). SEM employs conventional equipment including an excavator and cutting equipment to remove soil in small sections or bites.
This method provides a number of advantages for the Bellevue project over a tunnel-boring machine or digging a large cut-and-cover trench. Benefits include minimizing disruption to traffic, reduced noise and dust impacts to neighboring residents and businesses, and reduction of utility service disruptions. It also employs sophisticated continuous monitoring, which enables crews to react in real time to changing ground conditions and increases the safety and efficiency of the work.
SEM work is a carefully choreographed process. Following removal of soil, crews spray a custom mix of pressurized concrete, called shotcrete, on the tunnel’s sides, ceiling and floor. They install arc-shaped lattice girders to provide additional structural support for the tunnel. Work advances a few feet at a time, with crews carefully excavating in sections to create the tunnel’s ovoid shape.
A time-lapse video showing excavation of a segment of the tunnel using the SEM method is available at https://vimeo.com/229918007.
Crews have been working on the tunnel since February 2017. Excavation started at the south portal site at 112th Ave. NE and Main, and when complete, will be approximately one-third of a mile long, running between the future East Main and Downtown Bellevue stations under 110th Ave. NE, turning east near NE 6th St.
- Tunnel is 1,985 linear feet long, or approximately one-third of a mile.
- Height of the tunnel is 27 feet, 10 inches.
- Overall width (north- and south-bound sides) is 34 feet.
- Each side is 16 feet, 3 inches wide, with a center dividing wall, which is 18 inches thick.
- Tunnel is from 12 to 30 feet below the surface.
- Volume of soil removed: 72,000 cubic yards.
- Total number of lattice girders used: 479.
- Total volume of shotcrete applied: 9,000 cubic yards.
- Shotcrete contains 63 tons of polyethylene concrete fiber reinforcement.
In-kind contributions of $100 million by the city of Bellevue, including property and utility relocation, enabled the East Link project to include the tunnel. An MOU executed by the Sound Transit Board in April 2015 formalized this contribution.
“We are on the verge of a breakthrough in many senses of the word,” said Sound Transit Boardmember Claudia Balducci. “As the tunnel nears completion, we are moving ever closer to seeing light rail fully constructed, connecting the eastside to the rest of the region and giving our residents and visitors a fast, reliable, traffic-free way to get around.”
A video of the nearly complete tunnel is at https://vimeo.com/276911242.
East Link will extend light rail 14 miles from downtown Seattle to downtown Bellevue and the Overlake area of Redmond via I-90, with 10 stations. By the end of 2017, all segments of the East Link extension will be under construction, and the entire line will be operational in 2023. The following year Sound Transit will open a 3.7-mile extension further east to new stations in Southeast Redmond and downtown Redmond under funding approved in November 2016.
The Northgate Link light rail extension that is currently under construction will reach Seattle’s University District, Roosevelt and Northgate areas in 2021. Additional Link extensions that are nearing construction will reach north to Lynnwood in 2023 and south to Federal Way in 2024. Later extensions to form a 116-mile regional system will reach Everett, Tacoma, West Seattle, Ballard, South Kirkland and Issaquah. By 2040 Link trains should carry approximately 600,000 riders each weekday.
For more information on the East Link Extension see: www.soundtransit.org/Eastlink