A nearly 2,000-ft-long tunnel to carry light rail under downtown Bellevue is more than 90% complete, with an expected final break through to the site of the future Bellevue Downtown Station around July 20.

The tunnel is part of Sound Transit's plan to extend light rail 14 miles from downtown Seattle to downtown Bellevue and the Overlake area of Redmond with 10 stations. All segments of the East Link extension are under construction for a line expected operational in 2023. The following year, Sound Transit will open a 3.7-mile extension farther east.

The tunnel portion of the work, executed by Guy F. Atkinson Construction of Renton, Wash., for a contracted $133 million, is now on track. A mid-July completion would finish the project within 15 months, five months faster than forecast in the baseline construction schedule.

Crews used the sequential excavation method on the 1,985-ft project, a strategy of employing conventional equipment, including an excavator and cutting equipment, to remove soil in small sections. The method provided advantages for the Bellevue project over a tunnel-boring machine or a large cut-and-cover trench by minimizing traffic disruption and reducing noise and dust as well as utility service disruptions. The method required sophisticated continuous monitoring, Sound Transit says, which enabled crews to react in real time to changing ground conditions, increasing the efficiency of the project.

In the process, crews removed soil for the 27-ft, 10-in-deep tunnel over 33 ft wide and sprayed a custom mix of pressurized concrete—shotcrete—on the tunnel’s sides, ceiling and floor. Arc-shaped lattice girders provide additional structural support. Work advanced a few feet at a time to create an ovoid (egg) shape for the tunnel.

A time-lapse video showing excavation of a segment of the tunnel is available here. A video of the nearly complete tunnel is available here.

Work on the tunnel began February 2017 and runs from the future East Main and downtown Bellevue stations under 110th Avenue NE, turning east near NE 6th Street. Overall, crews removed 72,000 cu yds of soil, applied 9,000 cu yds of shotcrete, which contained 63 tons of polyethylene concrete fiber reinforcement, and installed 479 lattice girders.

“We are on the verge of a breakthrough in many senses of the word,” said Sound Transit board member Claudia Balducci in a statement. “As the tunnel nears completion, we are moving ever closer to seeing light rail fully constructed, connecting the east side to the rest of the region and giving our residents and visitors a fast, reliable, traffic-free way to get around.”

As work moves forward on East Link, the Northgate Link stretching from Seattle’s University District to Northgate is scheduled to open in 2021, 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.

Follow Tim Newcomb on Twitter at @tdnewcomb