Tunnel-boring machines bearing catchy nicknames are popular when it comes to local transportation infrastructure projects.
The queen of them all is Bertha, Washington State Department of Transportation’s 6,700-ton, 57.5-foot-wide machine burrowing a two-mile long tunnel to replace the Alaskan Way Viaduct through downtown Seattle. Two 600-ton machines (formerly named Pamela and Brenda, but renamed Machine 1 and Machine 2 after being confused with Bertha) are digging 3.6-mile-long tunnels to connect Sound Transit’s Link light rail route between Husky Stadium and Northgate in Seattle. And a 360-ton machine (nicknamed the “Emerald Mole”) was used about a decade ago to hollow out two nearly-mile-long tunnels through Beacon Hill and connect light rail service between Seattle’s Rainier Valley and SODO neighborhoods.
It’s a little different in Bellevue, where a contractor is using comparatively simple and conventional tools to dig a 2,000-foot-long tunnel beneath the city’s central business district. The project is part of the 14-mile, $3.7 billion East Link light rail extension that will connect Seattle to Mercer Island, Bellevue, and Redmond by summer 2023.
Digging is being completed through a process Sound Transit officials call the Sequential Excavation Method, which removes soil in small sections using an excavator and cutting equipment. Once the soil is removed, pressurized concrete, or “shotcrete,” is sprayed on the tunnel’s sides, ceiling, and floor. Additional structural support for the tunnel is provided by installing lattice girders. Sound Transit officials said this method of tunnel construction aims to minimize disruption to traffic, reduce noise and dust impacts to neighboring businesses and residents, and eliminate utility service disruptions compared to more conventional cut-and-cover tunnel construction.
Sound Transit awarded a $121 million contract to Atkinson Construction to dig the tunnel.
“We don’t have Bertha going through here, so we dig it out by hand, more or less,” said Bellevue Mayor John Stokes. “It’s not really true, but it’s a good story.”
Stokes joined Sound Transit, King County, and other City of Bellevue officials on Friday at the tunnel’s south portal construction site, located at 112th Avenue Northeast and Main Street, to mark the beginning of the tunnel-boring phase of the project. When completed, the nearly 28-foot-tall underpass will stretch approximately one-third of a mile between the future East Main and Downtown Bellevue stations under 110th Avenue Northeast, before the rail line hooks east near Northeast 6th Street and continues toward Wilburton.
“The schedule for this job is to turn under now and dig for about two years,” said Atkinson Construction’s Northwest Region senior vice president Bob Adams. “We’ll be advancing at a rate of about four feet at a time, secure that four feet, and then go ahead another four feet. That will take about two years to get to the other end of the tunnel, and then about a year to put a permanent, reinforced, concrete waterproof liner in the tunnel and turn it over to the next contractor to put in the rails, the finished electrical and mechanical systems, build the stations, and make it a finished product ready for the user.”
Sound Transit’s Central Link light rail service began in July 2009 with service between downtown Seattle and Tukwila, and expanded south in December 2009 with service to Seattle-Tacoma International Airport, north in 2016 with service to Capitol Hill and the University of Washington, and south again with service from Sea-Tac Airport to Angle Lake. Light rail service is scheduled to reach Seattle’s University District, Roosevelt, and Northgate neighborhoods in 2021. Sound Transit began light rail service in downtown Tacoma in August 2003.
“I’ve had a chance to talk to folks who live or work around the University District or Capitol Hill in the last year, and I can tell you that having this kind of fast, reliable, grade-separated transit that they have with light rail there, it changes people’s lives, it changes people’s day-to-day existence in how they get around and how they get from one place to another,” said former Bellevue mayor and city council member Claudia Balducci, who currently is a Sound Transit board member and King County council member. “That has been the vision here for Bellevue for over a decade now. I’m really excited to see the steps that are now happening with people out on the ground, digging the tunnel and making the light rail a real thing. I’m super excited to be here for this next big step as we move toward great transit on the Eastside.”