More than 700 feet of the 2,000-foot tunnel under downtown Bellevue has been carved out. Light-rail service between Seattle and the Eastside is scheduled to begin mid-2023.
The soil is holding firm and construction remains on schedule below downtown Bellevue, where Sound Transit’s future light-rail tunnel is one-third excavated.
More than 700 feet have been hollowed in the 2,000-foot grotto, between Main Street and Bellevue Transit Center.
From there, the trackway will emerge through the east-facing slope and cross over Interstate 405 on the way to Overlake. Service between Seattle and the Eastside is scheduled to begin mid-2023.
During a tour Tuesday, dirt was scraped out by a heavy excavator, with a rotating arm and five talons that gouged the gray surface ahead. Conversation was drowned out by a growling loader machine that carried scoopfuls outside to a waiting dump truck.
The laborious technique, called “sequential excavation method,” entails digging 4 feet, spraying concrete on the naked earth, then affixing an arched steel lattice, followed by another layer of concrete.
After a slow start, Atkinson Construction is advancing 5½ feet on good days. Digging began in February and will go until early 2019. After that, the team will attach a waterproof membrane, pour a smooth cast-in-place concrete wall and build a hard floor for the trackway, by June 2020.
There’s no Bertha-like mammoth tunnel drill, and not even a standard 21-foot tunnel-boring machine like the ones that dug to University of Washington Station.
It didn’t make sense, given the relatively short Bellevue passage, to build a launching pit as long as a football field, and assemble a boring machine — perhaps a year’s extra work, said Chad Frederick, a Sound Transit project manager. Also, some steel rods protrude into the path from nearby parking garages, an obstacle that can harm a boring machine but not the swiveling scraper.
The soil is behaving so well that crews have stretched out their sequence to scrape out an extra 3 inches per stage, for a total 4 feet, 3 inches, said Jake Cobion, an Atkinson project manager.
“The ground here is phenomenal compared to other parts of town,” Cobion said. “It’s not like building a tunnel along the waterfront, or next to the [sea] wall.”
In Seattle, Highway 99 tunnel contractors had to deal with waterlogged soil, high-pressure groundwater and a sinkhole at the south end, until conditions improved farther inland.
Bellevue favored a tunnel over a surface or elevated alternative, following the 2008 Sound Transit 2 vote approving the line called East Link.
City Council members were unwilling to disrupt traffic on broad east-west boulevards serving downtown. Since then, about 60 surface-level light-rail train collisions with cars or pedestrians in Seattle have demonstrated the value of grade separation.
But the prime corridor doesn’t come cheap.
Atkinson is being paid $123 million just to dig and line the tunnel, not including stations to be built by others. The whole route from International District/Chinatown Station across the Interstate 90 floating bridge to Bellevue and Redmond Technology Center near Microsoft is budgeted at $3.7 billion.
As for taxes, last year’s voter-approved ST3 plan doubles the median household’s annual bill to about $600, to build and run Sound Transit’s eventual network of 116 miles of light-rail plus commuter trains and express buses.
In return, passengers who ride East Link can travel from downtown Bellevue to the University of Washington in 31 minutes, or from Overlake to Bellevue in 10 minutes. A promised ST3 rail extension to downtown Redmond is planned by the mid-2020s.