Bellevue transit tunnel underway, but no giant drill for this job

Digging has begun on Bellevue’s future downtown transit tunnel, but there’s no Bertha or any other submarine-like giant drill with a famous nickname.

Digging has begun on Bellevue’s future downtown transit tunnel, but there’s no Bertha or any other giant drill with a famous nickname.

Instead, the team will use ordinary construction machines with carving attachments — and occasionally handheld power tools — to dig the tunnel. Every four feet, they’ll stop to spray fast-drying concrete before the soil crumbles, insert a steel lattice shaped like an arc, and spray more concrete into the lattice.

A bird’s-eye view of the East Link tunnel portal where light-rail trains will begin their trip under downtown Bellevue, Washington in 2023 connecting. (Courtesy of Sound Transit)

Atkinson Construction says it will need two years to go about 10 blocks, from Sound Transit’s planned East Main Street Station to the Bellevue Downtown Station. Then another year is needed to construct the final, impermeable tunnel wall.

Train service for the full $3.9 billion East Link route, connecting the International District/Chinatown Station in Seattle to Mercer Island, Bellevue and Overlake, is aimed for June 2023.

After years of route debate and engineering, the digging got underway early this month behind noise walls on the corner of Main and 112th Avenue Southeast.

Sound Transit chose a tunneling strategy called sequential excavation mining instead of its familiar method of drilling twin 21-foot-diameter tubes with rotary boring machines. Such machines recently traveled from Northgate to Husky Stadium, to create a light-rail extension by 2021.

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