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Why Bellevue didn’t pick a Bertha to dig its light rail tunnels

Bertha before the boring machine mined under Seattle. (WSDOT)

Sound Transit contractors are tunneling under downtown Bellevue, but you won’t find a big tunnel-boring machine like Seattle’s Bertha.

The light rail tunnel will connect the East Main light rail station at 112th Avenue SE and Main Street to the Downtown Bellevue light rail station at NE 6th Street, near the existing Bellevue Transit Center.

Sound Transit is using the Sequential Excavation Method (SEM), a way of digging shorter light rail tunnels. On the Eastside, they are looking at a shallow, half-mile-long tunnel — not like the Northgate Tunnel which will be 3.5 miles long.

SEM minimizes the disruption to surface streets and buildings while reducing noise around the construction site. The other big reason SEM was picked for the Eastside?

“SEM doesn’t require the same amount of muck-hauling (taking away dirt that has been excavated) that TBM (tunnel boring machine) mining does because far less dirt is being mined,” said Kimberly Reason with Sound Transit.

Bellevue vs. Seattle tunneling

People in the Northgate and Roosevelt neighborhoods know all about that, Reason points out.

“The Northgate Link tunnels, when open in 2021, will serve two underground stations 85-90 feet underground,” she said. “So we needed to use (tunnel boring machines) for those.”

Sound Transit used SEM to dig the Beacon Hill light rail station several years ago and now some of those same experts are working on the Bellevue tunnel, which is set to open in 2023.

According to Sound Transit, the Sequential Excavation Method works like this:
• First, excavators begin digging out a small section of the tunnel. That soil is removed.
• Reinforcing lattice girders are installed.
• Pressurized concrete, called shotcrete, is sprayed onto the tunnel sides, ceiling and floor.
• This process is repeated. Finally, the entire tunnel is covered with waterproofing and an additional layer of one-foot thick concrete.

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