February 27, 2017

Atkinson crews are digging $121M tunnel under downtown Bellevue

Image from Sound Transit [enlarge]

Crews are busy digging another downtown tunnel, but this one isn't in Seattle and no tunnel boring machines are required.

As part of Sound Transit's East Link extension, Guy F. Atkinson Construction is working under a $121.4 million contract to build a 2,000-foot-long light rail tunnel under downtown Bellevue. They are using the sequential excavation method, or SEM.

Crews are excavating at the south portal site at 112th Avenue Southeast and Main Street. So far, they have removed about 15 feet on the top heading section and are on pace to dig 3 to 4 feet of tunnel a day.

When complete, the tunnel will run between future transit stations at East Main and Downtown Bellevue. It will mostly go under 110th Avenue Northeast. The north tunnel portal will be at Northeast Sixth Street.

Project planners chose SEM because the tunnel is short and the method will have less impact on neighboring homes and businesses than using a TBM or digging a cut-and-cover tunnel.

SEM uses an excavator and cutting equipment to take out soil. As soil is removed, shotcrete is sprayed on the tunnel's sides, ceiling and floor. Lattice girders are installed for additional structural support in the tunnel.

A Sound Transit spokeswoman said Atkinson will complete excavation work in about two years, and then another year will be needed for finish work. Rails, systems and transit stations are not included in Atkinson's contract, but Atkinson will install the final tunnel lining.

The completed tunnel will be nearly 28 feet tall and run 30 to 60 feet below ground.

The tunnel was possible due to in-kind contributions of $100 million from the city of Bellevue that included property and permitting.

When East Link opens in 2023, it will extend light rail 14 miles from downtown Seattle to downtown Bellevue and the Overlake area of Redmond, and add 10 stations. All segments of the East Link extension are expected to be under construction by year end.

A video of the tunnel portal and route through downtown Bellevue is at https://vimeo.com/205622183.