Downtown Bellevue tunneling update

Publish Date
Body

Excavation of the downtown Bellevue light rail tunnel began in February 2017 and construction crews have been working 24 hours a day, 7 days a week on tunnel construction ever since. View the photos below for a peek behind the sound wall and a look at the tunnel progress to date. Visit the East Link Extension Flickr page to view construction progress across the 14 mile project.

54-inches in diameter, the large metal pipes provide fresh air ventilation for construction crews working inside the tunnel.

54-inches in diameter, the large metal pipes provide fresh air ventilation for construction crews working inside the tunnel.

Crew members stand at the south tunnel portal entrance, located at 112th Avenue NE and Main Street.

Crew members stand at the south tunnel portal entrance, located at 112th Avenue NE and Main Street.

Soil is removed with excavation equipment called a roadheader, which features a rotating cutting head.

Soil is removed with excavation equipment called a roadheader, which features a rotating cutting head.

Recently exposed soil is stabilized with pressurized concrete before excavation continues. The steel, rib-like structures forming a ring around the tunnel are called lattice girders and provide additional support.

Recently exposed soil is stabilized with pressurized concrete before excavation continues. The steel, rib-like structures forming a ring around the tunnel are called lattice girders and provide additional support.

Background

Sound Transit is building a light rail tunnel under downtown Bellevue between the future East Main and Bellevue Downtown stations. The tunnel will be approximately 2,000 feet long or approximately one-third of a mile. The south portal of the tunnel is located at 112th Avenue NE and Main Street and the north portal will be adjacent to City Hall at 110 Avenue NE and NE 6th Street. Construction of the north portal is anticipated to begin in the summer of 2017. Tunnel excavation will take approximately 24 months to complete.

Because of the short tunnel length, and to minimize impacts on neighboring homes and businesses, Sound Transit is constructing the tunnel using the Sequential Excavation Method (SEM) rather than a tunnel boring machine or digging a large trench that is later covered. SEM removes soil in small sections or bites using excavation equipment. Once the soil is removed, pressurized concrete, called shotcrete, is sprayed on the tunnel's sides, ceiling and floor. Steel lattice girders are installed to provide additional structural support for the tunnel. SEM minimizes disruptions to surface streets, homes and businesses while also reducing noise and dust. Sound Transit used SEM to successfully dig its Beacon Hill light rail station. Learn more about SEM.