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SR 99 tunnel’s lower deck arrives as upper deck nears completion

The tunnel’s on track to open in 2019

Crews building walls for the State Route 99 tunnel.
Courtesy of WSDOT

The tunnel that’s set to replace the Alaskan Way Viaduct is coming along beneath downtown, with the tunnel’s upper deck, which will eventually be a southbound roadway, 90 percent complete. Friday, the first pieces of the tunnel’s lower deck, which will eventually be the northbound road, arrived in Seattle for assembly.

While the upper deck was constructed using a cast in place method—crews assembled the structure and poured concrete on-site—the lower deck is different. The 1,152 panels that will eventually make up the deck were assembled in Tacoma over the course of 2017. Crews will start installing those panels from about two thirds of the way through the tunnel roughly underneath First Avenue and Virginia Street, and work their way south.

Each panel is about 32 feet wide, or as the Washington State Department of Transportation (WSDOT) explains, the width of two traffic lanes and a shoulder.

Four different kinds of panels all serve a different purpose. A specialized crane will lift the panels and place them on supports in the tunnel. Then crews will tighten the fit, add bolts, and pour concrete.

Sections of State Route 99’s lower roadway arrive in Seattle for assembly.
Courtesy of WSDOT

Bertha the tunnel-boring machine officially finished the length of the two-mile tunnel, which will take cars past downtown along State Route 99, on April 4. That was a few years behind the original schedule, which estimated the tunnel would open in December 2015.

The tunnel project was a point of political contention, with many alternative transportation activists, including mayoral candidate Cary Moon, pushing for a surface-street option with transit accommodations.

The new roadway is set to open in early 2019. After it opens, the aging Alaskan Way Viaduct will come down.