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SR 99 tunnel
Crews working for Seattle Tunnel Partners assemble rebar for a section of the future west wall of the northbound roadway inside the SR 99 tunnel. (WSDOT via Flickr)

Seattleites caught up in the twists and turns and starts and stops of Bertha, the SR 99 tunneling machine, might lose sight of the fact that trailing behind her boring efforts is a massive roadway-building project. A new video from the Washington State Department of Transportation sheds light on how the new double-deck highway is pieced together.

David Sowers, WSDOT deputy administrator, leads the tutorial on how a tunnel and a roadway can be constructed at the same time. It’s worth noting that Bertha has only travelled about a third of the 9,270 feet that she has to mine. But behind the machine, wall foundations, walls and elevated road surfaces are being put in place.

It all has to be done in a way that allows machinery and materials — specifically the concrete rings that make up the tunnel wall — to continue to be able to reach Bertha. The machine, by the way, is currently stopped for routine maintenance — take a look at the work that goes into that.

Sowers explains how the roadway foundations, or corbels, are built in 54-foot sections. The video shows rebar and concrete work and notes that after the formwork is removed a few days after pouring, the concrete actually cures under wet plastic blankets for another 28 days. The same process follows for the outer walls which eventually support the top deck of the roadway.

SR 99 tunnel
(Click to enlarge) Tunnel highway construction stages. (Via WSDOT)
SR 99 tunnel
(Click to enlarge) How much highway work is completed so far. (Via WSDOT)

The reinforcing steel in the top deck, or southbound lanes, is coated with epoxy to help guard against corrosion. An army of rubber-booted workers is seen in the video, pouring and massaging and smoothing the concrete into place.

The northbound roadway that runs along the lower deck is built at the end of the project and in the interest of time and efficiency, Sowers says, is the one piece of the whole operation that is precast offsite.

In the end there will be enough concrete in the SR 99 tunnel roadway structure to build nine Seattle football stadiums, Sowers notes, as the video shows animated cars cruising through the completed replacement for the Alaskan Way Viaduct.

SR 99 tunnel video
(Via WSDOT YouTube)
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