Progress on the SR 99 tunnel continues as crews work to build the road beds within the tunnel. The Washington Department of Transportation provides updates on the project via press release and the occasional time lapse video.

Their latest video shows how the work is done.

The tunnel's lower roadway carries the northbound lanes, hence the ceiling is the bottom of the southbound roadway. That configuration is the reverse of the traffic pattern on the current viaduct. To build the roadbed, trucks bring pre-made panels into the tunnel where a mobile crane lifts and installs each panel. The joints between each panel are sealed with grout. Once the grout has cured to full strength, crews lock the panels together with steel cables. Each panel weighs 22 tons, and the 1.7 mile long road bed is made up of 1,152 panels.

You can learn more at www.alaskanwayviaduct.com