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The double-decker highway that will eventually run beneath Seattle will sit inside a concrete tube that is as tall as a five-story building. The rings that comprise that 2-mile-long tube each have 10 segments, and like everything related to the SR 99 project, there’s a lot of precision that goes into constructing them.

The latest video from the Washington State Department of Transportation shows how the “building blocks” of the tunnel were formed as precast segments at Encon Washington in Frederickson, Pierce County, about 45 miles south of Seattle.

The video showcases the process each curved segment goes through from start to finish.

A 2,000-pound steel reinforcing cage acts as the skeleton for each 36,000-pound segment. Those cages are fitted into a mold which move down an assembly line, where they are filled with about 9 yards of concrete each.

Tunnel gif

Each segment is rigorously sealed to ensure no water can get in behind it and cause leakage in the tunnel. Samples are also taken from each concrete batch to undergo stress testing until they break so that engineers have an accurate measure of compressive strength.

According to WSDOT, Seattle Tunnel Partners crews have installed more than 5,000 curved segments in the tunnel as the outer wall takes shape. Bertha the tunneling machine is approaching the intersection of First Avenue and University Street as it mines north beneath downtown.

Tunnel rings
SR 99 tunnel rings are trucked into position. (Via WSDOT Flickr)
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