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Bertha now 70 percent done as 1,000th ring assembled

Watch two WSDOT videos to see what’s happening in Seattle’s basement


Congratulations, Bertha!

Well, Bertha isn’t really wearing those 1,000 rings. The SR 99 tunnel is built of a continuous series of interlocking rings. She helps deliver them like a very large, slow, underground Santa. The tunnel now has the first thousand rings installed. When finished, the tunnel will have 1,426 rings, which means the majority of them are installed. These would be hard to wear considering that most of the ten blocks that make up each ring weigh twenty tons. That’s a lot of concrete segments that have to travel more than a mile from the tunnel entrance to the ring builder rig.

WSDOT published a video describing the amount of effort that goes into placing each segment, about a forty minute job.

Bertha is continuing her climb to the tunnel exit at Sixth Avenue North and Thomas Street. She’s now in Zone 8 (of 10), under Second and Third Avenues. That’s 6,555 feet complete of a total of 9,270 feet.

The tunnel isn’t enough. Before the rest of us can drive through it (spring 2019?) they have to build a double deck highway inside it. That work’s in progress, too. So, even as Bertha digs and sets rings, work crews are following building a pair of stacked roads. Think it’s simple? Even as they build to roads, workers, equipment, and materials have to run back and forth from the face of the tunnel. To keep out of their way, the upper sections are built first. Check out WSDOT’s video to see that dance.

Turning a big, round tunnel into something square enough to drive on takes even more concrete, rebar, and lots of work - and it’s happening beneath Seattle’s feet.