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Bertha burrows into Zone 9 and under SR 99

Tunnels passing under tunnels, Seattle’s basement maze

Bertha has made it to Zone 9. Eight done. Two to go.

The tunnel replacing the Alaskan Way Viaduct continues to grow. As of Thursday, January 26, Bertha the boring machine has dug a tunnel 7,218 feet long. The tunnel will be complete when she’s exited after digging 9,270 feet. A little over 2,000 feet to go.


The work was broken up into ten zones of various lengths, depths, and soil conditions. Now, in Zone 9, Bertha is digging a tunnel for SR 99 under the SR 99 tunnel. The old SR 99 tunnel, more commonly called the Battery Street Tunnel was built in the 1950s, and was the underground extension of the elevated highway that has been the Alaskan Way Viaduct. Even then they saw the need for a tunnel, just not one as long as Bertha’s.

Bertha’s tunnel is not alone. She’s also had to dig in the vicinity of other tunnels: tunnels for buses, a railroad, and eleven foot diameter sewer lines. The maze gets more complicated because much of Seattle’s skyline and many of the landmarks are supported by pilings that reach far into the earth to support the city. Bertha’s had to dig below and around them, too. Check the (silent) WS DOT video for an early view of the underground structures that are so easy to ignore.

She should break through in a few months if she continues her pace of several hundred feet per month.


Digging the tunnel is the start. Filling the tunnel with a double-decker highway means work will continue into 2019. Crews are building the southbound roadway (39 percent complete) and the northbound walls (50 percent complete). The northbound roadway can’t start until Bertha hits open air, again; otherwise the tunnel crews wouldn’t be able to get materials to Bertha.

There’s a lot going on under our city.