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00000181-fa79-da89-a38d-fb7f2a000000Bertha, the world's biggest tunneling machine, is a five-story-tall monstrosity of drilling tasked with digging out the tunnel for State Route 99 to replace the Alaskan Way Viaduct. It's journey to the center of the earth underneath downtown Seattle began in July 2013, and since then the project has seen its fair share of successes and failures.Follow the progress of the $3 billion megaproject with KUOW.

Bertha Rests Before Big Push Under Viaduct

A view to the back end of Bertha, the SR 99 tunneling machine. The steel hooks on both sides of the wall of the tunnel will become part of the foundation that will support the decks and walls of the future roadway, according to the state.
Flickr Photo/Washington State Department of Transportation
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A view to the back end of Bertha, the SR 99 tunneling machine. The steel hooks on both sides of the wall of the tunnel will become part of the foundation that will support the decks and walls of the future roadway, according to the state.

Bertha has stopped again, but this time, it’s on purpose.

The tunnel boring machine rests in an underground concrete vault. Workers are putting the tunnel boring machine through complex tests before it pushes under the Alaskan Way Viaduct. 

What makes the tests complex is that workers operate in a highly pressurized environment behind the Bertha’s cutterhead. The air is pressurized so that water and mud doesn’t leak into the space and bury them. Workers have to acclimate to the pressure slowly, like divers.

Bertha’s drive under the viaduct will take a couple of weeks. The viaduct will close when the drill is beneath it, just to be safe.

We don’t know the exact dates of that closure yet.

Flickr photo by Washington State Department of Transportation.