The work to find out what's blocking Bertha the giant tunnel boring machine is causing the Alaskan Way Viaduct to sink just slightly.

The most recent WSDOT tests, conducted over the weekend, show the Viaduct has sunk a tenth of an inch since work began on lowering the water table near Bertha.

The Viaduct has already settled over its lifetime. So, a tenth of an inch is really negligible in terms of the stresses and how the Viaduct will behave, said David Sowers, WSDOT Engineering Manager.

The mystery of what is blocking Bertha gave rise to the sinking around her. When, the world's largest boring machine encountered something that stopped her, engineers devised a plan to get workers down into Bertha to check out the problem.

But Bertha's chamber is submerged in water. They're pumping water out of the soil and have already lowered the water table about 50 feet. They've got to get it down about ten more feet to get inside Bertha.

We've lowered the water table enough that we think we can get in there under atmospheric conditions... to see what's blocking our progress, said Sowers.

The Viaduct is being constantly monitored, manually and automatically. If all goes as planned, workers will get inside Bertha on Thursday to finally get a look at the mystery blockage.

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