Bertha will move slowly as it starts drilling beneath the viaduct

SEATTLE — As the tunnel machine Bertha begins digging beneath the Alaskan Way Viaduct on Friday, it will move very slowly, state officials said Monday.

Officials with the Washington State Department of Transportation distributed a handout during a visit to the tunnel on Monday that says the contractor, Seattle Tunnel Partners, plans to proceed slowly to start, since the first 10 feet of mining will be breaking through a concrete wall of a planned maintenance stop known as Safe Haven 3.

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Bertha has been at that stop since mid-March.

Two sinkholes on the project have occurred soon after the machine tunneled through similar walls.

Bertha will need to drill 385 feet to be clear of the viaduct.

It is currently 75 feet below ground and will descend to about 90 feet during the traffic closure on the roadway, which will last for two weeks.

WSDOT says the viaduct has been reinforced in places.

There is also an extensive laser monitoring system to check for ground movement.

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