The TBM that will build the SR 99 tunnel – nicknamed Bertha – began its journey beneath downtown Seattle on July 30. The machine with its 5-story tall cutterhead began its 2-mile by breaking through the north wall of its 80-ft deep launch pit. The TBM is expected to emerge in about 14 months near the intersection of Sixth Avenue North and Harrison Street.
Crews working with the Washington State Department of Transportation will push forward slowly at first, digging about 6 ft per day. By the time the machine is beneath downtown, it will dig up to 35 ft per day.
The tunnel route is divided into 10 separate zones, each with its own underground landscape. In the first zone, crews strengthened or replaced fill soils dumped there by the city’s early settlers while building protected areas where they can inspect the machine.
“We designed the project so that we would have opportunities to test the machine and make sure she’s functioning properly before we get beneath downtown,” said Linea Laird, WSDOT administrator for the Alaskan Way Viaduct Replacement Program. “If Bertha was learning to ride a bike, this initial section would be her training wheels.”
Bertha, who has been updating her progress on Twitter since December, said she is ready to go. “About to start tunneling,” @BerthaDigsSR99 tweeted. “I should say something profound, something Neil Armstrong-ish. Fortunately, I’m out of characters. Let’s dig.”