Bertha begins: digging officially underway in Seattle
This time, there were no crowds or fanfare. It was just Bertha, the world’s largest tunneling machine, finally getting the chance to do what she was built to do: dig.
Bertha got her first taste of tunneling on July 30, officially starting the 2-mile journey beneath downtown. Early Tuesday afternoon, Bertha’s 5-story-tall cutterhead broke through the north wall of her 80-foot-deep launch pit. She’s 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 feet per day. By the time the machine is beneath downtown, she will dig up to 35 feet 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.”