Bertha, the World's Largest Tunneling Machine

by / January 28, 2014

In July 2013, Bertha -- the world’s largest-diameter tunneling machine -- began digging the SR 99 tunnel beneath downtown Seattle to replace the city's Alaskan Way Viaduct.

Thus far, Bertha has traveled just over 1,000 feet of its 9,270-foot journey -- a journey that's been broken up into 10 zones. Currently Bertha is still in Zone 1, which starts in Seattle's Railroad Way South and ends at South Washington Street. This zone is considered training for Bertha, as crews have built three protected areas where they can crawl through the front of the machine to inspect it.

"Think of it this way: if Bertha were learning to ride a bike, Zone 1 would be her training wheels," according to the WSDOT website.

For a portion of January, Bertha has undergone inspections; crews were investigating the cause of Bertha's slowed progress (results are not yet in). But on Jan. 28, she was on the move again, albeit cautiously.