Getting Bertha tunneling under downtown Seattle after a December 2013 stoppage ended up being a roughly two-year endeavor. And not only is the world’s largest tunnel-boring machine again moving forward, but the 57.5-ft-dia machine is now moving again in native Seattle soil.
The machine first moved forward in its repair pit on Dec. 22, 2015, but it was the breaking through the northern wall of the 120-ft-deep repair pit that marked a key milestone in getting Bertha back on track under Seattle. Bertha ripped through the 15-ft-thick concrete wall the first full week of January and then on Jan. 7 mined into the native soil of the original route plan.
Since Dec. 22, Bertha has moved forward 73 feet, allowing the installation of 12 concrete tunnel rings. More mining will occur this week.
Bertha now approaches South Main Street near Pier 48 and is digging well below the area’s notorious fill soil. The top of the machine sits approximately 80 ft below the surface in a mixture of glacially compacted material.
Currently, the joint venture leading the charge, Seattle Tunnel Partners, has two tunneling crews consisting of about 25 members each. Each crew plans to work six, 12-hour shifts per week. Mining progress during each shift may vary, as there is a mix of straightforward mining and planned maintenance needed for Bertha.
According to the most recent schedule of mining put out by STP, Bertha should reach its next expected underground maintenance stop just south of Yesler Way in late January. Once there, crews will spend about three weeks performing final maintenance before the machine tunnels beneath the Alaskan Way Viaduct. Washington State Dept. of Transportation will fully close State Route 99 through downtown for approximately two weeks while the machine passes beneath the viaduct, likely sometime in March.
With Bertha once again moving forward, crews have started disassembling the crane used to repair the cutterhead. All parties hope there will be no need to ever reassemble the crane for another Bertha repair.
Follow Tim Newcomb on Twitter at @tdnewcomb.