Bertha, the world’s largest tunnel-boring machine, may once again bore under downtown Seattle. The machine, which has sat idle and undergoing fixes for just over two years, should move forward once again on Wednesday, Dec. 23.
The updated plan from Seattle Tunnel Partners, the joint venture trying to get the Alaskan Way Viaduct replacement project back on track, has Bertha moving a “short distance” before taking a break for Christmas. After the short break, the contractors plan to mine out the access pit toward a planned maintenance stop approximately 450 ft to the north.
Last week was spent backfilling the 120-ft-deep access pit in front of Bertha, which was used to access the 57.5-ft-dia machine for fixes. By Friday, only part of the machine’s shield and cutterhead were visible. Crews were forced to stop backfilling on Dec. 16 to repair a mixing arm that broke off inside the machine during testing of the cutterhead. Backfilling, though, resumed and continued through the weekend.
Bertha was sitting on a concrete cradle at the bottom of the pit, approximately 20 ft from the wall the machine will bore through when mining resumes. Contractors placed gravel under and beside the machine to fill gaps between it and the cradle, then added low-strength concrete-sand mixtures to the portion of the cradle in front of the machine to ensure good contact and a firm surface for the machine to mine through.
A sand and concrete mixture was then added to fill the remainder of the pit. Crews also used compact soil excavated from the area to top the backfill. Originally the plan called for just sand and then the final 51 feet to use compacted soil, but crews switched to the sand-concrete mix to reduce backfill timing.
While Dec. 23 marks an important milestone in the slow-going journey of Bertha, a milestone searched for since early December 2013 when Bertha overheated and was shut down, the next real test will come each day after as we watch and see how well Bertha moves under downtown Seattle.
Follow Tim Newcomb on Twitter at @tdnewcomb.