Maybe the restart of the world’s largest tunnel-boring machine will come as a Christmas present to the folks in Seattle, as the scheduled boring by “Bertha,” the 57.5-ft-dia machine currently sitting idle under downtown Seattle, was pushed back yet again, this time to Dec. 23, more than two years since it last mined. Bertha was shut down on Dec. 6, 2013, due to overheating after traveling just over 1,000 ft, or 11%, of its planned 9,270-ft route under the city to build a new roadway to replace the seismically vulnerable Alaskan Way Viaduct.

It took crews from Seattle Tunnel Partners (STP)—a joint venture of Dragados USA, a wholly owned subsidiary of Spain-based Dragados, and Tutor Perini Corporation, Sylmar, Calif.—months to determine the cause and then devise a plan to fix the main bearing on the 326-ft-long machine by digging a 120-ft-deep access shaft in front of the tunnel-boring machine. Crews this summer removed the front end of Bertha, bringing the cutterhead to the surface for repairs, including a new main bearing and seals from machine manufacturer Hitachi Zosen, Japan. Earlier estimates had crews resuming digging in November, but the latest report from Seattle Tunnel Partners calls for a Dec. 23 restart date, with the tunnel opening in April 2018.

The tunnel originally was scheduled to open by the end of 2015. “STP has told us the changes in the schedule reflect the current emphasis on giving crews the time they need to complete the [TBM] repairs successfully,” the Washington State Dept. of Transportation, the owner, said in a statement. Crews maneuvered the cutterhead back into the pit in August. Before tunneling can resume, crews are welding together pieces of the machine, reconnecting hundreds of wires and hoses, and preparing this “monumental effort” for final testing, says Chris Dixon, project manager for STP. Crews also must stabilize the ground above the tunnel as the machine exits the repair pit. Once tunneling resumes, plans call for Bertha to move forward 430 ft before a scheduled stop. It is the unscheduled stops that have folks wary.