SEATTLE — Sometime this fall, a huge vertical shaft lined with 84 concrete pilings, designed to hold back the slurry that defines underground Seattle, will be finished, and a Mr. Fix-It operation unlike any other will begin.

The world’s biggest tunnel-boring machine, nicknamed Bertha — which hit a pipe and was damaged in mid-December after only 1,000 feet of excavation — is down there in the dark, awaiting what may well be the world’s biggest industrial rescue operation.

“When you have such a big machine, you have a big intervention,” said Youssef Hashash, a professor of civil and environmental engineering who teaches tunneling at the University of Illinois at Urbana-Champaign. When things break, he added, “it all scales up, and it scales up the challenge as well that you have to overcome.”

On paper, the complex plan looks like a cross between a ballet and a monster-truck pull in its combination of delicate details and heavy-torque engineering. First, a rail-mounted crane will be inched up to the shaft’s edge. Then, a 2,000-ton piece of the boring machine’s front assembly will be raised up and laid down on the waterfront. There it will be repaired under the supervision of Japanese managers from the company that built it, reinforced with 200 or so tons of new steel and slowly lowered back down into the 120-foot-deep pit.