In the days immediately after the discovery that 33 workers trapped at the San Jose Mine were alive, Chilean officials cautioned that it might take up to four months to reach them with a rescue hole.
In the end, it took only about six weeks to do what mine safety experts say is a job with little precedent: drilling a precision shaft, wide enough to accommodate a man, to a spot nearly 700m underground.
“We have a number of mines that are this deep,” said John Urosek, chief of mine emergency operations for the US Mine Safety and Health Administration. “But we’ve never had a situation where we’ve had to rescue someone through a bore hole this deep.”
One reason Chilean officials may have thought the drilling could take far longer was that they were not familiar with the type of drill that carved the rescue hole.
Three efforts to bore through the abrasive volcanic rock went forward simultaneously — known as Plans A, B and C — but it was Plan B that broke through to the miners first.
“To tell you the truth, I don’t think anyone had a whole lot of faith in us,” said Brandon Fisher, president of Center Rock, a company in Berlin, Pennsylvania, that supplied the Plan B drills. “They didn’t understand the technology.”
Fisher and others lobbied the Chilean government to let them use the drills, known as downhole hammers, which have air-powered bits that pound the rock as the drill rotates.
The other two drilling operations, Plans A and C, used more conventional bits that work through rotation only.
The geology of the region — hard volcanic rock infused with other extremely hard minerals — favored the Plan B equipment, said Maurice Dusseault, a professor of engineering geology at the University of Waterloo in Canada.
“In very hard, brittle rocks, percussion drilling is indeed an excellent way” to make progress, he said.
The effort was also aided by the dry conditions encountered underground in the Atacama region of Chile, one of the driest deserts in the world.
“You can’t do percussion drilling if you have water or mud in the bore hole,” Dusseault said, as the liquids absorb too much of the percussive energy.
The Plan B drillers made use of one of the small bore holes that were drilled to locate the miners after the collapse, widening it first to 30cm and then to 71cm to accommodate the rescue capsule.
Using such a pilot hole “was really our only option,” said Fisher, who delivered the drills himself and stayed at the site for the entire drilling operation. “There were so many old workings in the mine that we couldn’t risk starting a new hole. We went with a sure thing.”
Even using the pilot hole, the 30cm drill encountered an old steel roofing bolt that heavily damaged the equipment and delayed the operation for a few days.
The use of a pilot hole also allowed the drillers to drop the rock cuttings down the existing hole, reducing the complexity of the operation, said Frank Gabriel, a vice president of Schramm, the Pennsylvania company that made the mobile drilling rig used by Plan B.
“Normally you’d have to flush all those cuttings back to the surface,” he said. “That takes a lot more air.”
Fisher said that now that they had seen how the technology worked, the Chileans were likely to incorporate it in many of their mining operations to drill ventilation shafts and other holes.
They were very grateful, he said, “thanking us for talking them into letting us come down and use this technology.”
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