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Drilling to Miners a Slow Slog

/ Source: Discovery Channel

The ongoing efforts to drill a 2,300-foot-long escape hole for the 33 mines tapped in a mine in Copiap, Chile, has once again this year brought the world's attention to bear on a field of work that usually escapes notice: Hard rock drilling.

The question this time is not how you drill into the seafloor and connect with blown-out oil well, but how do you drill a half-mile-long tunnel into very hard rock, then use that hole to extract the men?

"It is a long, slow process," said Robert Ferriter director of mine safety at the Colorado School of Mines. Ferriter has seen a lot of mining operations and rescues over the years, and has some insight into what Chileans must do to get the miners out.

The goal is to make a hole wide enough to allow a steel "torpedo" cage which can be fashioned to haul up one man at a time, Ferriter said. So right now the drilling is underway with a 26-inch-wide, three-combed drilling bit armed with industrial diamonds.

"They just put a bit in the hole and let the weight of it do the work as it turns," Ferriter told Discovery News. The diamond bit basically scratches the rocks into powder. That rock powder is then flushed out of the borehole with water.

This process goes on until the bit wears out and has to be replaced or they reach the miners, he said. The expectation that things will go slow at Copiap is based on the fact that the rocks there are very hard, he said.

The reason for those hard rocks is the same reason there's gold and other valuable metals found in them. Such valuable ore deposits are often found in what are called metamorphic rocks, said Ferriter. These are rocks that have been baked and put under a lot of pressure so that their very minerals grains are converted into more dense, harder kinds of minerals.

The forces that tortured Copiap's rocks into such durable stuff are the same that created the Andes mountain range: The collision of tectonic plates at the western edge of South America. The process also squeezed and heated a lot of water in the rocks, which dissolves metals and redeposits them in more concentrated veins of ore.

That said, the rocks are not invincible, but they don't give way easily.

Once the hole is drilled to depth, which could take months in this case, a special sprayer will likely be lowered to coat the walls of the hole with epoxy. The purpose of the epoxy, once it dries, is to keep rocks from breaking loose from the walls and make the walls as slick and smooth as possible.

"If you don't do that, you could wedge the torpedo," said Ferriter. That could permanently block the hole, which would mean they'd have to start all over.

As good as the drilling plan is, Ferriter hopes it's not the only avenue rescues are pursuing.

"I hope they continue to use other ways to get in there," like trying to digging around the rockfall that trapped the miners. "I don't know if they are pursuing a bypass kind of thing. One would hope that they are."

There was no official word from Copiap about alternate rescue efforts underway.