“We knew that already,” said Scott Brunsdon, chief financial officer for Revett Minerals Inc. “We knew that particular part of the ore body was a tougher mining area.”
Tougher, at least, relative to other Troy tunnels. But compared to most hard rock mines, Brunsdon said, this geology is easy.
And there just as surely is a much longer answer, one investigators still are considering, one with lots of technical variables and fancy formulas to back it up. That's the one everyone's waiting on.
But even that answer may come up short, Brunsdon warned, because the history of “ground fall,” as such cave-ins are known, suggests that the ultimate answer may also prove unsatisfactory: We don't know.
That is, after all, the most common answer, disappointing as it may be.
When the folks at the National Institute for Occupational Safety and Health (NIOSH) set out to review 15 years of mining ground fall, from 1984 to 1999, they found a staggering 13,277 cases nationwide. Of those, a whopping 40 percent fell into the “we don't know” category.
“It depends on so many things,” said Douglas Scott, a geologist and research scientist at NIOSH. “It can be very complicated.”
About 30 percent of the ground fall cases were found to be caused by human activity, by drilling and blasting and generally shaking the stone loose.
Another 15 percent were caused when roof support systems failed.
Nearly as many were caused by inadequate support systems, not built to handle the pressure in the first place.
But the bulk were attributed to “rock mass failures,” or “other weaknesses,” meaning, essentially, that for some unknown reason the rock fractured and gravity pulled it down.
Annually, many hundreds of miners, some years more than 1,000, are injured by ground fall. Sometimes, it comes from above. Sometimes, the walls splinter inward.
It is, Scott said, a matter of pressure.
“The more rock that is removed, the more stress that is available,” he said.
Remove the underpinnings, and the pressure must be picked up somewhere else. Miners know it too well.
“As you drill and remove the rock, the pressure shifts,” said Josh Peterson, who was with 55-year-old Mike Ivins when the roof came down Monday morning. Peterson and two others survived. Ivins did not.
“When you relieve pressure from one place,” he said, “it settles somewhere else. The weight's always shifting.”
The idea is to keep the weight spread out and balanced atop rock pillars, which are left standing to support the roof. In some places, miners add ceiling bolts, or screens, or spray-on cement, or sometimes all three.
“It depends on the area,” Brunsdon said. “We use it all at Troy.”
And then they bet on the rock above holding together, which sometimes it does not.
“I've seen little rock falls, and I've left areas because of minor rock falls,” Peterson said. “But I've never seen anything like that, nothing that big.”
He figures maybe 50 dump trucks of rock came down Monday, 30 or 40 feet of roof.
The rock at Troy - the geologic Revett formation - tends to come off in big blocks, Peterson said, and is shot through with broad horizontal layers of “mud.” Scott and other geologists suspect that “mud” might actually be argillite, which when soaked with groundwater becomes something approaching clay.
Either way, the place where quartzite ore meets the “mud” is a weak place, a fault along which things can come loose. “Faults are very dangerous geologic structures,” Scott said.
“I have to wonder if we didn't get some water in up above that weakened the rock,” Brunsdon said. “Because that can happen.”
Lots of things can happen, in fact, when you hollow out a mountain.
The size of the Troy Mine boggles the mind. A couple hundred miles of tunnels - 40 feet wide and upward of 70 feet tall - burrow underground. Miners haul out 4,500 tons of ore - that's
9 million pounds - every day.
The ore is just 0.65 percent copper, and there's less than an ounce and a half of silver per ton. And yet Troy Mine produces 2 million ounces of silver a year, 17 million tons of copper.
“It's a whole lot of rock,” Peterson said.
Remove enough, and some is bound to fall down.
“It's a fundamental issue that everyone deals with,” Brunsdon said.
But no one is suggesting that the mountain is unstable. Quite the contrary, “the geologic ore body is quite uniform and consolidated from top to bottom,” Brunsdon said. “Generally, the thinking is that we have very good ground conditions.”
But the problem remains: How to see through rock to what's inside?
“We know what it looks like when we mine through it,” he said, “and we know what the drill cores look like. But we can't know exactly what's up there in the roof.”
“The question is,” Scott said, “when you cut through that stress point, are you transferring the stress to a place where it can't be supported?”
There are technologies, Scott said, acoustic and physical machines that listen and feel for the explosive “pop” of rock shifting and releasing pressure, as well as a sort of “radar” that can find hidden fault lines. And coincidentally, Brunsdon has even worked at a mine that employed those tools, over on the other side of the continent, a mine with treacherous ground conditions, where they averaged one dead miner every year.
But all agree those technologies are extraordinarily expensive, and are often ineffective.
“It's like trying to predict an earthquake,” Brunsdon said, “and we all know how good we are at that.”
Which is to say, not very.
“No one,” Scott admits, “can predict a rock failure.”
Peterson's job when the roof came down, after all, was to install the bolts intended to keep it up. That bolting machine cost $1 million, and still the stone collapsed.
The best defense, Brunsdon said, may be a good offense, a highly trained crew skilled at watching for the warning signs. Before the ground fall Monday morning, everyone working at the site noticed the rock fall, the gravel dribbling down the walls, the larger blocks breaking loose.
Problem is, Peterson said, that's all fairly normal. And the line between normal and tragic is very thin.
“You can't stop work every time a rock falls,” he said. “If you did, you'd have to close the mine.”
And although that's not likely, Brunsdon is wondering about the future of this now notoriously tricky tunnel.
“Who knows?” he said. “That particular portion of the mine might never be reopened. Who knows?”
Investigators at the federal Mine Safety and Health Administration hope to know, but not for a while yet. Agency spokeswoman Amy Louviere provided a preliminary report on the tragedy Wednesday, but it contained just the barest of bones.
A meatier analysis could be weeks, if not months, out, she said. In Nevada, a miner was killed by ground fall a month and a half ago, she said, and that mine still has not reopened.
Brunsdon, meanwhile, is confident at least some parts of Troy Mine will be operational by next week, parts that aren't so tricky, and that aren't so susceptible to ground fall.
“My prediction is we'll be back doing some selective underground mining in other parts of the mine very soon,” he said. “But really, everything depends on the investigation.”
And upon gravity, that simplest of answers. Because according to researchers at NIOSH, ground fall remains the leading cause of fatal accidents in underground mines. Unfortunately, the leading cause of ground fall is unknown.
“Ground falls in underground mines in the U.S. continue to be a major cause of fatal or non-fatal injuries among miners,” the NIOSH report states.
That comes, perhaps, as no surprise, considering the nature of the work. But what truly is a surprise, Scott said, is that more aren't killed.
“You think about the hundreds of thousands of hours worked safely underground,” he said, “and what's really remarkable is that the mining industry is as safe as it is.”
But not for Mike Ivins. He was the 20th hard rock miner to die on the job this year.
Reporter Michael Jamison can be reached at 1-800-366-7186 or at mjamison@missoulian.com
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