Engineers debate risks involved in pulling Chile’s miners up through “live rock” or steel pipeBy Vivian Sequera, AP
Wednesday, October 6, 2010
Chile mine engineers weigh risks of encasing shaft
SAN JOSE MINE, Chile — It’s the nightmare scenario: The earth shifts just as a miner is being pulled to safety, jamming his escape capsule somewhere between the surface and the underground cavern where 33 men have waited for two months to be rescued.
A partial collapse in the shaft carved through nearly a half-mile of rock could trap the man in a spot where even the most powerful drills couldn’t free him.
With the rescue drill likely to reach the men by Friday, Chile’s government is planning to guard against such a disaster by inserting steel pipe that can withstand tons of pressure into the shaft, and giving the miners a smooth, ride up to safety.
But some experts fear that inserting the pipe is risky in itself.
Engineers must determine whether they can push the 40-foot (12-meter) sections of pipe through curves in the shaft without having the welds crack. An accident could send tons of steel hurtling downwards or cause a section to get stuck part way down, blocking an exit route that has taken more than a month to drill.
Rescue coordinator Rene Aguilar, among the most cautious of those involved, would put in the pipe. He said it would provide a virtually risk-free ride.
“The first choice is to put the casing,” Aguilar said Wednesday, speaking in English. “We have to put the casing at least for the first 100 meters. If we could do the lining for all the hole, of course, we are going to do it, of course. We have to reduce the risk of this operation.”
Casing the entire length would take up to 60 hours, but lining just the top 330 feet (100 meters) — which pass through a particularly fractured section — could be done in 15 hours, he said. That means a miner needing emergency care conceivably could be pulled out as early as this weekend, although Aguilar insisted the rescue won’t begin until after Oct. 15.
Others involved suggested that plan itself is risky, largely due to gradual curves in the “Plan B” shaft being drilled by the T130 machine, which is the closest to being completed.
It starts like a waterfall plunging off a cliff — gradual at first and then bending nearly straight down. And getting straight pipe through the curve is the challenge.
“So what happens? The pipe, by virtue of its own weight, bends a little; it has a certain flexibility. But when you weld two sections together and insert it in the hole, the weld doesn’t bend; it’s rigid, hard. And so the question is whether it will withstand” the stress of the curve, an engineer involved told The Associated Press, speaking on condition of anonymity because he is not authorized to discuss the drilling publicly.
“If they decide to encase it completely — all 624 meters (2,047 feet) — you’re talking about a tube that weighs 130 tons.” He predicted the first 100 or so meters will be encased, because that section curves through fractured rock.
There are other risks: “A pipe could get stuck in the shaft. And if you encase the whole length of the shaft, at the end, what would support all that weight? That’s what has to be determined,” the engineer said.
The pipe is made of half-inch-thick steel, 24-inches (61-centimeters) wide on the outside and 23-inches (58-centimeters) wide on the inside. Each section would be inserted into the shaft with an enormous crane that arrived at the mine on seven huge trucks Wednesday to applause from relatives who have held vigil on the dusty Atacama desert hilltop since their men were trapped on Aug. 5.
The risks cannot be weighed until the shaft is inspected with a video camera, said Brandon Fisher, president of Pennsylvania-based Center Rock Inc., which makes the drilling system being used on the T130.
Still, Fisher is concerned that installing pipe could complicate the rescue.
“My opinion is they’re going to try to run the casing, but when you’re running casing in an angled hole like this, it’s very, very difficult to make that happen,” he told the AP Wednesday. “This particular hole also has doglegs and turns. It’s not simple like a straight vertical hole.”
The T130 paused Wednesday at 1700 feet (519 meters) to change its drill bits, leaving just over 328 feet (100 meters) to go in a final push that will likely reach the miners on Friday.
The final section of the hole will narrow to 26 inches (66 centimeters) wide to minimize risk of a rockfall during the final drilling, leaving just four inches of leeway to push the pipe all the way through.
Omar Gallardo, a professor of mining engineering at the University of Santiago, favors inserting the pipe.
“With the available information, and to not run any risk at all, I would encase it. It would be less risky for the miners,” Gallardo said. “They’ve been down there for 61 days. Giving them a few more days down there in exchange for being able to pull them out safely is preferable.”
Pedro Buttazzonni, president of Geotec, the Chilean company that owns and operates the T130 drill, said that as miners, they can say that the shaft’s exposed “live rock” walls are very firm. “But for reasons of safety there are others who think otherwise,” he told the AP.
“If speed is your priority and it goes badly with one of them, you’ll be accused of not doing everything possible to ensure their safety … It’s a very difficult question,” he acknowledged. “There are human lives involved and this is something that none of us has had to confront before.”
Associated Press Writers Eva Vergara in Copiapo, Chile and Frank Bajak in Bogota, Colombia contributed to this report.
Tags: Accidents, Chile, Latin America And Caribbean, San Jose Mine, South America