Mine-safety experts say that two sections of the Crandall Canyon Mine that collapsed in March may have been an early warning sign

New questions arise about mine stability | By PAUL FOY and JENNIFER TALHELM, Associated Press Writers Tue Aug 14, 7:07 PM ET

HUNTINGTON, Utah - As frustration mounts over the slow pace of the digging to free six trapped miners, more questions arose Tuesday about whether risky mining methods may have left parts of the coal mine dangerously unstable.

Some mining companies consider the "retreat mining" methods used at Utah's Crandall Canyon so dangerous, they will leave behind coal rather than risk the safety of their workers.
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Mine-safety experts say that two sections of the Crandall Canyon Mine that collapsed in March may have been an early warning sign. They questioned whether the company — and the government agency that oversees its work — should have closed the mine then.
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But experts question that decision because the area is bordered by two outer sections that had already been mined and collapsed, using a technique that leaves behind unstable rubble.

That means the last pillars were bearing much of the weight of the roughly 2,000 feet of mountain above, and as they were pulled down, the pressure on the remaining pillars would have increased.

Larry Grayson, who worked in coal mining for nine years until 1984 and is now a professor of energy and mineral engineering at Pennsylvania State University, said retreat mining is so risky that the mining company he worked for would not use it between two sections of rubble.

The mining strategy at Crandall Canyon just didn't work, he said. "There was no advance notice, and just — wham-o." ...