The MSHA is set to release its findings tomorrow about the August 6, 2007 Crandall Canyon Mine collapse. It will be interesting to see what they say. A May, 2008 report entitled, Chairman Miller Statement on Findings of Investigation into Crandall Canyon Mine Disaster, by U.S. Representative George Miller (D-Calif), states that MSHA approved a faulty plan for retreat mining.
Retreat mining is a practice used in many coal mines; it involves the pulling of rock pillars as the miners retreat from an open room-and-pillar underground mine area. The pillars are ore; that's why they are pulled. The use of the room-and-pillar underground method, at least in coal mines, leaves more than 50% of the coal in the pillars. That's a lot of ore (coal in this case) to leave behind.
It doesn't matter whether the pillars in an underground mine are in gold ore or in a coal seam: they are there to hold up the mine. Pulling them involves an engineered or planned collapse of the mine section or room from which the pillars are pulled. It is not a mining method to be undertaken lightly or to be undertaken at all by anyone more concerned about profit than for miners' safety.
NIOSH has several links to information about safe use of retreat mining. According to their studies, reasons for failure of the practice wherein so-called "bumps" occur, as were reported at Crandall Canyon [these are actually rock bursts] usually have to do with "relatively thick overburden and extremely rigid strata occurring immediately above and below the mine coalbed" - but also have to do with certain mining practices that they recommend avoiding.
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