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For the most part, the media coverage of this event has been about as accurate as it could be, given the lack of knowledge about mining in the reporting corps. You saw what may have been a first, TV coverage underground in the midst of a mine rescue effort. MSHA leadership is going to take some serious flack over this risky behavior in the aftermath of this multiple disaster.

I have been in the mining business over 34 years. After a disaster, you usually see the same group of “experts” and politicians on TV criticizing the operator, criticizing MSHA and calling for more regulations, without regard to the real facts of the particular situation. I listen to their positions, but filter their conclusions through the prism of my own experience. Rarely do I agree with the usual cadre of instant “experts” and always wait for the facts to come out, but in this case I find myself in general agreement with some of the critics regarding the mining plan.

The map of the Crandall Mine is posted on the MSHA site. The mine was developed out to the boundary with main headings using the room and pillar technique. The pillars are blocks of coal that are left to support the roof and overburden. The rooms are formed when the coal is extracted. The rooms in these main headings were the main travelways, haulageways, and ventilation ways.

Having reached the mining boundary, the Crandall Mine was pulling back, using a technique called “retreat mining”, or in the WV coal fields “pillaring”. During retreat mining, the roof support pillars are mined. The pillars can be removed because the main headings no longer need to be maintained. There is no need to get through them after all of the coal is mined. Another term for this technique is “robbing” pillars.

This is a technique that is commonly used in the coal industry. As enough of the roof support pillar is mined, the roof collapses “inby” where the miners are actually working. You want the roof to collapse to relieve the stresses on the pillars that remain where you are actually working, and in the travelways between you and the outside of the mine. The pillars support the roof, or the overburden. The deeper below ground (vertically)the mine is, the greater the load (stresses) on the support pillars. If the roof does not cave (fall) after the support pillars are removed, the load these pillars were supporting is transferred onto the remaining pillars, increasing the load on them.

In the eastern coal fields, where the cover is relatively shallow over the mine, the roof caving after pillar extraction generally is effective in controlling the load and stress on the remaining pillars, so it is generally safe to mine using this technique. The type of rock strata above the mine is also a factor.

Out in Utah, the mines are much deeper, so the load is much greater on pillars. If there is a thick band of hard sandstone in the rock strata somewhere above the mine level, this tends to not fail when the rock below it might collapse after pillar extraction. Therefore, the load shedding on remaining pillars that usually accompanies roof caving during retreat mining is not as effective. You have two factors working against you during retreat mining in a situation like Crandall. First, you have high stresses in the support pillars due to the depth. Second, you have stresses building up in the remaining support pillars as pillars “inby” are removed. The bumps referenced at Crandall are caused by the mountain trying to find equilibrium. At some point when the stresses become high enough, something has to give. In some mines the floor cracks and heaves up. I have seen a room that was 5’ high on Friday be 3 or 4’ high on Monday. In some cases the stresses build up in the pillars. There is a lot of stored energy. Rock under pressure can explode, and it does’t take much to relieve the pressure. The shock wave from a mountain bump could do it if the stresses are high enough in the pillars. An earthquake could do it. A shock from blasting can do it. We call this sudden pressure release an outburst or rock burst. This appears to be what has happened at Crandall, on a scale that I have never seen. The mine roof did’t collapse, the walls and perhaps the floor have exploded inward and filled the mine main headings. This is what the rescue team was mining through when the second accident occurred.

To add complexity with the above scenario, very large areas on each side of the main travelways where retreat mining was being conducted were mined out using the longwall technique. Longwall mining differs from room and pillar mining in that all of the coal is extracted in a longwall panel. The roof is designed to cave behind the longwall unit as the coal is extracted. No support pillars are left in place. To the extent that roof caving behind the longwall unit did not fully relieve the stresses in the roof immediately above, the remaining stresses would have been transferred out to the support pillars in the main headings, increasing the load on them.

This third factor, longwalled areas immediately adjacent on both sides of the rooms where support pillars were being extracted, likely made a significant contribution to this disaster. It appears support was removed systematically transferring additional load to the remaining pillars, where they failed catastrophically due to the pressure.

Is there anything illegal about this practice? No. In fact, MSHA had to approve the written mining plan, and did so about a month or so before the disaster. However, to me and to others I have talked to in the mining industry, this entire setup seemed risky.

In the end it may turn out to be an earthquake which triggered the rockburst, but I think it is just as likely, perhaps more likely, the mountain shifting to equalize stresses created the shock wave that released the stored energy in the pillars.