Retreat Mining
by Andy McAllister, Watershed Coordinator, and Bruce Golden, Regional Coordinator
Coal mining has always been an occupation fraught with danger. We have been reminded of that stark reality in recent days due to the collapse of the Crandall Canyon Mine in Utah where six miners have been trapped since August 6th and 3 workers have already perished in the attempt to rescue those trapped. Since the collapse, there have been many reports indicating a form of mining known as “retreat mining” may have contributed to the problem.
In Pennsylvania, coal mining has been going on for about 200 years and retreat mining has also been going on for just as long. But in order to understand retreat mining, we must understand something of the mining technique known as “room and pillar” mining. In extracting coal, voids in the coal seam are created. However, substantial blocks of coal must be left behind to support the mine roof. The voids are called rooms, while the supporting blocks of unextracted coal are called pillars. Roughly half the coal in the seam remains in place in the form of pillars for the safety of the miners and equipment. As mining in a seam progresses, a gridwork of voids punctuated by pillars emerges.
Retreat mining is the process of removing the supporting pillars (a.k.a pulling the pillars) once the traditional room and pillar mining is completed. A carefully drawn-up and regulated retreat mining plan outlines the systematic removal of all or parts of pillars to safely extract as much coal as possible. The retreat process begins in the furthest reaches of the mine and progresses in the opposite direction of the original mining. As retreat mining progresses, the roof is intended to collapse following pillar removal. This collapse should happen soon, but not too soon. Understandably, this is an inherently dangerous undertaking, even for the most experienced of miners. However, the technique yields a return of around 70% of the coal originally in the seam.
Each mine has its own conditions that dictate how aggressive the retreat mining can be. For instance, deeper mines will have greater stresses on their roofs (and thus pillars) than shallower mines. Safety dictates leaving more of the pillars in highly stressed situations. Without sufficient supporting material in the pillars, a “bump” may occur in which a pillar literally explodes outward from excessive stress. Indications are the Utah mine is highly stressed with bumps a not uncommon occurrence.
Room and pillar mining still occurs in the Bituminous region of Western Pennsylvania along with retreat mining. Although retreat mining can still be a dangerous practice, it is generally safer today than in years past. Indeed, all mining methods can be very dangerous if applicable laws, regulations, and approved plans are not followed.
Today, mining operations that wish to legally practice retreat mining must have a federally approved plan containing proper safety precautions.
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