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Retreat mining is the last phase of a common type of coal mining technique referred to as room and pillar mining. In room and pillar mining a mining machine known as a “continuous miner” bores a network of chambers or “rooms” into a coal seam, leaving behind an unexcavated pillar of coal in each room to support the roof of the mine. Room and pillar mining advances inward, away from the entrance of the mine. When the coal seam runs out or the mine’s property line is reached, retreat mining is a process that recovers the supporting coal pillars, working from the back of the mine towards the entrance, hence the word “retreat.”
Room and pillar mining leaves behind approximately 57% of the mine’s coal for support. This presents a strong financial incentive for retreat mining as a way to recoup more material and maximize profits. Retreat mining is a dangerous operation, however, guided by strict safety regulations.
When pulling support pillars several techniques are used to shore up mine ceilings and prevent roof falls. Mobile roof supports relieve pressure on adjacent pillars during this precisely conducted mining phase. In some cases wooden cribs or hydraulic jacks are secured in place. Working from the back of the mine towards the entrance, roof falls are expected in the wake of retreat mining, though collapsed portions should already be mined and vacated.
One of the dangers associated with retreat mining is coal bursts. As stabilizing pillars are removed pressure increases on the walls and remaining pillars. In the same way a stick will snap when enough pressure is applied, pressure can intensify to the point that a wall will explode or a pillar will burst, shooting material into the mine. The result can be lethal to nearby miners and can be complicated by localized roof falls. A massive pillar collapse can also trigger a domino effect on adjacent pillars causing catastrophic failure.
Retreat mining came to the forefront of the news in August 2007 when six miners became trapped in Crandall Canyon Mine, Utah. The miners were trapped at the rear of the mine on August 6th in a collapse that generated a seismic reading of 3.9 to 4.0 recorded as far away as Nevada. In the rescue attempt that followed a secondary collapse occurred ten days later, killing three and injuring six. The original miners were never found and rescue operations were called off due to safety concerns. Though retreat mining had been conducted in portions of the mine, co-owner Robert E. Murray publicly stated it was not taking place at the time of the collapse.
In the United States, the U.S. Department of Labor’s Mine Safety and Health Administration (MSHA) is responsible for regulating mining operations including retreat mining. Research to improve mine safety is ongoing.