Retreat mining is a coal mining technique that recovers more material and maximizes profits by removing support pillars. This can cause roof falls and coal explosions, making mining dangerous. The US Mine Safety and Health Administration regulates mining operations and ongoing research aims to improve safety. The Crandall Canyon Mine collapse in 2007 resulted in six miners being trapped and three killed during rescue attempts.
Mining is the last stage of a common type of coal mining technique called room and pillar mining. In room and pillar mining a mining machine known as a “continuous miner” drills 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. The room and pillar mining progresses inward, away from the mine entrance. When the coal seam is depleted or the mine property line is reached, retreat mining is a process that recovers the coal support pillars, working from the rear of the mine towards the entrance, hence the word “ withdrawn”.
Room and pillar mining leaves about 57% of the mine’s coal for support. This provides a strong financial incentive for mining retreat as a way to recover more material and maximize profits. Mining is a dangerous operation, however, driven by strict safety regulations.
When pulling support pillars different techniques are used to shore up mine ceilings and prevent the roof from sagging. The movable roof supports relieve the pressure on the adjacent pillars during this precisely conducted extraction phase. In some cases wooden cradles or hydraulic jacks are fixed in place. Working from the rear of the mine towards the entrance, roof falls are expected in the wake of mining, although the collapsed parts should already be pulled out and vacated.
One of the dangers associated with mining is coal explosions. As the stabilizing pillars are removed, pressure builds on the remaining walls and pillars. In the same way that a stick snaps when enough pressure is applied, the pressure can build up to the point that a wall explodes or a pillar explodes, shooting material into the mine. The result can be lethal to nearby miners and can be complicated by localized roof falls. A massive collapse of a pillar can also trigger a domino effect on adjacent pillars causing catastrophic failure.
Retreat mining made the headlines in August 2007 when six miners were trapped at the Crandall Canyon Mine in Utah. Miners were trapped in the rear of the mine on Aug. 6 in a cave-in that generated a 3.9 to 4.0 seismic reading as far north 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 canceled due to safety concerns. Although mining had been conducted in parts of the mine, co-owner Robert E. Murray has publicly stated that 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 mining. Research to improve safety in mines is ongoing.
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