Room and pillar
Room and pillar (variant of breast stoping), also called pillar and stall, is a mining system in which the mined material is extracted across a horizontal plane, creating horizontal arrays of rooms and pillars. The ore is extracted in two phases. In the first, "pillars" of untouched material are left to support the roof overburden, and open areas or "rooms" are extracted underground; the pillars are then partially extracted in the same manner as in the "Bord & Pillar method". The technique is usually used for relatively flat-lying deposits, such as those that follow a particular stratum.
The key to successful room and pillar mining is in the selection of the optimum pillar size. In general practice the size of both room and pillars are kept almost equal, while in Bord & Pillar, pillar size is much larger than bord (gallery). If the pillars are too small the mine will collapse, but if they are too large then significant quantities of valuable material will be left behind, reducing the profitability of the mine. The percentage of material mined varies depending on many factors, including the material mined, height of the pillar, and roof conditions; typical values are: stone and aggregates 75 percent, coal 60 percent, and potash 50 percent.
Room and pillar mining is one of the oldest mining methods. Early room and pillar mines were developed more or less at random, with pillar sizes determined empirically and headings driven in whichever direction was convenient.
Random mine layout makes ventilation planning difficult, and if the pillars are too small, there is the risk of pillar failure. In coal mines, pillar failures are known as squeezes because the roof squeezes down, crushing the pillars. Once one pillar fails, the weight on the adjacent pillars increases, and the result is a chain reaction of pillar failures. Once started, such chain reactions can be extremely difficult to stop, even if they spread slowly.
Room and pillar mines are developed on a grid basis except where geological features such as faults require the regular pattern to be modified. The size of the pillars is determined by calculation. The load-bearing capacity of the material above and below the material being mined and the capacity of the mined material itself will determine the pillar size.
If one pillar fails and surrounding pillars are unable to support the area previously supported by the failed pillar they may in turn fail. This could lead to the collapse of the whole mine. To prevent this the mine is divided up into areas or panels. Pillars known as barrier pillars separate the panels. The barrier pillars are significantly larger than the "panel" pillars and are sized to allow them to support a significant part of the panel and prevent progressive collapse of the mine in the event of failure of the panel pillars.
Retreat mining is often the final stage of room and pillar mining. Once a deposit has been exhausted using this method, the pillars that were left behind initially are removed, or "pulled", retreating back towards the mine's entrance. After the pillars are removed, the roof (or back) is allowed to collapse behind the mining area. Pillar removal must occur in a very precise order to reduce the risks to workers, owing to the high stresses placed on the remaining pillars by the abutment stresses of the caving ground.
Retreat mining is a particularly dangerous form of mining; according to the Mine Safety and Health Administration (MSHA), pillar recovery mining has historically been responsible for 25 percent of American coal mining deaths, even though it represents only 10 percent of the coal mining industry. Apparently retreat mining was in use at the Crandall Canyon Mine and may have caused the August 2007 collapse that trapped six miners. On November 21, 2007, the mine was sealed, entombing the deceased miners. The bureau of land management says retreat mining was being used. Robert E. Murray, CEO of Murray Energy (owner of the mine), says the technique was not being used at Crandall Canyon.
- Mullineux (1973), p. 50.
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