Bord 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 room and pillar system is used in mining coal, iron and base metals ores, particularly when found as manto or blanket deposits, stone and aggregates, talc, soda ash and potash.

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.

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Wikipedia