Bumps in Coal Mines:
Coal bumps generally occur in deep mines. These are sudden violent bursts of coal from pillars, usually accompanied by an air blast and shooting of coal pieces at terrific speed. The amount of coal ejected in a bump may be a few hundred tonnes.
Coal bumps are the result of a sudden release of elastic strain energy stored in the pillars. They are in this respect similar to rock bursts. Coal bumps are found only in coal seams with specific geological and mining conditions.
There are two types of coal bumps; shock and pressure bumps.
Shock bumps occur where a strong massive stratum lying immediately above the coal ruptures as a beam or flat arch and sets up a shock wave that is transmitted downward to the coal pillars. If the underlying coal pillar is already highly stressed, the transmitted shock wave may cause the pillar to bump.
Pressure bumps occur when weak pillar is stressed beyond its strength and fails suddenly and violently. It will be obvious that bumps occur when stresses larger than coal strength are induced, whatever be the cause.
Bumps occur usually under the following conditions:
(1) A strong and brittle coal that does not crush easily,
(2) Depth of over 150 m
(3) A strong overlying rock stratum such as sandstone that overhangs into the goaf,
(4) A strong floor rock that does not heave readily, and
(5) Improper mining methods that that create localized extensive stress concentration.
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