Rocks available in the coal mining areas consist of sand stones, shales, clay and coal seams. Rocks of sills or dykes constitute irregularities which are not present at all the mines. Rocks are called massive when bedding planes in them are more than 1.2 m apart, bedded when between 75 mm and 1.2 m, and flaggy when less than 75 mm.
Rock mechanics is the theoretical and applied science of the mechanical behaviour of rock and rock masses; it is that branch of mechanics concerned with the response of rock and rock masses to the force fields of their physical environment. This definition of Rock Mechanics is widely accepted by the US National Committee on Rock Mechanics in 1966 and subsequently modified in 1974.
Rock, mechanics itself forms part of the broader subject of Geo-mechanics which is concerned with the mechanical responses of all geological materials including soils.
It is generally assumed that the weight of the rocks in coal, mining areas exerts a pressure of nearly 0.2 kg/cm2 for every metre depth from the surface. When working a mine and driving roadways in it, it is, however, the immediate roof, some 6 m above the seam, and the immediate floor, some 3 m below the seam, that matter. Timbering, stowing and other artificial supports have the object of supporting the immediate roof and sides and prevent sagging of the roof and heaving of floor of the coal seams.
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