Coal Mechanics / Угольная механика

Coal is a type of solid-state combustible organic rock, which is transformed from plant remains by complex biochemistry, physical chemistry and geochemistry effects also known as the coal-forming process. Through deposition, coal eventually forms a seam within coal measure strata. Compared to the ordinary sedimentary rocks, coal has the following features: the spacing of coal’s structural surface (cleat) is much smaller, when viewed under a microscope; the density of coal cleat can reach several to hundreds of cleats/m; coal can be classified as a discontinuous body consisting of fractures (cleats) and matrix; the average density of coal is about 50% of the rock; the coal strength is much lower than the strength of a typical sedimentary rock which is reflected in lower compressive strength, tensile strength and elastic modulus as well as larger Poisson’s ratio; coal is usually porous with well-developed pore structure where the specific surface area of coal micropores and minipores can exceed 80% of the total surface area, providing abundant space for gas adsorption; the maximum adsorption capacity of coal can reach dozens of cubic metres per tonne; after gas adsorption, the sorption-induced swelling effect will lead to weaker coal and reduced permeability. Coal with large gas content is generally weaker with larger Poisson’s ratio, leading to greater deformation and increase in strain energy storage under the same stress state. During mining activities in a coal mine, the stress changes occur in coal ahead of the working face, making it easier for the coal body to destabilize causing rib failure, spalling, coal extrusion or bumps, and under the extreme high gas pressure conditions, coal and gas outbursts may occur. <...>