Mechanical properties of coal deformation and its influence on permeability

By the mechanical tests of the coal samples,the physical and mechanical properties of coal and the law of permeability during the complete stress-strain process were studied.It is shown that,compared with the rocks from coal roof and floor,the coal is more prone to plastic deformation with the property of lower mechanical strength,lower elastic modulus and higher Poisson's ratio.For the coal samples which have obvious strain-softening character during the complete stress-strain process,first,the coal sample volume is compressed and the permeability of coal with stress increases slightly or permeability changes little in the micro-cracks closure and elastic deformation stage.After the stress increases higher than the elastic limit,the coal sample gets into the crack propagation stage.The volume strain of coal changs from compression to expansion.The permeability of coal first slowly and then sharply increases with an increasing of crack extension.During the strain-softening stage,the coal permeability achieves the maximum and follows by a sharp decrease.The coal permeability after the peak strength is generally greater than that before the peak.For coal samples which have little strain-softening character or strain-hardening during the complete stress-strain process,the maximum permeability is mainly achieved in the plastic deformation stage before the peak.During the strain-hardening stage after the peak strength,the permeability decreases with an increasing of stress.Commonly,the coal permeability after the peak strength is generally less than that before the peak.