Effects of the Deviatoric Stress Ratio on the Coal Failure and Permeability During Deep Mining

Practices show that the triaxial stress change of mining-disturbed coal deeply affects its failure and permeability characteristics and easily cause the coal and rock dynamic disasters during deep underground mining process. The combined mechanic-damage-permeability experiments of gas-bearing coal were carried out under the mechanical paths of conventional triaxial compression (CTC), fixing axial pressure and unloading confining pressure (FAPUCP) and loading axial pressure and unloading confining pressure (LAPUCP). Then the effects of the deviatoric stress ratio (DSR) on the damage and permeability characteristics of coal were analyzed. The results proved that the distinguishing deformation and strength characteristics of coal were showed due to the different mechanical paths and the coal specimens were converted from brittle deformation to plastic deformation with the effect of higher confining pressure. The coal specimens of the three mechanical paths generated the main shear failure, tensile-shear failure and ‘X’ conjugate shear failure, respectively, and the fracture angles decreased in order. The damage and permeability of coal specimens could be more and more severe in the order of CTC, FAPUCP and LAPUCP paths, which was due to the different DSR. Thus, the deep mining-disturbed coal of lager DSR is seriously prone to cause the coal and rock dynamic disasters. The corresponding measures should be taken to reduce the DSR of mining-disturbed coal and then avoid the dynamic disasters.