Investigation on confining pressure effect of coal mass containing orthogonal fractures based on synthetic rock mass approach

The distribution of discontinuities in coal mass is characterized by orthogonality.To clarify the confining pressure effect and failure mechanism of coal mass,a numerical model of coal mass containing orthogonal fractures is constructed based on discontinuities sampling campaign and parameters calibration tests.The conven tional triaxial compression tests under different confining pressures are conducted using synthetic rock mass (SRM) approach.The joints activation ratio is put forward.The influences of confining pressure on failure characteristics,strength properties,deformation behavior and joints activation for coal mass are analyzed.The joints activation mechanism and failure mechanism of coal mass under confining pressures are analyzed and verified.The results show that:① the mechanical properties of coal mass containing orthogonal fractures show heavy confining pressure effect.With the increase of confining pressure,the compressive strength,residual strength and elastic modulus are increased,and the ability of resistance to expansion deformation is enhanced.The mechanical behavior transits from brittleness to ductility.The failure form of intact coal transits from tension to shear.The activation ratio of joints is reduced,and the influence of joints on the mechanical properties of coal mass is weakened and even disappears.② The difficulty of joints activation increases sharply when the confining pressure exists and the confining pressure effect is more significant at a low confining pressure.③ When no confining pressure,the coal mass containing orthogonal fractures exhibits splitting failure and the fractures are composed of activated joints and wing fractures.At a low or middle confining pressure,the shear failure occurs with an inclined shear fracture through the coal mass.At a high confining pressure,coal mass ruptures by means of plastic flow.④ When no confining pressure,the failure of coal mass containing orthogonal fractures is controlled by joints.At a low or middle confining pressure,the fracture geometry is influenced by joints,but the influence becomes weak with the increase of confining pressure.At a high confining pressure,the failure is not affected by joints.