Mechanical charactristics of deformation and damage mechanism of sandstone under different confining pressure

The sandstone of the roof of coal seam 21 in Zhaogu mine was tested under uniaxial compression to obtain its mechanical parameters. The meso-mechanical parameters were got by Particle Flow Code(PFC), in combination with fish program, the test was conducted to study sandstone strength, deformation characteristics and damage mechanism under different confining pressures. It is concluded that the sandstone yield stage increases with the increase of confining pressure, the variation of peak strength with confining pressure is more sensitive under 40 MPa confining pressure, the sensitivity is reduced when confining pressure is greater than 40 MPa. As the confining pressure increases, sandstone failure mode gradually changes from brittle model into ductile failure; sandstone volumetric strain experiences three stages:linear compression, linear expansion and nonlinear expansion. The linear expansion stage shortens and the nonlinear expansion stage lengthens with confining pressure. There is a exponential variation among dilatation index, the axial strain of initial point and confining pressure. Sandstone damage evolution in failure process is divided into four stages:dispersion distribution, damage gathered in some zone, formation of local fractures and formation of rupture surface. Sandstone is destroyed in compression and shear mode along the single rupture surface under lower confining pressure, and destroyed in X shearing failure mode under higher confining pressure.