Study on effect of fracture network on mechanical properties of rock specimens in uniaxial compression

In view of the influence of crack distribution on the strength and stability of rock pillars or coal pillars，synthetic rock mass modeling technology (SRM) was used to reconstruct the rock pillar model to study the influence of crack density and crack inclination on the stability of rock pillars. The flat joint bonding model (FJBM), smooth joint model (SJM) and fracture network model (DFN) were used to simulate rock block, fractures, and fracture network in the SRM model respectively. The macro-mechanical parameters of rock blocks (uniaxial compressive strength, tensile strength, Young's modulus and Poisson's ratio) of rock block were calibrated by uniaxial compression tests and uniaxial tensile tests. The comparison between the numerical simulation calibration results and the experimental results shows that the microscopic mechanical parameters of the numerical model can well match the macroscopic mechanical parameters of laboratory rock specimens.The results show that: the cracks in the rock mass significantly reduce the compressive strength of rock mass, and the strength of rock pillar decreases with the increase of fracture density P21; with the increase of fracture density P21 from 1 m/m2 to 5 m/m2, the average strength of rock block decreases from 103.5 MPa to 69.5 MPa, which shows that the compressive strength of rock mass is negatively correlated with fracture density; with the increase of fracture inclination angle from 0° to 90°, the compressive strength of the rock pillar shows a "U" change; when the fracture density is small, the rock pillar is easy to form "X" compression-shear failure, and when the fracture density is large, the “X” compression-shear failure of the rock pillar gradually disappears; the micro-cracks produced in rock pillars are negatively correlated with the density of rock fractures; when the dip angle of the fracture is less than φ or greater than 45°+φ/2, the rock pillar is prone to form “X” compression-shear failure, while when the dip angle of the fracture is between φ and 45°+φ/2, the rock pillar is prone to form shear-slip failure. The SRM modelling technique can provide an idea for determining the strength of rock pillars.