Failure Characteristics of Rock Mass Under the Coupled Action of Three-Way Isotropic Confining Pressure and Blast Loading

In this work, the effects of three-way confining pressure on the force distribution, fracture expansion and damage extent in the blasted rock mass were observed. With the fluid attenuation law introduced, the effect of blasting gas on crack propagation was primarily considered. Based on the ideal gas equation of state, the initial propagation pressure of the cracked zone as a function of the peak pressure on the blast hole wall was established. The method for determining the internal stress intensity following blast loading of the rock mass under confining pressure was derived utilizing the principles of elastic mechanics. A forecasting model of the extent of the cracked zone was proposed, taking into account the effect of confining pressure on the strength of the rock mass. Subsequently, the force distribution within and the extent of the cracked zone in the blasted rock mass were analyzed. Using Polymethyl Methacrylate (PMMA) specimens as an equivalent replacement for rock specimens, blasting experiments under different initial confining pressures were conducted on the in-house triaxial hydraulic loading rock blasting test jig, and the morphology, crack distribution, fractal dimension and damage of the blasted specimens were examined. Finally, the stress distribution and damage extent in the near-field, far-field blast zone in the blasted rock under the confining pressure action were analyzed by the FEA software LS-DYNA. Results showed that the initial confining pressure applied to the rock mass would reduce the fractal dimension, the average damage and the extent of the cracked zone of the rock mass. As the confining pressure increases, the ratio of such reduction gets smaller. As the confining pressure further increases, the tensile failure zone of the rock mass gets smaller, and the confining pressure was found to suppress the tensile failure of the rock mass and cause the transition of the rock mass to compression failure. The blast loading predominates in the near-field blast zone, where the initial confining pressure has little impact on the blast stress field; in the far-field blast zone, the initial confining pressure begins to impact the coupled stress field as the blast stress wave further attenuates. As a consequence, the tangential peak stress in the rock mass arrives at a time lagging behind the radial peak stress, and the tangential tensile peak stress would decrease as the confining pressure increases, which in turn restrains the crack propagation.