Study on the Variation Characteristics and Laws of Hole Formation in Coal-Rock Masses under HPWJ Impact Fracturing

To investigate the characteristics and regularities of borehole evolution during coal-rock mass fracturing induced by high-pressure water jet (HPWJ), multifactor coupled perforation experiments involving jet pressure, confining pressure, and time were conducted, and the SPH-FEM method was employed to analyze the coal-breaking process of HPWJ. The breakage mechanism of coal-rock mass under HPWJ impact is elucidated, along with the key influencing factors and characteristics governing cavity evolution. The results demonstrate that increasing jet pressure significantly enhances the expansion of both the external diameter and depth of holes; appropriately extending perforation duration improves the effectiveness of jet-induced coal-rock fragmentation, while elevated confining pressure exerts an inhibitory effect on both the rock-breaking process and borehole formation quality. The dynamic evolution of voids exhibits typical phased characteristics: during the rapid expansion stage, geometric parameters of voids sharply increase, leading to accelerated formation; in the stable adjustment stage, the growth rate of these parameters significantly decelerates, resulting in gradual development trends. An increase in jet pressure promotes the growth of the number of damaged elements, enhances the degree and rate of damage to the coal-rock mass, and induces expansion of cavity morphology parameters, whereas an increase in confining pressure significantly inhibits the growth of damaged elements, affects the failure mode, reduces coal-breaking efficiency, and restricts the expansion of cavity morphology parameters.