Controlling mechanism of fractures for failure patterns of borehole-containing specimens under cyclic loads

Gas drainage is one of the main methods for preventing gas accidents in coal mines. However, disturbance induced by coal mining may influence the stability of gas drainage boreholes and thus weaken the gas drainage effect. To solve the problem, loading and unloading experiments with different lower limits of cyclic stress were conducted on specimens containing a hole. At first, the fracture development and the acoustic emission characteristics of the specimens were analyzed. Then, the influences of loading and unloading under different lower limits of cyclic stress on the failure modes of the specimens were studied. Finally, the influencing factors of failure patterns of the hole were discussed. Research results show the effect of loading and unloading with different lower limits of cyclic stress on the degree of deformation and failure of the specimen. The larger the number and the more uniform the fractures around the hole in the specimens are, the larger the shrinkage of the hole at failure. In addition, the loading process of the specimens is divided into a stable period and an unstable period according to the acoustic emission characteristics in the failure process of the specimens. Research results help to determine failure modes and failure patterns of gas drainage boreholes under disturbance induced by different coal mining conditions in advance according to development characteristics of fractures around boreholes and then ascertain gas drainage states of boreholes. This prevents the occurrence of gas accidents and provides theoretical support for the safe production of coal mines.