Experimental study on mechanical response characteristics of coal specimens with boreholes under cyclic loading

To investigate the mechanical response characteristics of coal specimens with boreholes under different loading rates and cyclic loading, mechanical tests were conducted on coal specimens with boreholes selected from a working face of Changcun Coal Mine under different stress paths (monotonic loading and cyclic loading–unloading) and loading rates (0.5 and 2.0 kN/s). A non-contact full-field strain measurement system and an Acoustic Emission (AE) monitoring system were used to examine the strength, deformation, failure modes, and fracture evolution characteristics of the coal specimens with boreholes. The results showed that: ① Compared with that of coal specimens without boreholes, the uniaxial compressive strength of coal specimens with boreholes decreased by 9.50%–26.24% under the specified loading paths and rates. ② Under monotonic loading, when the loading rate increased from 0.5 to 2.0 kN/s, the uniaxial compressive strength of coal specimens with boreholes increased by 7.70%. Significant strain concentration occurred around the boreholes, and the high-strain zones exhibited a butterfly-shaped distribution and extended along the axial loading direction, forming a typical X-shaped failure dominated by tensile failure. At the low loading rate, the fractures exhibited dispersed initiation and local coalescence. At the high loading rate, the fractures were activated earlier and exhibited burst-like characteristics of rapid propagation and sudden coalescence. ③ Under cyclic loading–unloading, at the low loading rate, the cumulative strain increased only slightly and the cumulative AE energy increased gradually. At the high loading rate, the AE energy growth rate increased markedly, and the fractures coalesced during the early loading cycles. These results provide theoretical support for the stability control of gas drainage boreholes and optimization of sealing processes.