Study on the instability characteristics and precursor signals of pressure relief in large diameter boreholes in coal seam

To investigate the mechanisms of instability and precursor information related to coal seam large diameter borehole depressurization. Based on the fine-scale numerical experiment with Universal Distinct Element Code (UDEC) and the macro failure experiment in the laboratory, the influence of borehole parameters on coal sample damage and failure was studied from the perspectives of stress, micro cracking, failure mode, and acoustic emission (AE) information. The results show that the borehole parameters mainly affect the micro damage mechanism of coal samples in the form of shear cracks. The secant modulus and compressive strength of coal sample decrease with the increase of borehole diameter, the shear crack decreases rapidly, and the degree of cavity deformation increases. The compressive strength and impact properties of coal can be effectively reduced after borehole. According to the failure pattern of coal samples, it is found that adjusting the borehole parameters can control the fracture development of coal samples and determine the final failure pattern to a certain extent. The continuous expansion of small energy micro fracture events will lead to large energy local fracture events, and then produce high stress concentration around boreholes, accelerate the intersection of local fracture areas, and lead to the overall instability of coal samples. The research provides a scientific basis for the selection and optimization of the parameters of large-diameter depressurization borehole in coal seam, and has a good guiding significance for the prevention and control of coal burst and the monitoring and early warning.