Research On Macro- And Micro-mechanisms And Precursor Information Of Large-diameter Drilling Pressure Relief In Coal Seam

Since the advent of deep mining,coal deposits located deep within the earth have been subject to a complex and challenging mechanical environment,characterized by the presence of"three highs and one disturbance".This has resulted in a significant increase in the occurrence of disasters like rock bursts,which have a severe impact on the safety and efficiency of mining operations.Large-diameter drilling,a widely adopted technique in the coal industry,is effective for transferring the high stress of surrounding rock and reducing the risk of regional shock.However,there remains a dearth of research on the macro and micro pressure relief mechanisms specific to coal seams.Through theoretical analysis,numerical simulation,and laboratory experiments,this paper investigated the quantitative relationship between drilling parameters and pressure relief effects.Furthermore,we examined the impact of drilling parameters on the macro and micro damage mechanisms of the drilled coal and analyzed the fracture characteristics.Finally,the precursory of drilled coal instability was proposed.The following conclusion can be drawn:（1）The influence of large-diameter borehole control layout parameters,namely diameter,spacing and depth on the unloading effect of coal seams was investigated using theoretical analysis and FLAC^3D simulations.Orthogonal design was employed to systematically set the parameter values.The effect level of these three drilling parameters on pressure relief was assessed by FLAC^3D simulation.The results reveal that the most significant factor affecting pressure relief effectiveness is borehole diameter,followed by spacing and depth;The stress distribution of the coal after drilling was analyzed by slicing using Tecplot,a post-processing software.The results show that with the increase of borehole diameter,the range of stress reduction zone around the hole increases.When the diameter is fixed,reducing the spacing between holes can effectively connect stress-reduced areas and create a pressure-relieving weakened zone.Meanwhile,there is a positive correlation between drilling depth and stress transfer distance,when a pressure relief weakening zone is formed.（2）Through secondary development of UDEC numerical software,a damage evaluation index for drilled coal was constructed,namely,stress,microcracks,and failure modes,and from the perspectives of these indexes,the effects of borehole parameters on the macro and micro-damage mechanisms of coal and the unloading laws were explored.The results show that the influence of drilling parameters on mesoscopic damage mechanism of the coal sample is mainly reflected in the shear cracks;Increasing the diameter of borehole results in a reduction of the modulus of elasticity and compressive strength of coal.Additionally,this increase causes a rapid decrease in shear cracks,while increasing the degree of hole deformation.The unloading effect is also significantly improved;The unloading effect increases initially but eventually decreases with the increasing spacing between boreholes;There is a negative correlation between row spacing and unloading effectiveness,with increasing row spacing leading to reduced unloading effectiveness.（3）By conducting acoustic emission experiments on coal samples under uniaxial loading in the laboratory,the relationship between drilling parameters and pressure relief effect were investigated based on stress,modulus of elasticity,failure mode.The fracture evolution characteristics of coal samples were analyzed by acoustic emission information during the entire experiment process.The results show that the compressive strength and impact properties of the coal samples are effectively reduced after drilling,and the influence of variable drilling parameters on the uniaxial compressive strength was basically consistent with the simulated laws;According to the failure modes of the coal samples,it is found that adjusting the drilling parameters can control the development of surface cracks and determine the failure mode;Small-scale energy micro-cracking events that continue to expand can cause large-scale energy local rupture events,resulting in high stress concentration around the borehole and accelerating the intersection and connection of local rupture zones,ultimately leading to instability.（4）Using the monitored data from acoustic emission events and the normalization method,six active multidimensional information indicators,including total fault area,acoustic emission activity,time information entropy,algorithm complexity,spatiotemporal diffusion,and Z-Map value,were identified and compared.The accuracy of the six indicators for early warning was analyzed.The results show that the warning accuracy of acoustic emission activity,spatiotemporal diffusion,and Z-Map value are high,all above 88.9%;When the values of these indicators rapidly increased to 0.75 or higher,the coal would face peak fracture or instability,which means these indexes have a significant early warning effect.