Study On Mechanical Mechanism Of Coal-Rock Fracture Evolution And Gas Seepage Law Under Complex Stress

During the underground coal mining process,due to the influence of mining stress,the coal and rock mass under the steady state will be unstable and destroyed,which results in redistributing of the internal stress in coal and rock mass,and changing of the internal structure of coal body,while causing the change in permeability of coal body in front of working face,especially in complex geological environment such as high geostress and high gas pressure,which is more likely to cause disasters such as coal and gas outburst,which will bring heavy losses to human beings and society.Therefore,how to eliminate the occurrence of disasters such as coal and gas accidents is one of the most concerned problems for scholars at home and abroad.In this paper,the 2+3#outburst coal seam in Shamushu Coal Mine of Sichuan Coal Group was taken as the research object.Combining with the field test,laboratory test,theoretical analysis and numerical simulation,the 2+3#coal and gas outburst coal seam in Shamushu Coal Mine was studied thoroughly through on-site mining stress test and gas drainage flow monitoring,and the results were achieved below:?1?The stress relief method was used to test the in-situ stress of the S3012 working face of the 2+3#coal seam in Shamushu Coal Mine,and the in-situ stress distribution law of S3012 working face was obtained.?2?Conventional triaxial loading and unloading seepage test,adjusting gas pressure seepage test under different stress levels and post-peak loading and unloading seepage test were carried out on raw coal samples.It is considered that the peak strength decreases linearly with the increase of gas pressure,and increases exponentially with the confining pressure.The relationship between fitting gas pressure and internal friction angle,gas pressure and cohesion are in line with linear function.Constant axial stress and unload confining pressure in the target value,the effect of constant pressure and increasing flow appears in the sample.The relationship between permeability of the sample and deviatoric stress in the process of fitting unloading confining pressure and pre-peak loading axial stress satisfies the ExpDec1 function;the seepage velocity of coal sample increases exponentially with the increase of gas pressure,and the curve by fitting seepage velocity and axial pressure,seepage velocity and confining pressure all satisfy the negative exponential function;the relationship curve between seepage velocity?permeability?and effective stress satisfies the ExpDec1 function in the process of post-peak loading and unloading confining pressure,and the seepage velocity?permeability?increases after each loading and unloading confining pressure compared with the previous loading and unloading confining pressure.?3?Conduct the test of alternating graded loading axial stress,loading and unloading confining pressure,constant axial stress and confining pressure rate,alternating loading axial stress and unloading confining pressure,and loading axial stress and unloading confining pressure under variable confining pressure unloading rate.It is proposed that the axial strain change rate?₁?,radial strain change rate?₃?and permeability change rate k?of the specimen under unit stress state were used to express the sensitively of the axial stress and confining pressure to the deformation and permeability characteristics of the specimen during loading and unloading.The axial strain variation rates?₁??,radial strain variation rates?₃??,permeability variation rates k??of each specimen in unit time were used to characterize the effects of variable axial loading rate on axial strain,radial strain and permeability of each sample.?4?Conduct the mechanical and permeability tests of rock specimens under conventional triaxial loading and unloading,and the mechanics and permeability tests of fractured rock mass under step-by-step loading axial stress and cyclic loading and unloading confining pressure.It is proposed to use the rock sample permeability damage rate and the maximum permeability damage rate to evaluate the degree of recovery and decrease amplitude of rock sample permeability.It is considered that the greater the permeability damage rate is,the worse the recovery degree of permeability will be.The larger the maximum permeability damage rate is,the larger the decrease range of permeability of rock sample will be.It is proposed to use the volume expansion rate and the maximum volume expansion rate of the rock sample to evaluate the expansion degree and increase range of the rock sample volume.It is considered that the larger the volume expansion rate of rock sample is,the larger the volume expansion degree of rock sample will be;the larger the maximum volume expansion rate is,the larger the volume increase of rock sample will be.?5?By using numerical simulation method,analysis the three-dimensional stress evolution law,three-dimensional plastic zone evolution law,the three-dimensional deformation law of the roof rock stratum,the“O”shape evolution law of the coal seam roof displacement of the coal-rock mass around the working face in the coal seam mining process,and the evolution law of the fracture field of the overlying strata during the coal seam mining process.?6?Establish a mathematical model of fluid-solid coupling of gas extraction coal and a mathematical model of dynamic change of permeability of gas containing coal,and analyze the migration law of coal seam gas drainage by COMSOL Multiphysics numerical simulation software,and the evolution law of gas pressure and permeability of coal body in front of working face.The borehole stress gauge and gas flowmeter are arranged in the coal body in front of the S3012 working face,and the distribution law of coal seam abutment stress and gas drainage flow during the mining process of working face was obtained.