Research On The Dynamic Process And Instability Mechanism Of Coal And Gas Outburst

With the continuous extension of coal mines to the depths,the conditions faced by deep mining are more complicated.Influenced by the“three hights and one disturbance”the possibility of coal and gas outburst has also greatly increased.Therefore,it is of great significance to carry out research on the mechanism of coal and gas outburst,and to master the occurrence and development lows of coal and gas outburst.In this paper,guided by rock mechanics,seepage mechanics,elastic mechanics,thermodynamics,and other related basic theories.Taking the Ji _15-17 coal seam of 13th mine of Pingdingshan Tian’an Coal Industry Co.,Ltd.,as the research coal sample.The dynamic process of coal and gas outburst is studied by comprehensively using experimental research,simulation research and theoretical analysis methods,and is verified with examples.The main research results are as follows:（1）The stress-strain curve of raw coal and briquette coal samples during conventional uniaxial and triaxial compression can be roughly subdivided into the compaction,elastic,plastic,and failure stages.During uniaxial compression process,the raw coal samples shows a stepped drop after peak stress.Except for the MD group briquette coal containing 7%rosin,the other groups of briquette coal all show a stress yield plateau near the peak stress.In the process of triaxial compression,the peak strength,peak strain and elastic modulus of raw coal and briquette coal all shows a good positive correlation with the loading confining pressure.It is further found that the strength of raw coal shows strain wekening after reaching the yield point,while the strength of briquette coal shows strain strengthening under high confining pressure.Based on the similarity measure of mechanical parameters and acoustic emission（AE）characteristic parameters of the two kinds of coal samples,it is concluded that the mechanical properties of the MD group briquette coal containing 7%rosin are the most similar to the raw coal.（2）Dynamic impact tests show that the dynamic stress-strain curves of coal samples can be divided into four stages:approximate linear segment,nonlinear convex seagment,plstic deoformation seagment and stress drop seagment.The results of dynamic compressive experiments show that the dynamic compressive strength,ultimate strain and train rate of coal samples increase with the increase of impact velocity,and the threshold velocity of dymaic compressive strength increase is 8.10 m/s.In the dynamic tensile test,the impact velocity has no obvious effect on the tensile strength of coal samples.However,with the increase of impact velocity,the dynamic strain and strain rate of coal samples gradually increases,and the dynamic deformation modulus gradually decreases.When the impact velocity is greater than 4.75 m/s,under the same impact velocity,the dynamic tensile strength of the coal sample in the vertical bedding direction is higher than that in the parallel bedding direction.In the mean time,the impact angle mainly affects the propagation of parallel bedding cracks,but has less effect on the tensile strength.（3）According to the measurement results of permeability,when the gas pressure is equal,with the increase of hydrostatic pressure,the permeability of raw coal gradually decreases.Under the condition of constant effective stress,with the gas pressure increases,and the permeability of raw coal decreases gradually.Under that same condition,the permeability of the raw coal unloading point is smaller that that of the loading point.Under the action of axial and confining pressure,the permeability of the briquette coal unloading point is smaller than that of the corresponding conditional loading point.Based on the simplified dual pore structure of coal,combined with effective stress and matrix adsorption expansion deformation,the permeability model of coal in the elastic stage and plastic deformation stage is established.Combined with the permeability test data,it is proved that the model has good applicability.（4）The results of the coal and gas outburst simulation experiments shows that the coal powder particles in the outburst interval have obvious sorting properties.In the same outburst range,the coal particles with particle size less than 0.28 mm account for the largest proportion,and the outburst hole presents an inverted pear shape.The gas pressure decreases first near the outburst hole;The coal powder and spallation characteristics are most obvious in the outburst cavity.The study shows that the importance of the three factors in coal and gas outburst is:coal strength>gas pressure>in-situ stress.Under the contidions of this experiment,the initial gas pressure value of coal and gas outbsrut is 0.45 MPa.When the gas pressure is greater than 0.45 MPa,the relative outburst strength also increase as the gas pressure continues to increase.When the in-situ stress increases continuously the relative outburst strength shows the characteristic of decreasing fluctualtion.The coal strength is negatively correlated with the relative outburst strength.The lower of the coal strength,the greater the damage range of the coal at the outburst holes,and the greater the amount of outburst coal.（5）The dynamic stress change characteristics of the coal body in front of the excavation face are studied,and the mechanical instability criterion of coal and gas outburst is established.The energy change of coal and gas outburst is discussed,the expression of outburst energy is improved,and the energy instability criterion of coal and gas outburst is established.The outburst orthogonal simulation experiment verified the rationality of the mechanical instability and energy instability criteria.And the coal and gas outburst cases also verified the validity of the two instability criteria.The paper has 110 figures,34 tables and 195 references.