Study On Precursory Law Of Micro-fracture Of Rockburst And Dynamic Instability Mechanism Of Surrounding Rock

With the normalization of deep coal mining,rockburst disasters occur frequently,which seriously threatens the safety of mine production.In this paper,aiming at the scientific problem of‘Precursory law of micro-fracture of rockburst and dynamic instability mechanism of surrounding rock’and relying on Xiashijie Coal Mine in Shaanxi Province as the engineering background,the microseismic monitoring method is used to deeply excavate and analyze the micro-fracture precursor law of rockburst and the focal mechanism,and a new indicator of rockburst early warning is proposed.At the same time,based on the rock real fracture process analysis software RFPA^2D-Rockburst,the whole process simulation analysis of coal and rock mass from static energy accumulation to dynamic energy release in Xiashijie Coal Mine is realized,and the influence of different coal seam dip angles and lateral pressure coefficients on the mechanical response of surrounding rock in rockburst roadway is discussed.The main research results obtained in this paper are as follows:（1）Based on microseismic monitoring technology,the precursor law and source mechanism of micro-fracture of rockburst in Xiashijie Coal Mine are excavated and analyzed.The fracture of the hard main roof in the far field of the gob side roadway will lead to intensive high-energy microseismic events.The occurrence time is closely related to the periodic weighting,and there is an obvious periodic law,thus causing strong dynamic load disturbance to the gob side roadway Low energy microseismic events gather and distribute near-field surrounding rock of adjacent roadway,forming high apparent stress zone and high energy release zone.Based on energy index（EI）and cumulative apparent volume（CAV）,a new indicator lgCAV/lgEI for rockburst early warning is proposed,which can effectively evaluate the risk degree of rockburst.The larger the value of lgCAV/lgEI is,the higher the rockburst risk is.When the value of lgCAV/lgEI increases sharply and exceeds 10,it indicates that the rockburst risk increases.The high energy microseismic events caused by the fracture of hard roof in Xiashijie Coal Mine are mainly tensile failure.（2）Based on RFPA^2D-Rockburst,the mechanical response characteristics of coal and rock in Xiashijie coal mine from static energy accumulation period to dynamic energy release period were analyzed.In the static energy accumulation period,acoustic emission is tensile-shear mixed failure,and the number and energy release increase of AE sharply.In the dynamic energy release period,the coal and rock mass first enter the quiet period of acoustic emission,and then the number and energy of AE increase sharply.The newly generated AE is mainly tensile failure.The surrounding rock stress of the gob side roadway shows‘double peak’characteristics,and the peak stress occurs in the static energy accumulation period and dynamic energy release period respectively.The horizontal displacement of the coal side of the gob side roadway fluctuates steadily first,then increases slowly and then increases sharply;the vertical displacement of adjacent roadway floor first fluctuates smoothly and then increases sharply.In the dynamic energy release period,along the top and bottom corners of the roadway to the middle,the closer to the top and bottom corners,the greater the PPV（particle peak velocity）.On the contrary,the smaller the PPV,the rockburst risk area can be divided into:top angle position>bottom angle position>middle position.（3）The surrounding rock mechanical response characteristics of rockburst roadway under different coal seam dip angles and lateral pressure coefficients were discussed.During the incubation period of rockburst,the roadway failure areas under different coal seam dip angles are mainly concentrated in the left and right sides of the coal sides.With the increase of coal seam dip angle,the damage degree of roadway decreases gradually.When the lateral pressure coefficient K is 0.4 to 0.8,two symmetrical cracks are mainly formed downward obliquely along the upper left corner and the upper right corner of the roadway.When K exceeds 0.8,four symmetrical cracks are formed downward obliquely and upward obliquely along the top and bottom angles of the coal sides of the roadway.In the occurrence period of rockburst,secondary failure of surrounding rock of roadway under different coal seam dip angle and lateral pressure coefficient,and the‘splitting’tensile cracks are produced in the left and right sides of the roadway.During the occurrence period of rockburst,with the gradual increase of coal seam dip angleθ,the rockburst risk on the left side of roadway increases sharply first,then decreases rapidly and finally tends to be constant.Whenθis within 20°,the rockburst risk is higher near the upper left corner of roadway.Whenθexceeds 20°,the closer to the lower left corner of the roadway,the higher the rockburst risk.Whenθis within 15°,the rockburst risk of the left side of the roadway is higher than that of the right side.Whenθexceeds 15°,the rockburst risk of the right side of the roadway is higher than that of the left side.There is a positive correlation between the（PPVx）_max of coal wall and coal rock failure degree of two sides of roadway under different dip angles.With the increase of the lateral pressure coefficient K,the rockburst risk of the left side of the roadway first increases,then decreases and then increases,and the closer to the upper left corner of the roadway,the higher the rockburst risk.With the increase of K,the rockburst risk of the right side of the roadway first decreases and then tends to be stable.When K is within 0.8,the rockburst risk of the right side of the roadway is higher than that of the left side.When K exceeds 0.8,the rockburst risk of the left side of the roadway is higher than that of the right side.The damage degree of the left side of the roadway is negatively correlated with the（PPVx）_max of coal wall under different lateral pressure coefficients,and the influence distance of the right side of the roadway is positively correlated with the（PPVx）_max of coal wall.