Study On Mechanism And Prevention Of Coal Rock Burst Based On Stress Drop Under Hydraulic Coupling Regulation

With the coal resources of shallow layer in the China gradually drying up,coal resources mining will move further deep by years.Many experts and scholars at home and abroad have done a lot of research on rock burst,and obtained rich technical achievements.However,the theoretical achievements are relatively few,especially the mechanism and model of critical transition mechanics in static-dynamics are still lacking.At the same time,rock burst occurs suddenly,it is difficult to obtain the characteristics before the occurrence,so prevention and treatment become difficult.Therefore,the deep research on the mechanism and prevention of rock burst in deep wells has become an urgent safety problem.Based on LNMRI technology,the distribution characteristics of coal rock pore-fracture structure,time-space evolution of water transport and seepage law of coal rock self-imbibition are obtained.The SHPB system is used to carry out the impact mechanical tests of coal rock under lateral constraints and active confining pressures,and the dynamic mechanical responses of coal rock with different water content under tensile and compression conditions are revealed.The geometric and topological parameters of the stress drop model are obtained based on the analysis of the dynamic evolution characteristics of the fracture network.The probability density function of Weibull distribution and critical condition formula of catastrophic failure of brittle coal rock are put forward.The stress drop model of macroscopic failure and rockburst evaluation model based on stress drop are established.The established model has good application effect.The main research results are as follows:(1)Study on pore fracture structure and microscopic seepage characteristics of coal rock based on LNMRI technology.LNMRI technology is used to monitor the experimental coal samples,and the T2 and HSE maps are obtained,as well as the one-dimensional water signal distribution of the coal samples at different times and locations,which can intuitively display the pore and fissure structure characteristics and distribution rules of water in the coal body.The time-space evolution of water absorption and water content is analyzed from the microscopic point of view,and the microscopic seepage characteristics of self-imbibition are further revealed.It is found that the proportion of micropores and mesopores in coal samples is generally high,and the water absorption is fast in the early stage,while the proportion of microcracks is small and the water absorption capacity is strong in the middle and late stage.The smaller the micro fissure content of coal rock is,the higher the proportion of isolated pore fissure is,and the slower water absorption is.At the same time the higher the water injection pressure is,the stronger the water absorption capacity of coal rock is.(2)Experimental study on impact dynamics of natural and saturated coal rockby SHPB system.Based on SHPB system,dynamic tensile and compression experiments of coal rock under natural and saturated conditions are carried out.It is found in the experiment that under the action of highspeed impact load,the dynamic tensile strength,dynamic compressive strength and elastic modulus of coal rock under the saturated state are smaller than those under the natural,and the peak strain is larger,indicating that water has a significantweakening effect on coal rock.Under both natural and saturated conditions,coal rock have obvious loading rate effect,and the dynamic tensile strength of coal rock shows strong linear correlation to the loading rate.The energy absorption value per unit volume of natural coal rock is significantly larger than that of saturated,and the energy dissipation capacity of natural coal rock is stronger.High-speed photography shows that the failure of the specimen can be divided into four stages:crack initiation,propagation,secondary crack and failure.There are three types of fragments:large fragment of unilateral tensile fracture surface,fragments of bilateral tensile fracture surface and fragments of shear fracture surface.With the increase of strain rate after tensile failure of coal rock,the fracture section develops in the above order.The geometric and topological parameter evolution of fracture network evolution during coal rock failure is compared to reveal the dynamic failure mechanism of coal rock.(3)Experimental study on impact dynamics of natural and saturated coal rock under lateral constraints and active confining pressures.It is found that the dynamic compressive strength and elastic modulus of saturated coal rock under lateral constraint are higher than those of natural coal rock,and the peak strain is lower than that under uniaxial compression,which is contrary to uniaxial compression.The constraint effect is significant.Under lateral constraints,the impact pressure,axial stress and lateral stress are positively correlated,and the lateral stress of saturated coal is higher than that of natural coal.Under active confining pressure,the dynamic stress and elastic modulus of saturated coal rock are higher than those of natural coal rock.After the stress drop,the natural coal sample appears obvious plastic flow under stress superposition.Under the condition of active confining pressure,saturated coal sample is more sensitive to impact load rate,and the influence of active confining pressure is more obvious.(4)Study on macroscopical and microscopical failure laws of natural and saturated coal rock under impact.The macroscopic failure modes of coal rock are affected by the impact load rate and confining pressure environment.The greater the impact rate is,the greater the failure degree is.Under dynamic tensile loading,the particle size distribution of coal rock fragments is affected by water state,and large size fragments are more easily generated in dry coal rock.Under dynamic uniaxial loading,both coal samples show comminding failure under two water-bearing states,and the weighted average of fragmentation of natural coal samples is higher than that of saturated.Compared with confining pressure,the damage degree of coal sample under active confining pressure is higher.The damage degree of coal rock under active confining pressure is between uniaxial impact and lateral constraint,and the damage degree of saturated coal rock is greater.(5)Stress drop theoretical model and stress drop model considering dynamic impact effect.Based on the quantitative definition of brittleness and parallel bar model,the Weibull probability density function ω(kx)=1-exp[-(kx/ω)m]and the critical condition formula Kf=-dw0/dsm of catastrophic failure of brittle materials such as coal and rock mass are obtained considering the damage distribution and critical condition,and the theoretical models of sudden or catastrophic stress drop and progressive stress drop are obtained.Based on The Coulomb strength theory,the strength degradation model is established and the theoretical formula Δσθ=σθ-σθf,of stress drop is obtained.The time variable is introduced into the damage model to obtain the stress drop model considering the dynamic impact effect.(6)Rockburst model based on stress drop considering hydraulic coupling induced action is established,verified and applied in field.Considering the disturbance mode of rock burst and other factors,the rock burst evaluation mode Φ=αΔσdropRσ0/(σt)2 is put forward considering the stress drop,and the established model has been proved effective in several mining areas.