Mechanical Properties And Instability Mechanism Of Loaded Fractured Coal-Rock Combined Body

With the increase of coal mining depth and the expansion of mining scale,the frequency and intensity of mine geological disasters increase significantly,and the rock burst is the most serious.According to the investigation,the rock burst mostly occurs in the fractured coal and rock body.In underground engineering,there are often one or more irregular micro-cracks in coal and rock body.This kind of fractured coal and rock body under the action of mine pressure,the cracks gradually develop and expand,which eventually lead to the instability and failure of coal and rock body,and then lead to coal mine power disasters.The existence of cracks in coal and rock has a great influence on the stability of coal and rock.The main conclusions are as follows:The occurrence of rock burst is not the result of the single action of coal or rock body,but the result of the joint action of coal and rock.This paper takes the fractured coal-rock combined body as the research object,uniaxial compression test and acoustic emission experiments are carried out for coal-rock combined body with different crack length,angle and width,and numerical simulation experiments are carried out for coal-rock combined body with different fracture number and position by using PFC^2D simulation software.To explore the evolution law of mechanical parameters,acoustic emission energy and ringing count of coal-rock combined body with different fracture length,angle,width,location and quantity;the macroscopic failure patterns of fractured coal-rock combined body are compared and observed,the crack propagation law and failure mode are analyzed,and the instability mechanism of fractured coal-rock combined body is discussed.（1）When the coal and rock body with cracks are subjected to certain pressure,the cracks will germinate at the crack tip,there are four kinds of cracks:tension crack,anti-wing crack,shear crack and shear wing crack,different cracks cause different failure modes.（2）Based on the results of laboratory and numerical simulation experiments,it is found that cracks can obviously weaken the mechanical parameters of the combined body.The length,width and number of cracks are negatively correlated with the mechanical parameters of the combined body,the larger the length,width and number of cracks are,the smaller the mechanical parameters of the combined body are;When the fracture angle is 0°～45°,the degree of deterioration increases continuously,when the angle is 45°～90°,the deterioration effect of fissures on the mechanical properties of the combined body is gradually weakened.Through the experimental data of coal-rock combined body at different fracture positions,it is found that the closer the fracture is to the middle position of coal body,the smaller the mechanical parameters of the combined body.In addition,the acoustic emission characteristic parameters of different fractured coal-rock combined body have different rules.When the crack length increases continuously,the peak energy,cumulative energy and cumulative ring count in the fracture process of the combined body increase at first and then decrease.The crack angle is positively correlated with the peak ring count,and increases at first and then decreases with the peak energy and cumulative energy.The peak energy is the highest when the angle is 60°and the cumulative energy is maximum when the angle is 30°.When the crack width is increasing,the peak energy and cumulative energy of the combined body are also increasing,and the peak ring count decreases at first and then increases.（3）Fissures have a certain influence on the failure form of coal-rock combined body.The length and width of the crack have little influence on the crack propagation and failure mode of the combined body,which is the mixed failure of tension and shear.The angle,quantity and position have great influence on the crack propagation and failure mode of the combined body.Shear failure occurs when the angle is 0°～30°.When the angle is 45°～60°,the combined body is tension-shear mixed failure.When the angle is 90°,the combined body shows compression-shear failure.The failure mode of coal-rock combined body with different fracture number and location gradually changed from tension-shear mixed failure to shear failure.（4）In this paper,the mechanical model of fractured coal-rock combined body is constructed,and the stress is analyzed,The instability and failure process of fractured coal-rock combined body under loading is divided into five stages.Analysis of the influence of cracks on coal-rock combined body from the point of view of energy,it is found that there is an energy concentration zone at the crack tip,the energy transfer mechanism of instability and failure of coal-rock combined body is discussed,under the loading condition of coal-rock combined body,coal accumulates more energy,after reaching the strength limit of coal body,the failure of coal body releases energy to rock body,when the strength limit of rock body is reached,the rock body is destroyed.The existence of cracks reduces the ultimate strength of coal and rock body,the energy transfer rate and transfer efficiency are improved,so fractured coal and rock are more prone to instability and failure.