Mechanics And Acoustic Emission Characteristics Of Coal-rock Combined Body Subjected To Pre-static And Cyclic Disturbing Loading

The essence of the underground coal pillar instability is the instability of the combined system of "roof-coal pillar-floor",and the instability of any medium in the system will cause the overall instability of the system.Therefore,the study of the instability mechanism of the "roof-coal pillar-floor" combination system can reveal the essential characteristics of coal pillar instability more than just studying the instability of coal body.In addition,the "roof-coal pillar-floor" combination type in the coal mine in-situ engineering is diverse,and it is of great significance to study the mechanical characteristics of the combined body with different combination types and rock proportions to scientifically evaluate the stability of the in-situ coal-rock masses and ensure the safe production of the mine.Based on the conventional uniaxial compression test results of 16 upper coal,roof and floor specimens in Yangcun coal mine,the mechanical characteristics of coal rock combined body under conventional uniaxial compression and pre-static and cyclic disturbing loading were studied by using PFC numerical simulation and theoretical analysis methods.The main conclusions are as follows:(1)The strength and elastic modulus of sandstone-coal-sandstone(SCS)and mudstone-coal-mudstone(MCM)combined bodies increase with the increase of rock proportion according to the exponential function law in the conventional uniaxial compression test.(2)In the conventional uniaxial compression test,the axial strain corresponding to the stress peak point of SCS and MCM combined bodies shows the evolution characteristics of "first decreasing,then increasing" with the increase of the rock proportion.(3)In the conventional uniaxial compression test,the bursting energy index of SCS and MCM combined bodies shows the characteristics of "two ends large and the middle small",that is,the main factor determining the bursting tendency of coal-rock masses is the ratio of coal-rock height.(4)Pre-static and cyclic disturbing loading have two effects on the coal-rock combined body,namely: plastic softening and plastic strengthening,and the mutual restriction and struggle of these two effects determine the final strength of the coal-rock combined body;and its strength has a good exponential function relationship with the rock proportion.(5)For the cyclic disturbing loading path designed in this paper,in the process of cyclic disturbing loading with the same stress increment,,when the irreversible strain increases at the end of a disturbing loading cycle,and the elastic modulus decreases in the subsequent disturbing loading phase,it indicates that the specimen is about to be destroyed.(6)The maximum acoustic emission hit count of SCS and MCM combined bodies shows the characteristics of "two ends large and the middle small" with the rock proportion.