| With the increase of mining depth and the complexity of geological structure,dynamic disasters such as rock burst and coal and gas outburst occur frequently and seriously.Geological structure plays an obvious role in promoting the occurrence of dynamic disasters.Among them,the coal seam erosion zone is a common geological structure in coal mine.The coal-rock system near the erosion zone is not only affected by the structural stress,but also by the cyclic load caused by the engineering activities such as chamber blasting,roadway driving and working face mining.The coal-rock system will inevitably cause damage under the action of similar cyclic loading and unloading,reduce the bearing capacity of coal-rock structural body,and lead to the instability of roadway and coal pillar.Therefore,this paper takes the coal seam erosion zone as the engineering background,the methods of the theoretical analysis,laboratory test,numerical simulation and field practice was used to study the mechanical characteristics,energy evolution characteristics and failure mechanism of coal-rock structural body with different lithology and coal-rock height ratio under the cyclic loading and unloading.These studies have important guiding significance for the scientific evaluation of coal-rock system stability and the prevention of coal mine dynamic disaster.The main research contents and achievements are as follows:(1)According to the characteristics of coal-rock structure in coal seam erosion zone,the mechanical model of coal rock structure was established.The mechanical properties of the coal-rock structural body with different lithology and coal-rock height ratio were analyzed.By using FLAC3D simulation software,the numerical model of coal seam erosion zone was established to study the influence of slope dip angle,coal-rock height ratio,coal-rock strength ratio and confining pressure intensity on the accumulated energy in coal seam erosion zone,and the sensitivity of influencing factors is analyzed.(2)The coal-coarse sandstone structural body,coal-fine sandstone structural body and coal-mudstone structural body with the coal-rock height ratio of 1:3,1:2,1:1,2:1 and 3:1 were tested by uniaxial compression,and the influence of different lithology and coal-rock height ratio on the mechanical characteristics of the structural body was analyzed.The peak strength and elasticity modulus of coal-rock structural body are between coal and rock monomer,but closer to the peak strength of coal monomer.With the increase of rock strength,the mechanical properties of the structural body is slightly enhanced.The crack initiation stress and failure mode of coal-rock structural body were analyzed.The coal specimen in coal-rock structural body was the first failure body,while the rock fails before it reaches its failure strength,mainly due to the rapid crack growth and sudden release of energy in the coal specimen.(3)The coal-coarse sandstone structural body,coal-fine sandstone structural body and coal-mudstone structural body with the coal-rock height ratio of 1:3,1:2,1:1,2:1 and 3:1 were tested by uniaxial cyclic loading and unloading test.The mechanical characteristics of the coal-rock structural body with the different coal-rock height ratio and different lithology under uniaxial cyclic loading and unloading were analyzed,and the peak strength under uniaxial cyclic loading and unloading was compared with that under uniaxial compression.Through the CT image,AE system and the DVC system,the crack development and failure mode of coal-rock structural body during the cyclic loading and unloading were analyzed.The influence of coal-rock height ratio and rock lithology on the failure mode of structural body was obtained.(4)The energy evolution and AE characteristics of coal-rock structural body under uniaxial cyclic loading and unloading are studied.The damage variable and cumulative damage variable of coal-rock structural body in the process of loading and unloading were calculated by dissipative energy method and AE method respectively.The same and different points of the two calculation results were analyzed and compared.On this basis,the joint calculation method of damage variable was proposed to describe the evolution of damage variable of structural body under cyclic loading and unloading more accurately.(5)The triaxial cyclic loading and unloading tests with confining pressure strength of 3Mpa,6Mpa and 9Mpa were carried out for the coal-rock structural body with the height ratio of 1:3,1:2,1:1,2:1 and 3:1.The effects of different confining pressure strength and different coal-rock height ratio on the mechanical properties,energy variation and damage variables of the structural body under the triaxial cyclic loading and unloading were studied and analyzed.And the effects of confining pressure strength and coal-rock height ratio on the crack initiation stress and failure mode of the structural body were obtained.(6)Based on the characteristics of energy storage in front of working face,the concept of energy judgment coefficient(Q)was proposed from the perspective of energy release.The law of energy variation in the process of coal seam erosion zone was analyzed by using energy judgment coefficient.And the reliability of energy judgment coefficient was verified by microseismic monitoring method and drilling cuttings method,which provides a new way to predict the occurrence of rock burst. |
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