Study On Microscopic Pore Structure Variation Characteristics Of Structurally Anisotropic Coal Under Impact

The microscopic pore characteristics of coal and rock mass are used as an important theoretical basis for coal mining,coalbed methane occurrence state prediction and gas outburst prevention and treatment.It has important engineering significance.This paper takes the coal body of No.3 coal seam in Chengzhuang Coal Mine,Shanxi Province as the research object.Based on the field explosion impact test device and Hopkinson pressure bar(SHPB)system,three different types of impact tests were carried out on coal with different structures.The microscopic pore structure of coal samples before and after impact was analyzed,and the following main conclusions were obtained:(1)In the field explosion impact tests with different impact loads,the results show that the trend of stress curve is basically the same.The peak stress of structurally heterogeneous coal is positively correlated with impact load.(2)After the coal samples are impacted by the SHPB compression bar system with different impact strengths in uniaxial and triaxial,the stress-strain curve trends are basically the same,and they are all composed of linear elastic stage,elastoplastic stage,and plastic softening stage.The curve of elastic-plastic stage in triaxial SHPB impact test is more gentle than that in uniaxial SHPB impact test.Peak stress is positively correlated with the average strain rate.Compared with uniaxial SHPB impact test and triaxial SHPB impact test,triaxial SHPB impact test can better improve the dynamic mechanical properties of coal samples.The peak stress and average strain rate of coal samples in different bedding directions are compared.When the impact load is the same,the vertical bedding direction is greater than the parallel bedding direction.(3)In the low-temperature liquid nitrogen adsorption test,the adsorption-desorption isotherms of coal samples with different structures in raw coal,field explosion impact test,and triaxial SHPB impact test all belong to type IV liquid nitrogen isotherms,and the single-layer-multilayer isotherm mainly occurs.However,the adsorption-desorption isotherms of coal samples after uniaxial SHPB impact test belong to the type III liquid nitrogen isotherm,which mainly occurs in multilayer adsorption for nitrogen.The pore size distribution,specific surface area and pore volume ratio of coal samples before and after different impact tests remained stable.The proportion is between 0.3% and 5%,and the specific surface area and pore volume of the coal sample show a negative correlation as a whole with the increase of the impact load.(4)In different impact tests,the impact load of coal samples has a negative correlation with the adsorption of nitrogen and micro specific surface area.The overall trend of reduction is almost the same in different impact tests.The results show that the micro pore structure of coal sample changes obviously after being impacted.