Study On Mechanical Properties And Acoustic Emission Characteristics Of Water-bearing Coal Under High Stress

Water is one of the important factors affecting the safety and efficient production of coal mine.The mechanical properties of coal and rock are different whether it contains water or not.In recent years,with the implementation of the"One Belt,One Road"strategy and the gradual westward shift of China’s key coal production capacity,the development of Jurassic coal resources in the Ordos Basin has become an important support for the sustainable development of China’s coal industry.The 4~#coal seam is mainly mined in Binchang mining area,and the water content of the coal seam is weak.However,under the influence of mining movement,the water-conducting fracture zone will directly spread to the overlying conglomerate aquifer during the mining process,so that the coal seam will be flooded.In addition,the ground of coalbed methane development in the process of hydraulic fracturing,as well as the role of the ground and underground drilling engineering and other external conditions,all can lead to coal seam flooding again,after flooding the change of the mechanical properties of coal will directly influence surface between the coal pillar and working face coal wall,the stability of coal roadway etc,at the same time,with the increase of mining depth,efficient water coal mining will be more complicated.Therefore,it is of great significance for safe and efficient coal mining and mine flood prevention and control to deeply study the mechanical characteristics of water-bearing coal under high stress and its variation law.In this paper,the mechanical characteristics and acoustic emission characteristics of water-bearing coal under high stress are systematically studied by taking the coal samples from Dafosi No.4 coal seam as the research object.The main conclusions are as follows:(1)Uniaxial compression tests of coal and rock under wet and dry conditions were carried out,and acoustic emission tests were carried out simultaneously.The results show that the uniaxial compression process of coal can be divided into compaction stage,elastic deformation stage,stable fracture development stagend unstable fracture development stage.At the same time,the peak stress decreases,the peak strain increases and the elastic modulus decreases after water saturation.The brittleness characteristic of failure is weakened.The peak value and cumulative ringing count of coal rock decreased after water saturation.The damage evolution process of water-bearing coal under uniaxial compression is divided into three stages:primary damage,stable damage development and accelerated damage development.(2)The Brazilian splitting test of coal and rock under wet and dry conditions was carried out,and the acoustic emission test was carried out simultaneously.The results show that the Brazilian splitting failure process of water-bearing coal can be divided into four stages:compaction stage,elastic stage,yield stage and failure stage.The tensile strength,elastic modulus and peak strain of coal rock decreased after water saturation.The failure mode changes from tension failure to"tension+shear"failure.The accumulative ringing count of saturated coal decreases obviously.The damage process of dry coal can be divided into three periods:quiet period,stable growth period and sharp rising period,while saturated coal samples can be divided into two periods:stable growth period and sharp rising period.(3)Triaxial compression tests of water-bearing coal under different confining pressures were carried out.The results show that in the triaxial compression test,the compaction stage of coal is obviously shortened,and the elastic stage,yield stage,failure stage or post-peak stage can be clearly seen.The peak strength,peak axial strain,peak lateral strain and peak volumetric strain of coal rock all show an increasing trend with confining pressure increasing.The measured Angle of failure fracture will increase with confining pressure increasing.The cohesion and internal friction Angle are 9.8MPa and16.43°,respectively.(4)Triaxial unloading tests of water-bearing coal under different initial confining pressures were carried out,and acoustic emission tests were carried out simultaneously.The results show that the peak axial strain,peak lateral strain and peak volumetric strain of coal all increase with the increase of initial confining pressure.At high confining pressures,axial strain,lateral strain and volumetric strain are not significantly affected by unloading.The unloading confining pressure difference will increase with the increase of initial confining pressure,but the increase amplitude will decrease continuously.Compared with the conventional triaxial test,the cohesion and internal friction Angle of water-saturated coal are obviously decreased.According to the variation law of AE parameter RA value,it is found that the internal cracks of water-saturated coal are mainly in the form of tension in the early stage of development,and in the form of shear after unloading.The damage evolution process of water-saturated coal rock under triaxial unloading can be divided into two types: progressive damage increasing type and sudden damage increasing type.The research results of this paper can provide theoretical basis for safe and efficient mining of deep coal and mine flood prevention and control.