| In the "three-lower" coal mining,strip coal pillar are often used to support the roof and control surface subsidence.The instability of the coal pillar will induce a variety of mine disasters,maintaining the long-term stability of the coal pillar is crucial for the movement control of the overburden and subsidence of the ground.As part of the whole mining system of"roof-coal pillar-floor",in order to study the stability and failure characteristics of coal pillar,the coal pillar should be placed in the whole mining system.In this paper,theoretical analysis,laboratory testing,acoustic emission monitoring,numerical simulation methods are used to study the instability characteristics of coal pillars under different lithologic roof and floor rock stratum,and the following results are obtained:Based on the interaction of strip coal pillars and roof and floor,combined with the theory of composite rock-mass and superposition principle,the stress and strain of coal-rock with cohesive force between the layers were analyzed,and the friction effect model of "roof-coal pillar-floor" system was established,which considers the joint force of the strip coal pillar and the roof and floor.The uniaxial tensile tests,uniaxial compression tests,and triaxial compression tests of medium sandstone,red sandstone,shale,and coal samples were carried out.The basic mechanical parameters of coal and rock specimens were obtained.A monitoring device for progressive failure test of strip coal pillar has been designed and developed,long-term bearing test of "roof-coal pillar-floor" coal-rock system was carried out.It was found that the uniaxial compressive strength and elastic modulus of the coal-rock system sample increased with increasing strength of the roof and floor.The experiment found that the instability process of the strip coal pillar can be divided into initial compaction,stable growth,step deformation and rapid growth stage.With the increase of the roof and floor hardness,the lateral deformation of coal pillar is decreases,the upper position of the central of the coal pillars was deformed first,and the lateral deformation of the upper part and the upper part of the coal pillar is obviously larger than the middle lower part and the lower part.The lateral deformation of strip coal pillar has obvious step type increase,showing a step and mutation characteristic.The axial stress applied to the loading system is transformed into elastic potential energy,strip coal pillar occurs elastic deformation.Energy accumulated by the coal-mass causes the expansion slip of the crack,which leads the lateral deformation of the strip coal pillar showing a step and mutation characteristic.The AE signal of strip coal pillar instability process is divided into initial silence stage,steady growth stage,fast growth stage and unstable stage.Before the complete failure of strip coal pillar,there is obvious precursor signal of AE,the quiet period of AE signal can be regarded as a precursor criterion for the failure of coal pillar.The compressive stress of the roof and floor strata equivalent to the coal pillar restricts the instability of the coal-rock interface,which results in the localization of the AE events in the in coal is concentrated in the middle part.The AE events location technique can qualitatively analyze the progressive failure process of the coal pillar,and can study the distribution of AE events of the coal pillar under different lithologic of roof and floor.The FLAC3D numerical simulation was used to study the evolution of plastic zone,vertical stress and horizontal displacement of coal pillars under different lithologic of roof and floor.Simulation results show that,with the smaller the strength of the roof and the floor,the greater expansion of the plastic zone of the coal pillar,the greater the maximum stress,the location of the stress peak is closer to the edge of the coal pillar,and the horizontal displacement of the strip coal pillar is also gradually increased. |
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