| Most of China's coal mines have complex geological conditions.The deformation of surrounding rock under mining disturbance is easy to induce coal and gas disasters such as coal and gas outburst and rock burst.The rupture of overburden and the development-expansion-connection process of cracks induced by instability of surrounding rock are closely related to many factors,such as key layer characteristics,working surface propulsion distance and coal seam dip.In order to ensure the safety and efficiency of coal mining,it is necessary to study the laws of movements and deformation of overburden,and the distribution characteristics of original rock stress and reservoir fractures.In this thesis,mechanical tests on coal are carried out on the basis of engineering background of Sihe Coal Mine and microscopic structure of coal,theoretical analysis and numerical simulation method are combined to study the evolution laws of overlying rock fissure during mining.The structure of this master's thesis is as follows:The first chapter introduces the research status of dynamic laws of overburden deformation by collecting and analyzing the related literature at domestic and abroad,then summarizes the evolution laws of the stress and displacement fields of overlying strata under the influence of mining.The second chapter uses microscopic analysis and macroscopic mechanical tests to study coal samples.(1)The tested coal samples have low porosity and poor connectivity,and the pores are mainly micro pores(with a volume ratio of 42.75%)and small pores(with a volume ratio of 41.78%).(2)Under different uniaxial loading conditions(from 0.05mm/min to 0.5mm/min),the peak intensity increases by more than 50%.(3)The results of triaxial and acoustic emission experiments of coal under different confining pressures show that the cumulative number of acoustic emission rings has experienced the initial increase phase,the medium-term stationary phase,and the late intermittent growth phase;The activity of acoustic emission increases with confining pressure,while the initial stress damage of the coal sample decrease with confining pressure.The third chapter studies the mechanical characteristics of typical overlying strata sandstone.(1)Through uniaxial compression tests and Brazilian tests,the uniaxial compressive strength,elastic modulus,Poisson's ratio and tensile strength of sandstone are obtained as follows: 103.37 MPa,4.77 MPa,0.23,7.25 MPa.(2)In the direct shear test,the shear strength of the sandstone increases with the increase of loading rate and the lateral expansion of the crack decreases.(3)In the multi-step shear test,the shear strength of sandstone increased by 27.8% as the loading rate increased by an order of magnitude.The magnitude of acoustic emission signals during the loading section and the constant pressure section differ greatly,spanning several orders of magnitude,but the overall probability density function during loading stage,constant pressure stage,and late stage have uniformity satisfying the scale-free distribution of power law.The fourth chapter introduces the engineering overview,geological structure and coal seam occurrence of Sihe Mine.Then the stress distribution of overlying strata,the distribution of mining-induced fracture and the spatial shape of mining-induced fracture after coal seam mining are summaried.Finally,based on the key layer theory and elastoplastic mechanics,the initial pressure distance of the roof of the 3402 working face of Sihe Mine is calculated to be 19.34 m,and the periodic pressure distance is 16.2m,which is consistent with the measured data in the field and can provide reference for numerical calculation.The fifth chapter uses the discrete element software(UDEC)to simulate the evolution law of cracks in overlying strata under mining.(1)Under different propulsion distances,the trend of vertical stress of the top and bottom of coal seams are consistent;The stress concentration areas appear at both ends of the open-off cuts and working faces,and the trend along the coal seam conforms to the characteristics of “horizontal three zones”,and the relative position remains with the advancement of the working face.(2)Under different propulsion distances,the trend of vertical displacement of coal seam roof and floor is consistent,the roof subsides,the displacement curve is “V” shape,the drum on the bottom plate has a “?” shape;After the coal seam is mined,successive fractures and bends occur on the overburden layer and show obvious "three belts" characteristics.(3)Under different dip angles of coal seams,the failure process of overburden during mining: Shearing displacement-bending deformation-tensile fracture-separation collapse-filling compaction;dip angle increase,support stress in front of coal wall and the stress concentration factor increase gradually.(4)Under different dip angles of coal seams,the inhomogeneity of stress distribution in surrounding rock is significant,the collapse of stratum is linear,the upper strata falls first,the lower goaf is filled with vermiculite,partially filled area,completed filled area and compaction area are formed along vertical direction.(5)Using the binarization to extract the overburden fractures and study the evolution law of the cracks of overlying strata.Under different propulsion distances,the porosity of surrounding rock fractures first increases and then decreases,while the surface subsidence changes are not obvious in the early stage,and increase in the later stage.Under different dip angles and different propulsion distances,the variation trend of surrounding rock fissure porosity is the same;The porosity of surrounding rock fissure decreases with the increase of dip angle;when it is 80 m away from the open-off cut,the surface subsidence gradually increases from zero,and increases as the dip angle increases.The sixth chapter summarizes the research results and conclusions of the general paper,and proposes the prospects,such as similar simulation and large-scale field test,to further analyze the effect of key layers,mining height,propulsion speed and other factors on the cracks of overlying strata under mining. |
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