Research On The Mechanism Of Cave Instability Induced By Mining In Shallow Coal Seams In A Mine In Guizho

The occurrence of coal seams in the southwest mining area has the characteristics of shallow burial depth and developed karst.When underground coal mining activities are carried out in the shallow buried coal seam with developed karst,the expansion of roof cracks is easy to connect with the karst cave,which causes the cave water to become the water source of water filling for the mining face,and may even induce Mine floods,surface subsidence and other disasters.At present,the research on mining-induced karst cave instability is still in the exploratory stage,especially under the condition of shallow burial depth of coal seam in the southwest mining area.Theoretical derivation analysis,similar simulation test,numerical simulation test and other methods are used to study and analyze the instability mechanism of karst caves induced by shallow-buried coal seam mining,aiming to provide theoretical basis and reference for underground coal mining activities under similar conditions.The main results of this study are as follows:(1)The structural characteristics of the karst strata were analyzed,the composition of the karst strata was studied through physical experiments,and the mechanical properties such as uniaxial compressive strength,tensile strength,elastic modulus and Poisson’s ratio of the karst strata were measured.(2)The stress environment of circular cavern under the influence of mining is analyzed,and the criterion of cavern instability with or without weak structural plane is obtained,The roof load calculation model is built,the load of each rock layer is calculated,the specific position of the key layer is determined,and the expression of the primary and periodic breaking step of the basic roof is derived.(3)Based on the strata occurrence conditions of a mine in Guizhou,a similar simulation model of karst cave instability induced by shallow-buried coal seam mining was established to invert the deformation,fracture,fracture development and karst cave of the stope overlying rock under the action of mining pressure under actual conditions.Destruction features.The breaking movement law of the overlying rock,the evolution law of fissures,and the failure characteristics of the karst cave under the mining disturbance are studied.The results show that the overall subsidence of the overlying rock is distributed in a "U" shape,and the maximum subsidence position is basically located near the position directly below the karst cave;the overall fracture network of the overlying rock is It develops upwards in a "stepped" manner,and the maximum development height of fissures appears in the karst cave.(4)Based on the UDEC numerical simulation software,a numerical simulation mining model was established,and the overlying rock fracture and fracture development laws,and the instability and deformation of the karst cave were analyzed.The results show that the development law of cracks is basically "cracks appear-cracks are compacted and closed-new cracks appear",and the overall development trend of crack heights is "gentle-severe-gentle";after the working face is mined,the vertical stress behind the working face is usually less than The original stress of the rock mass,the maximum value usually appears in front of the working face and will move forward with the working face.When the mining reaches the position of the karst cave,the stress will be unloaded in the karst cave area,and the phenomenon of the vertical stress peak value will decrease;When the working face enters the affected area of the karst cave,there will be a sudden increase,and it will gradually become stable as the working face moves away from the karst cave.Through comparative experiments,the influence of the size of the karst cave span and mining height on the instability and deformation of the karst cave is obtained.