Study On Development Characteristics And Formation Mechanism Of Fractured Rock Mass In Water Flowing Fractured Zone Of Shallow Coal Seam

There are many shallow coal scams in the northern Shaanxi mining area.Due to the shallow depth and thickness of the coal seam in this area,it is easy to form cracks in the mining process of the coal seam.The cracks form a good water diversion channel inside the rock,connecting the aquifer and the working face,which makes the mine water disaster accidents occur frequently,resulting in the loss of life and property,and also leading to the decline of groundwater level.The surface is characterized by water shortage and land desertification.In order to further understand the disaster-causing mechanism of shallow coal seam,based on the geological background of Lucaogou working face in Hongliulin Coal Mine,adopts the methods of similar material simulation and numerical simulation,and applies the monitoring methods such as dial indicator,optical fiber sensor and VIC-3D to study the development characteristics and formation mechanism of fractured rock mass in water flowing fractured zone of shallow coal seam.The following results were achieved:(1)With the mining of coal seam,the development height of water flowing fractured zone presents ladder development,and the height finally reaches 24m.The fractured rock mass in the water flowing fractured zone finally presents irregular shape.According to the different stress modes,the fractured rock mass in the water flowing fractured zone is divided into tensile fracture zone and extrusion fracture zone.(2)The fracture grid of fractured rock mass in water flowing fractured zone presents obvious fractal characteristics.With the advance of working face,the fractal dimension of fractured rock mass fracture grid in water flowing fractured zone is divided into three stages:dimension rising stage,dimension stable stage and dimension decreasing stage.The cracks expand in the rising stage of dimension,the cracks are compacted in the decreasing stage of dimension,and the cracks expand-close dynamic changes occur in the stable stage of dimension.The fracture network of mining rock mass in tension fracture zone and extrusion fracture zone has different fractal dimensions.The fractal dimension of tension fracture zone is greater than that of extrusion fracture zone.(3)The development of fractured rock mass in water flowing fractured zone is controlled by medium grained sandstone.Fiber optic technology was used to monitor the strain of medium-grained sandstone during mining failure.The results show that the failure mode of medium-grained sandstone is basically consistent with the development mode of fractured rock mass in water flowing fractured zone,and the rock layer is the key layer affecting the development of water flowing fractured zone.(4)With the mining of coal seam,the vertical stress of rock is symmetrically distributed in the goaf,and the stress concentration area is formed at the open-cut and stop-mining line of the working face,and the stress release area is formed at the strata above the goaf.With the advancement of the working face,the stress concentration at the open-cut and stop-mining line increases.The closer the goaf is to the coating rock of the coal seam,the more obvious the stress release is,and the greater the failure degree of coating rock is.The concentration and release of rock stress forms tensile fracture zone and extrusion fracture zone,which is the fundamental reason for the development of fractured rock in water flowing fractured zone of shallow coal seam.