Research On The Evolution Law Of Overlying Fissures In Shallow Coal Seam Mining Under Karst Cave Landform

There are abundant coal reserves in southwest China.The coal seam has the characteristics of shallow burial,thin bedrock and karst development.Under the karst cave development landform,the shallow buried coal seam mining is carried out,and the disturbed rock fracture is easy to expand to the karst cave to form a water diversion channel,which leads to the instability of the karst cave,the surface subsidence and the loss of karst water,which threatens the safety production of the working face.At present,the mining overburden fractures are mainly concentrated in the plain,mountainous area and gully landform.However,the evolution law of mining fracture propagation and the characteristics of strata movement and deformation under the condition of karst cave in the overlying strata are not clear.In view of this,this paper takes the shallow buried coal seam with typical karst cave development landform in Guizhou as the engineering background,and combines a variety of research methods to explore and analyze the evolution law and fractal characteristics of overburden fracture under mining disturbance.The main research results are as follows :（1）The structural characteristics of the overlying strata are analyzed.Based on the key layer theory,the position of the key layer of the overlying strata is determined.It is pointed out that the overlying strata have the structural characteristics of multi-key layer and thin loose layer.Combined with elastic mechanics and material mechanics,the rock beam mechanics analysis model is established.The initial and periodic weighting fracture distance of the main roof are calculated,and the important factors affecting the development height of the mining overburden fracture are analyzed.（2）The influence of karst cave on mining-induced fracture propagation and rock strata movement and deformation was analyzed by conducting similar simulation test.The results show that when karst cave occurs above the coal seam,the integrity of rock structure is destroyed,and the disturbance stress transfer is blocked and interrupted at the position of karst cave.The mining influence is mainly concentrated on the left side of karst cave,resulting in uncoordinated deformation between the two sides of karst cave,and the mining-induced fracture reaches the maximum development height in the cave side.The separation zone appears in the bending zone,and the separation fractures under the cave are more developed,and the overall distribution of mining overburden fractures is ’ platform ladder ’.（3）On the basis of similar simulation experiment,combined with fractal geometry theory,the box-counting dimension method was used to calculate the fractal dimension of two-dimensional mining-induced overburden fracture extraction image,and the fractal effect of mining-induced overburden fracture network evolution was quantitatively analyzed.It was concluded that the variation law of fractal dimension with working face advance can be divided into four stages.The first and second stages show an exponential growth trend,in which the second stage has a large increase,and the third and fourth stages show a linear change with slopes of-0.007 and 0.0014,respectively.The fluctuation of fractal dimension is small,and the correlation coefficients of fitting curves in each stage are all above 0.90.At the same time,it is concluded that overburden fractures have high self-similarity in the ’ three zones ’ area,and the fractal dimension of fracture zone is greater than that of caving zone and bending zone.（4）By establishing a numerical mining model,the movement and deformation law of overlying strata,the distribution characteristics of vertical stress and the evolution law of fracture field in shallow coal seam mining under karst cave landform are analyzed.The results show that the karst cave has a guiding effect on mining fractures.The vertical stress of the main roof increases sharply below the working face advancing karst cave,and the stress reaches the first peak,and the mining overburden fractures reach the maximum development height.When the mining face is 150 m,the vertical displacement of the main roof and the key layer II reaches the maximum.At this time,the mining cracks extend to the roof of the cave,and the separation cracks under the cave begin to close.According to the distribution map of micro-fracture field under different mining distances,the functional relationship between fractal dimension of overburden fracture network and advanced peak pressure of working face is established.