Physical simulation on spatial distribution of void fraction in overburden due to repeated mining in North Shaanxi Mining Area

Due to the shortage of water resources, the mining area in Northern Shaanxi uses the goaf to store mine water for reuse.The water storage in goaf is closely related to the spatial distribution of mining-induced fractures. To accurately predict the goaf water storage, based on the working plane strata fracture zone, it is determined that the main water storage space of underground reservoir in coal mine is the crack space of caving zone and fracture zone, and the calculation method of the spatial distribution of mining-induced fracture rate in overburden is established. In addition,the subsidence curve of overlying rock after repeated mining of 1-2 coal and 2-2 coal in Ningtiaota Mine was obtained through similar simulation experiment, and the spatial distribution of fracture rate of overlying rock in vertical and horizontal directions after repeated mining was obtained.The results show that along the vertical direction, the fracture rate of overburden decreases gradually with the increase of height. After 1-2 coal mining, the fracture rate of strata decreases gradually from 1.87% to 1.18%, and after 2-2 coal mining, the fracture rate decreases from 12.85% to 4.11%; Along the horizontal direction, the fracture rates of caving zone, fracture zone and bending subsidence zone appear two peak values near the edge of goaf, and the peak values gradually move to the center of goaf. After 1-2 coal mining, the maximum fracture rate of the middle compaction zone is 1.21%, and the maximum fracture rate of the separation zone on both sides is 2.19%. After 2-2 coal mining, the maximum fracture rates are 16.1% and 6.54%, respectively, in the compacted zone and stratified zone; After repeated mining, the fracture rate of overlying strata increases significantly, which is conducive to the storage of more mine water in underground reservoir. The maximum fracture rate of separation zone increases from 2.19% to 16.1%, and the maximum fracture rate of middle compaction zone increases from 1.21% to 6.54%, which increases by 6.4 times and 4.4 times respectively. The fracture rate of overlying strata increases significantly, which increases the water storage capacity, so more mine water could be stored in the mined-out area of the lower coal seam after mining.