Quantitative correlation between fracture fractal and overburden deformation due to the multiple layers backfill mining

The overburden fractures evolve with the backfill mining process shows a stage characteristic which plays a key role in understanding the quantitative correlation between fracture fractal and overburden deformation. In this study, the evolution of fracture network and the fracture fractal induced by the multiple layers backfill mining are firstly investigated based on the results of scale model and fractal dimension analysis. Then the quantitative correlation between fracture fractal and overburden deformation is achieved accordingly. The results show that the fracture network develops in 3 stages with high asymmetry during the upper layer mining, and 4 stages with the reopening of the previous vertical fractures during the lower layer mining. The fractal dimension changes with the height of water-conducting fractured zone show an exponential relationship and can be divided in 4 stages during the upper layer mining, and 3 stages during the lower layer mining, which is consistent with the evolution characteristics of fractal dimension changes with the mining process. The fracture ratio expressed by the ratio of the area occupied by the fracture to the analysis area exponentially increase with the advancement of the multiple layers backfill mining, and linearly with the fracture area. The fracture ratio, fracture area and fractal dimension jointly reflect the development degree and evolution characteristics of fractures, which can effectively represent the deformation and failure type of the overlying strata of the coal seam during the mining process. The result is an important basis for taking measures to mitigate the overburden failure and prevent the water and sand inrush or water-sand mixture inrush disaster.