In the process of ascending repetitive extraction of multilayer coal seams, the evolution and expansion of excavation-triggered cracks dominate the structural instability of overlying rock layers and the deterioration of overall rock mass quality. In this study, a similar material model containing two coal seams (No. 2 upper coal seam and No. 4 underlying seam) was constructed. Through model excavation, the entire process of overburden movement was captured, and a box-counting program was developed for quantitative analysis to investigate the dynamic evolution law of overlying strata fractures under upward repeated mining disturbances. The results show that during the mining of the No. 4 coal seam, the fractal dimension of mining-induced fractures initially increases and then decreases, with the peak value occurring at an excavation distance of approximately 40 cm. For the No. 2 coal seam, the fractal dimension exhibits fluctuating evolution characteristics, with the global peak appearing at approximately 55 cm and a local peak at approximately 70 cm. The rate of change in fractal dimension during coal seam mining exhibits alternating fluctuations of increase and decrease. The initially mined No. 4 coal seam, subject to higher in situ stress, exhibits rapid fracture development with an earlier peak, while the No. 2 coal seam, influenced by pressure relief, presents progressive fracturing characteristics with a delayed peak. The fractal dimension can effectively characterize the evolution characteristics of the mining-induced fracture network in overlying strata and provide reference for strata control and disaster prevention.
Gao et al. (Sat,) studied this question.