Evolution law and fractal characteristics of overburden fissures in extra-thick coal seam mining

PENG Baoshan; LIU Qianjin; YANG Xuemeng; LI Zhifeng

doi:10.11799/ce202512018

Abstract：The mining of extra-thick coal seams induces more intense responses in the overlying strata.To clarify the evolution laws of mining-induced fissures in the overburden,ensure safe and efficient production at the working face,and protect the surface ecological environment,taking the 2303 working face of Yongning Coal Mine in Xinjiang as the engineering background.The discrete element numerical simulation is employed to visually characterize the development features of overburden fissures under the extra-thick coal seam mining,revealing the evolution patterns of the water-conducting fissured zone and the caved zone as the working face advances.Combining with fractal geometry theory,the box-counting method is used to precisely quantify the fissure development intensity.The results show that after mining working face,the development heights of the caved zone and the water-conducting fissured zone are 51.10 m and 148.11 m,respectively,with caving-to-mining and fracturing-to-mining ratios of 5.77 and 16.63.As the working face advances,the height of the fissured zone undergoes a three-stage development of initiation-growth-stabilization,while the height of the caved zone undergoes two cycles of"sudden increase-gentle decrease"accompanied by the fracture of the main roof and sub-key strata.The development of mining-induced fissures in the overburden exhibits significant fractal characteristics.The overall fractal dimension dynamically changes in a saddle-shaped pattern("high at both ends,low in the middle")as the working face advances:the average fractal dimensions on both sides of the goaf(main fissured zones)are 1.001 and 0.978,respectively,constituting the primary water-conducting channels of the goaf.The fractal dimension in the bed separation development zone 150 m above the floor is 0.904,which is prone to forming groundwater accumulation spaces.By fitting the evolution curve of the fractal dimension,it is estimated that the fractal dimension peaks when the advance distance reaches 71.18 m,which precisely corresponds to the stage of large-scale roof caving.