Progressive failure analysis and fractures space model construction of overlying strata in shallow multi-seam overlapping mining

The comprehension of the collapse law of overlying strata and the corresponding evolution of overburden fractures is the foundation for solving mine disasters related to rock mass dynamics and fluid flow in shallow multi-seam mining, and the correlation study is inadequate. In this work, a three-dimensional (3D) model of shallow multi-seam is developed based on discrete element method to simulate the whole process of overlapping mining. The overburden caving, fracture propagation and strata subsidence are investigated from a spatial perspective, and the evolution and distribution of highly fractured areas are assessed. Based on the stability and transformation characteristics of the collapsed overburden structure influenced by geological features and the disturbance of secondary mining, the propagation mechanism of overburden fractures is determined. Additionally, a calculation criteria of overburden collapse and fractures development height in shallow multi-seam mining with separation height as a single judgment factor is proposed. The spatial distribution models of overburden fractures in full mining stage of upper and lower coal seams are established singly, and regions are divided based on the degree of fracturing. Finally, the spatial models of overburden fractures is verified and supplemented by field investigations of morphology and distribution features of surface cracks.