Investigation on the deformation and crack evolution of overburden rock in downward repetitive mining of multi-coal seams with near distance

With the gradual depletion of easily exploitable coal resources in the shallow crust, multi-coal seams mining under complex geological conditions will become the new normal. As multi-coal seams mining will cause great repetitive disturbance to the overlying rock mass, resulting in a more complex collapse, movement and crack evolution law of the overburden rock compared to single-coal seam mining. In order to study the collapse, deformation and crack evolution law of overburden rock under the condition of downward repetitive mining of multi-coal seams, this paper takes the four coal seams mining of 4⁻², 4⁻³, 4⁻⁴ and 5⁻² in Zhangjiamao Coal Mine, Shenfu Mining District, North Shaanxi Province, as the research subject, and carries out a similar modeling experimental study and a UDEC numerical simulation model with a geometrical similarity ratio of 1∶100 . The results of physical similarity simulation and numerical simulation analysis of downward repetitive mining in the fourth coal seam of Zhangjiamao Coal Mine show that the cyclic pressure step distance gradually decreases with downward mining, and the height of the caving zone is positively correlated with the thickness of the coal seam. After the mining of the 4⁻² coal seam, the water conducting crack zone runs through the surface. After the mining of the 4⁻³, 4⁻⁴, and 5⁻² coal seams is completed, the water conducting crack zone runs through the interlayer overburden rock, and the fragmentation of the roof surrounding rock gradually increases under repetitive stretching and compression, and most of them are unclosed cracks. The findings of investigation can provide a theoretical level of reference and guidance for roof management and mining design under multi-coal seam mining conditions.