Characteristics of stratum movement induced by downward longwall mining activities in middle-distance multi-seam

With the development of mining depth, multi-seam mining activities is becoming more and more frequent. Correspondingly, the study on stratum movement characteristics induced by multi-seam mining activities is of great significance for preventing water inrush disaster and forecasting support pressure, surface deformation, gas drainage and other mining problems. According to the interaction among the caved zone, the fractured zone and the continuous deformed zone caused by the multi-seam downward mining activities, multi-seam can be divided into three types, i.e. the short-distance, middle-distance and long-distance multi-seam. This paper carries out a case study on the middle-distance multi-seam mining activities in a coal mine in China to investigate the characteristics of stratum movement caused by downward longwall coal mining activities in the multi-seam by means of borehole water level observation, digital borehole camera test, microseismic monitoring, surface crack observation of the surface cracks and distinct element modelling. By analyzing the in-site monitoring results and the numerical simulation results, it can be found that the failure of the key stratum between the lower and upper seams resulting in the connection between the failed zone in floor induced by the upper seam and the fractured zone induced by the lower seam. After the failure of the key stratum, the mining activities of the lower seam has a great influence on the secondary stratum movement in the overburden. These effects are mainly reflected in the rapid increase of horizontal displacement and vertical displacement, which caused the caved and fractured zones extend upward in the overburden and then surface cracks increase significantly. The above research results are of great significance for the prevention and control of disasters induced by downward longwall mining activities in middle-distance multi-seam.