Discrete element numerical simulation of crack evolution in multi-coal seam mining

Based on characteristic of the associated mining of multi-coal seam and the engineering geological characteristics of overburden, through establishing a discrete element numerical simulation model by using type of UDEC, the overburden failure mechanism induced by multi-coal seam mining was analyzed. By employing fractal theory, the evolution of the cracks in the overburden strata, which were obtained from discrete element numerical calculation, caused by multi-coal seam mining was analyzed. The stress distribution of multi-coal seam mining has been analyzed, the stress concentrated location are at the roof, bottom and surround rock. Stress concentrated maximum are on the coal wall lines, the bottom stress tend to transfer into the coal pillar. The development height of caving zone and water flowing fractured zone of multi-coal seam mining were calculated by use of the stress verification method. They were 32 m and 81.5 m respectively. The results of evolution law of fractal crack network show that the opening ratio of the cracks and development height increased in the overburden with the increase of the seam mining layer number. Increasing of the mining width, the fractal dimensions of fractal crack network increase generally. From our investigation it can be concluded that there exists the possibility of water and sand inflow during mining operations; therefore some measures to prevent and control water flow should be used, and mining thickness should be locally strictly limited.