Crack Development and Interconnected Characteristics of Closely Spaced Shallow Coal Seams Under Overlapping Mining

Overlying strata of closely spaced shallow coal seams influenced by the repeated mining which cause cracks superposition development and form connection of mined-out area and surface. Taking typical closely mining coal seams as an example, we use the method of physical experiment and numerical analysis to simulate crack evolution laws and stress distribution of overlying strata during the process of slice mining. The results show that after the mining process of two coal seams, there forms obvious stress concentration region on both sides of trend and tendency. Fracture height exhibits hump shape distribution, the maximum vertical fracture height is 75 m, but not connected to surface after the higher slice has been mined. With the mining process of the lower slice, cracks developed again, crack height gradually increased with the advance of working face and the both sides of fracture connected to surface firstly.