Formation and development mechanism of ground crack caused by coal mining: effects of overlying key strata

Ground cracks caused by coal mining is a typical man-made geological hazard which severely damages the ecological environment and ground structures (such as buildings, pipelines and bridges, etc.). Therefore, it is necessary to study ground crack development law in order to fully understand the mechanism of ground crack formation and to prevent damage. Current research in China is mainly focused on the mining areas in eastern China, but there have been few studies on the law and the mechanism of ground cracks caused by high-intensive mining in western China and aeolian sand areas. Additionally, the western mining areas are in an arid and semi-arid climate zone with a fragile eco-environment, where ground cracks can have a strong and irreversible destructiveness. Therefore, research on mining-induced ground cracks in western China has great practical significance. Using the mining-related ground crack in the 12,406 fully mechanized coal mining working face of the Bulianta Coal Mine in western China as an example case and performing field surveys and analyses, we determined that the dynamic ground cracks caused by high-intensive mining exhibit secondary development characteristics. The cracks have two developmental cycles and two peak values. In another words, the development law is M-shape with double peak values–double cycles, which is different from the conventional research result of “single peak value–single cycle”. In order to identify the internal mechanism of this special development law, we conducted our research from two perspectives of key strata theory and mining subsidence theory by adopting the methods of theoretical analysis and physical/numerical simulation, and then combining the results with the actual survey data of the 12,406 working face. The results indicate that the special phenomenon involving the secondary development of ground cracks meet the following conditions: (1) compound key strata exist in the covering rock (i.e., more than two layers of key strata); (2) the mining range makes the ground surface over-mined, and the flat floor is revealed in the subsidence basin; (3)the mining range achieves breaking of the span of the compound key strata, which then causes the fracture of the compound key strata. The development characteristics of mining-induced ground cracks are related to the periodic breaking of the compound key strata. The compound key strata fractures generate the tensile deformations on the secondary surface, which cause secondary ground cracks, resulting in the cracks exhibiting secondary development characteristics. The results of our study comprehensively display the internal mechanism of ground crack development, which can provide guidance in the areas of accurate ground crack prediction, land environment damage control, and ecological restoration.