Investigating the damage characteristics of overburden and dynamic evolution mechanism of surface cracks in gently inclined multi-seam mining: A case study of Hongliu coal mine

In this paper, based on the mining project of NO.2 coal seam and NO.3 coal seam in the Hongliu coal mine, the stress evolution characteristics of overburden, subsidence, the characteristics of fissures developed in strata, and the evolution mechanism of surface cracks were systematically investigated by numerical simulation, theoretical analysis, and field investigation. The results showed that the stress, the subsidence, and the fissures exhibited significant asymmetric characteristics. The features of multi-period combined dynamic evolution were present in the fissures developed in strata during the gently inclined multi-seam mining. In the mining of the upper coal seam, the frontmost surface crack was always located behind the mining position (lagging crack). Nevertheless, the frontmost surface crack in the lower coal seam progression was consistently found in front of the mining position (advanced crack). Three different types of variable characteristics in the surface crack width were observed as a result of the gently inclined multi-seam mining. A formula for characterizing the formation of tensile and collapsed cracks was established. The surface crack activity mechanism was elucidated, in terms of stress evolution, movement, and deformation of the surface. The rationale and accuracy of the research findings were confirmed by investigating results of the surface cracks. In this work, scientific guidance for ecological environment protection caused by coal mining in western China was provided. • The stress of overburden, surface subsidence and overburden fissures have asymmetric evolution characteristics. • The surface crack width has multi-period dynamic change characteristics. • The type of surface crack is closely related to soil properties and periodic weighting step of the roof. • The dynamic characteristics of surface crack width is controlled by the surface stress state and soil movement.