Patterns of overburden failure and ground fissure development of thick loose layer and thin bedrock

Studies on mining-induced overburden rock movement, damage and surface deformation cracks are essential to the safety production of coal mines and mining damage protection. Taking the 12203 working face of a mine of Zhengzhou Coal Group as an example, this study analyzed the evolution mechanism of overburden rock affected by mining via theoretical analysis, on-site measurement and numerical simulation. We investigated the functional relationship between back-mining distance and height of overburden development and between sufficiently destructive depth of overburden and back-mining distance. Moreover, we analyzed the distribution of the ground cracks in this geological conditions. Results show that: ① The overburden destruction of thin bedrock in thick loose layer encompasses 4 stages: the beginning, rapid increase, slow increase and stabilization; the overburden destruction height shows a Logistic relationship with back-mining distance, and a linear relationship with the thickness of the coal seam. The back-mining distance is equal to the depth of burial when the overburden is sufficiently destroyed in this geologic condition. ② In the evolution of overlying rock fracture, bedrock is more violently damaged than the loose layer. ③ The cracks in the ground are mainly tensile cracks interacting with step cracks. The cracks at the edge of the working face show an "elliptical-circular-elliptical" dynamic variation pattern along with the back mining of the working face; the development of cracks in the ground occurs at a certain distance in front of the working face. The distance of the dynamically overrunning cracks is 84.07m, and its angle is 73.15°.