Study on Fracture and Seepage Evolution Law of Stope Covered by Thin Bedrock under Mining Influence

Abstract

Aiming to better understand the fracture evolution characteristics of thin bedrock affected by mining, a program was developed to establish a numerical calculation model for the fracture evolution of the overlying rock in the stope under the coupled seepage-stress condition. The fracturing law of mining overburden during the advancing process of the coal seam working face has been deeply studied. The dynamic change process of the development height of the overburden fissure zone is analyzed. The results show that with the advance of the working surface, shear and tension compound rupture occurs in the overlying rock layer bottom-up. The rupture penetrates into the sand-water layer and forms a stable rupture zone, which terminates at the bottom of the clay layer in the vertical direction and no longer develops upward. The equivalent stress concentration area is obviously separated at the bottom of the clay layer. Additionally, there is no obvious damage to the clay layer, indicating that the integrity of the clay layer has been protected. This pattern is consistent with the field monitoring results. Under the dual action of mining stress and pore water pressure, the bedrock aquifer ruptured in a wide range, and gradually caused water to flow to the goaf. The low pore pressure zone runs through the entire bedrock layer and ends at the bottom of the clay layer; also, the effective velocity of pore fluid shows a consistent pattern. The on-site water inflow monitoring results found that the main source of water inflow was the sandstone aquifer in the bedrock section, and the shallow groundwater and surface water did not enter the working face in large quantities with coal mining. This shows that the clay layer has a good water barrier effect, effectively blocking the inflow of shallow groundwater or surface water into the working face. It also shows that the “soft–hard” roof layer combination feature greatly buffers the impact of mining on the water isolation layer and has a good water separation effect.