Study on the failure mechanism of clay layer overlying thin bedrock in coal seam mining

The water-blocking property of clay at the bottom of the Cenozoic overburden is an important factor for mining safety and protecting underground latent water resources in thin bedrock coal seam mining. The failure mechanism of such a clay is studied based on the actual engineering background of the SanYuan Coal Mine. The failure of the overlying clay layer is due to the reduction in the supporting space of the clay layer with the progression of coal mining, and the overlying clay layer will subside due to the self-weight load. Therefore, the vertical stress, horizontal stress and shear stress of the soil change during subsidence, and the change in these stresses determine whether the clay layer fails. Then, the failure criterion of soil expressed by the vertical stress, horizontal stress and shear stress is derived based on Mohr–Coulomb shear failure theory. Next, the function relating the stress and subsidence magnitudes are established by fit analysis. Finally, a failure criterion expressed in terms of soil subsidence amount is presented. Based on the failure criterion expressed by soil subsidence amount, a method for soil failure discriminant is proposed, in which only the subsidence amount is necessary to judge the soil failure state. This method is applied to the engineering of the SanYuan Coal Mine to get the failure subsidence amount curve of clay layer I. The results show that the failure subsidence amount curve plots below the subsidence amount curves of clay layer I at any advancing distance; that is, the clay layer did not fail. An investigation of the source and amount of water inflow to the 3301 working face also verifies that the clay layer does not fail, and the failure discriminant method used to judge the failure state of the soil is appropriate and convenient for use in engineering applications.