Study on synergistic water-resisting stability of upper and lower soil layers and bedrock layer in shallow coal seam group

The roof water inrush of shallow buried coal seam is caused by the water flowing fractured zone through the aquifuge. With the downward mining of coal seam group, the mechanism of synergistic water isolation between soil layer and bedrock layer needs to be clarified. In order to analyze the failure mechanism of aquiclude under the background of downward mining of coal seam group, so as to accurately predict the water inrush problem of coal mine roof, taking the downward mining of 2⁻², 3⁻¹ and 4⁻² coal seams in Han Jiawan Coal Mine of Shenfu Mining Area in Northern Shaanxi as the research background, the differences in the properties of aquiclude at different depths were obtained by in-situ borehole coring experiments. The development form of cracks is described by establishing the mechanical model of seepage expansion of upper aquifuge soil layer. Using the mechanical model of soil-rock composite aquiclude, the mechanical criterion of the lower aquiclude in the overall stable state is calculated under the condition of ‘given deformation’. The research shows that the aquiclude can be divided into upper aquiclude and lower aquiclude according to its different properties. By comparing the total load with the tensile strength of the upper aquifuge, when the total load is less than or equal to the tensile strength of the upper aquifuge, the crack no longer develops downward to obtain the crack development length. Under the condition of ‘given deformation’, when the tensile stress generated at the middle position of the long side of the aquiclude is less than or equal to the tensile strength threshold, the overall structure of the lower aquiclude is in a stable state, and no tensile failure will occur. According to the geological conditions of Han Jiawan coal mine, the numerical simulation results are consistent with the theoretical analysis results. The research can provide reference for roof water inrush prediction under similar geological conditions.