Analytical stress analysis method of interbedded coal and rock floor over confined water: a study on mining failure depth

Deep mining commonly causes redistribution of the stress-strain field for coal seam floor strata. The study of this distribution law was of great significance to analyzing the characteristics of floor deformation and failure and predicting the risk of floor water inrush. In this paper, the damage of a coal seam floor with strong water pressure in the Junggar Basin in western China was examined, and the stress analytical formula for every point of floor confined aquifer was constructed using a stress model in the direction of coal seam strike. Combined with Mohr-Coulomb failure criterion, the maximum depth of floor failure was 17.5 m. According to geological and stratigraphic data of the stope floor, a 3D numerical model of fluid-structure coupling was built. Simulation results showed that under a 1.5 MPa water pressure, the maximum floor failure depth was 18 m. The internal stress of the coal-rock interface and soft rock increases significantly, and the weak coal seam intensifies the transfer of stress and strain at depth to some extent. Water pressure had a certain influence on floor deformation and failure. The joint simulation results of stress-seepage-fracture were closer to the real stope measurements. In the boreholes, two types of sensors, distributed fiber and electrodes were arranged. When the coal floor broke, the compressive strain increased with a peak of 4152 με. Meanwhile, the apparent resistivity of the rock seam reached 500–700 Ω•m. The comprehensive analysis showed that: the floor failure depth in the monitoring area of the 61,303 working face was −18 m, and the floor failure disturbance depth was −35 m. The results presented in this contribution can provide reference for future examinations of coal and rock interbedded floor failure over deep confined water. •Study on mining failure of deep interbedded coal and rock floor.•The analytical formula of stress at any point of floor is derived.•The influence of weak coal seam on stress transfer is studied.•Combined application of borehole strain and apparent resistivity imaging.