Failure mechanism of composite structure floors of deep coal and rock strata and its application

To study the failure mechanism of floor rock mass in deep rock mining working faces of Pingmei mining area, a mechanical model of plastic slip line field for three-layer composite structure floor was constructed, and the theoretical solution of the maximum failure depth of the floor under five working conditions was derived on the basis of the traditional plastic slip line field theory of single rock floor. The stress field distribution law and plastic deformation characteristics of floor rock mass under different advance degrees were simulated and analyzed. Finally, vibrating wire strain gauges were used to monitor the micro-strain of floor rock mass in No. 15-31040 rock mining working face of No.12 Mine in real time, and the deformation development pattern and failure domain of the floor before-during-after mining were obtained. The results show that the depth of the active failure zone of the floor of the working face is located between the middle and the lower structural strata, which belongs to the third working condition with the theoretical solution of the maximum failure depth of 17.08 m. According to the simulation experiment, the failure and damage of floor rock mass are mainly concentrated in the open-off cut and under the two lanes at the depth of 17.1-17.9 m, mainly in the form of plastic slip failure. The height of uplift of cold ash confined water is less than the thickness of bauxite mudstone at the bottom of carboniferous system, and the effective waterproof layer can resist the uplift of cold ash water. The measured data reveal that the initial position of floor failure is 7.9 m ahead of the mining face, mainly in the form of compression-shear failure, while that the rock mass near the mining face turns into tensile-shear failure after entering the goaf and the failure depth can reach 16.5-18 m. According to the temperature monitoring, the lower rock mass in the mining failure zone is still impermeable. The theoretical calculation is consistent with the simulation experiment and measured results. The research could provide theoretical guidance and practical reference for prevention and control of floor water disasters in coal and rock mining working faces under similar geological conditions. © 2022, Science Press. All right reserved.