Study on failure mechanism and application of double-layer structure floor with large buried depth and high confined water

The first mining of nearly whole rock lower protective layer working face in Pingdingshan coal mining area is used to liberate the Ji group coal resources of its upper threatened by the gas outburst. The mining of the rock layer at a depth of nearly 1000 meters is bound to increase the depth of the floor damage. Once the L₅ weak water-rich aquifer in the aquifuge is disturbed, the indirect recharge channel of the cold ash water is formed, which affects the safety and stability of the rock floor. Firstly, the theoretical model of plastic slip line of double-layer structure floor is established, and the analytical solution of maximum failure depth of double-layer floor under three working conditions is derived. Then through the self-designed similar simulation experimental platform of pore water pressure (spring) and stratum effective stress (jack), the deformation form and failure characteristics of stope roof and floor are simulated and analyzed based on digital image correlation technology. Finally, the borehole strain measurement method was used to carry out on-site monitoring of floor fracture development morphology in Ji₁₅-31040 nearly whole rock working face of Pingdingshan No.12 Coal Mine. The results show that the maximum failure depth of Ji₁₅-31040 nearly whole rock working face floor is 16.59 m by using the plastic slip line theory of double-layer structure floor. The similar simulation experiment reveals that the floor failure is concentrated at both ends of the open-off cut and the working face, with obvious lagging failure characteristics. The maximum failure depth is 17.8 m. After the working face advances 159.9 m into full mining, the floor stress gradually recovers. The field measurement results show that the floor rock mass has a compression-shear slip failure at 7.9 m in front of the working face. The floor before and after the working face is pushed through the borehole shows compression-shear and tension-shear failure, respectively. The maximum failure depth of the floor is between 16.5 m and 18 m. The results of field measurement are in good agreement with theoretical calculation and similar simulation test.