Delayed Failure Characteristics of Deep Coal Seam Floor in Goaf

To study the delayed failure characteristics of deep coal seam floor in goaf, the deformation and failure characteristics of the locking section between the crack surfaces are analyzed by using the cantilever beam model. The relationship between volumetric strain and partial stress in the unloading and reloading process of rock samples after loading into the plastic stage is analyzed by laboratory test. Finally, the change of permeability of deep coal floor rock mass in the process of goaf recompaction is analyzed with the measured data of water injection test in coal seam floor. The results show that the deformation and failure degree of the interlock section between the crack surfaces increases gradually in the process of mining shear slip and mining unloading; the rock sample in the plastic state is loaded to the unloading point of about 81 MPa after the axial stress unloading, and the volume strain increases from 0.000 56 mm to 0.001 1 mm. Through the field measurement, it is found that the crack propagation and deformation failure appears in the rock mass 18.19 m of the coal floor in the process of the working face pushing through the measuring point from 3.6 m to 15.8 m. Meanwhile, the deformation failure of the coal floor appears again in the process of the working face pushing from 20.6 m to 30.9 m, which indicates that the deep coal seam floor still has the possibility of deformation and failure in the process of unloading and reloading after loading to the plastic state.