Tensile fracture mechanism of rock mass induced by the unloading seeping of confining pressure in deep coal mining

Under the influence of stress unloading after mining,the confined water with high pressure in deep rock cracks can easily penetrate into the longwall face and goaf,which may cause water inrush disaster. In order to study the fracture mechanism of high stress rock masses induced by the confining pressure unloading seeping process (CPUS) in deep mining,the tensile stress evolution in rock mass was obtained based on the evolution equation of stress concentration and the principle of effective stress,and the tensile fracture conditions and mechanical mechanism of rock masses induced by the CPUS were established. Combined with the definition of the stress unloading level (SUL) and the deformation formula of Bandis hyperbolic cracks,the critical values variation of the stress unloading level in the CPUS was calculated under different mining depths. It was found that the abrupt change point of permeability of fractured rock mass under the CPUS is exactly the critical value of SUL. The relationship between the tensile and shear fracture under the CPUS was comparatively analyzed and verified by experiments. The results show that:under the influence of confining pressure unloading and confined water seepage,the additional tensile stresses ΔσT in deep fractured rock mass paralleled to the unloading direction are generated;the peak value of ΔσT increases with the crack scale of the rock mass;and ΔσT increases with the mining depth and confined water pressure. When ΔσT is greater than or equal to the sum of confining pressure and the tensile strength of crack,tensile fracture occurs in the rock mass,which will lead to nonlinear increase for the permeability of rock masses,and the abrupt change point is exactly the critical value of SUL. In addition,with the increase of mining depth and confined water pressure,the SUL of confining pressure of permeability abrupt change point decreases significantly. Affected by the structure and property of fractured rock masses,the critical value of SUL of the tensile fracture can be higher or lower than that of shear fracture,and the fracture modes of rock masses will change from shear to tensile with the increase of mining depth.