True Triaxial Experimental Study of Anisotropic Mechanical Behavior and Permeability Evolution of Initially Fractured Coal

During deep underground coal mining, coal is typically fractured under high in situ and mining-related stresses, thus increasing the risk for dynamic disasters. However, knowledge regarding the mechanical and permeability characteristics of initially fractured coal remains limited. In this study, laboratory investigations were performed to examine the anisotropic strength, failure modes, and permeability of initially fractured coal subjected to true triaxial stresses. The results indicated that initially fractured coal can be classified into semi-brittle Class I failure modes. A new failure plane can be formed for initially fractured coal samples after failure, which was different from the intact coal samples’ failure mode. The peak strength of initially fractured coal was typically observed when the maximum principal stress was subjected perpendicular to the bedding planes, while minimum values were obtained when the maximum principal stress was subjected parallel to the butt cleats. The brittleness of initially fractured coal was increased with intermediate stress. The permeability evolution curves for initially fractured coal exhibited an L-shaped trend. The pulverized coal grains in initially fractured coal can block the gas flow channel contributing to the continuous decrease in permeability. Intermediate stress exerts a more significant effect on permeability reduction when deviatoric stresses are applied parallel to cleats in coal.