The permeability evolution law of fractured sandstone under triaxial cyclic loading and unloading

The permeability change of coal and rock induced by repeated mining during coal seam group exploitation is a key factor restricting mine gas drainage. To explore the permeability evolution law under repeated mining, the permeability tests on fractured sandstone with different prefabricated dip angles were conducted under cyclic loading and unloading. The permeability evolution mechanism was revealed. The results indicated that: ① Fractured sandstone permeability generally declines, then slowly increases and suddenly surges during cyclic loading and unloading. In each cycle, permeability changes from "decrease-increase-decrease" to "increase-increase-decrease". The end of the unloading phase can be used as a pre-extraction point for gas drainage. ② With more cyclic loading and unloading, instability and failure of fractured sandstone with different dip angles intensify, and permeability increases more significantly. ③ Fractured sandstone with a dip angle of 45° and 60° has higher deviatoric stress sensitivity coefficients and irreversible permeability damage rates, and the former decreases with increasing confining pressure. ④ Fracture dip angles affect the stress field around fractures, influencing fracture initiation and propagation, thus altering permeability. At the final failure stage, the permeability of fractured sandstone with different dip angles increases by 2-9 times. The research findings have significant implications for gas drainage in coal seams and safe mining operations.