Experimental study on stress-permeability evolution of rocks under complex mining-induced loading: Insights into water inrush risk in floor strata

Water inrush hazards from the floor strata of longwall workingface are commonly encountered in North China coalfields, which essentially result from the evolution of permeability in the floor rock under complex mining-induced stress conditions. Current research rarely addresses the evolution of rock permeability under such complex stress paths. Describing this evolution using only one stress parameter, such as effective stress, deviatoric stress, axial stress, or confining stress, is highly challenging. In this study, we developed a laboratory loading scheme that simulates mining-induced stress evolution. Hydro-mechanical experiments were conducted to investigate the evolution of rock permeability under mining stress. The mechanism on the change of stress-permeability relationships in mining-disturbed rock is revealed, supporting to the analysis of management strategies for floor water-inrush disasters. The results show that rock permeability evolves through four stages, including rapid decline, gradual fluctuation, sharp increase, and slow attenuation. 1–2 permeability surges occurred during mining-stress loading, closely linked to the emergence and reversal of deviatoric stress in magnitude and direction. With the first permeability surge, the deviatoric stress within the mudstone reached approximately 1.7 MPa, whereas that of the sandstone was about 1 MPa. The second permeability surge in the mudstone corresponded to the secondary rotation of the principal stress direction. CT and ultrasonic tests suggested an increase in microcracks in both rocks during the first permeability surge. However, the deviatoric stress-permeability plot before and after mining indicated that the fracture of mudstone sample changed significantly, while that of the sandstone remained unchanged. The permeability surges observed at different stages are interpreted as resulting from shear-induced reopening of pre-existing fractures and the formation of new shear-failure fractures. A stress-permeability model jointly governed by effective mean stress and deviatoric stress was established. Furthermore, two strategies are proposed for the floor water-inrush disasters prevention, (i) timely backfilling to reduce deviatoric stress, (ii) grouting after the first permeability surge. This work provides insights into stress-seepage behavior in rocks under complex stress evolution and offers new perspectives for identifying potential water inrush pathways in the floor strata of coal seam during longwall mining.