Experimental study on damage characteristics and permeability evolution of K1/J2 paleoweathered rocks under triaxial unloading-loading confining pressure path

Mud-sand inrush accidents in coal mining under extremely thick Cretaceous in Ordos Basin seriously threaten the safety of mine production and miners' life. Paleoweathered rock in the K1/J2 unconformity contact zone (K1/J2 PWR) of the Ordos basin are key aquifuge under the Cretaceous aquifer and new material source of mud-sand inrush during coal mining. In this paper, the damage characteristics and permeability evolution of K1/J2 PWR under triaxial unloading-loading confining pressure path (TULCPP) were studied by MTS8.04 testing machine, which is of great significance for mine disaster prevention and control. Results indicate that in the TULCPP process, with the increase of initial stress and clay mineral content (CMC) of the sample, the peak strength increases, and it is more difficult to expand, as well as the peak volume strain at the expansion point generally increases. The variation trend of permeability coefficient, AE counts and stored counts with axial strain is corresponding to different deformation stages, and it changes dramatically in the pre-peak plastic yield stage, showing a sharp increase trend. The permeability coefficient of K1/J2 PWR shows a power-law and negative-exponential relationship with confining pressure. With the increase of initial stress, the relationship will change from power-law to negative-exponential, and this change first occurs in the samples with low CMC. With the increase of CMC, the permeability coefficient increase range expanded in the confining pressure unloading failure process, the permeability coefficient decline range decreased in the confining pressure loading process, and initial stress enhancement could restrain this trend. Based on the experimental data, the peak strength and permeability coefficient empirical equations for the K1/J2 PWR considering confining pressure and CMC were established.