Experimental research ofthe influence of different unloading stress paths on permeability evolution ofsandstone

In order to study the influence of the initial confining pressure and the unloading rate on permeability evolution of sandstone during unloading process, triaxial unloading seepagetests werecarried out under different high initial confining pressures(10, 20, 30MPa) and different unloading rates(0.1, 0.5, 1.0, 1.5MPa/min). The results show thatthe sandstone occurs brittle failure and that the lateral deformation and the volume expansion increase significantly with increasing the initial confining pressure or the unloading rate. The permeability evolution is closely related to the unloading stress and strain characteristics. The permeability increasesslowly atthe elastic deformation stage, rapidly atthe yield deformation stage and sharply atthe stress dropping stage, and eventually decreasesto a stable valueatthe residual strength stage. The initial confining pressure and the unloading rate have a significantinfluence on the permeability evolution of sandstone. With increasing the initial confining pressure, the variation of the unloadingpressure from the beginning of unloading to the moment of failure increases while the ratio between the final and initial confining pressures decreases. With ascending the unloading rate, the variation of the unloading pressure increasesand the peak value of the permeability increasesdue to a lower confining pressure at failure. The results could provide a good reference for permeability and safety evaluation of surrounding rocks in deep underground rock engineering. © 2019, Science Press. All right reserved.