Experimental study on mechanical behavior and permeability evolution of weakly cemented sandstone under unloading conditions

The excavation of underground engineering usually leads to the unloading of rock stress, causing stress concentration and resulting in the instability and failure of the surrounding rock. Surrounding rock with low strength, weak cementation, and poor water-holding capacity is more sensitive to stress unloading. In this study, unloading confining pressure-seepage tests under constant axial pressure with different unloading rates and initial confining pressures were carried out on weakly cemented sandstone specimens to study soft rock's dilatancy and fracturing behavior. The results show that the unloading rates and initial confining pressure significantly impact the deformation of weakly cemented sandstone, and the volumetric strain is mainly related to radial deformation. During the unloading process, the greater the initial confining pressure was, the smaller the initial permeability and post-peak permeability of the sandstone, and the confining pressure had an apparent inhibitory effect on the permeability. With the decrease in the unloading rate, the unloading degree of the confining pressure increased, and the post-peak permeability increased. With the decrease in the initial confining pressure and unloading rate, the volume-expansion phenomena were more apparent, and the dilatancy angle was larger. By calculating the crack strain, it was found that the crack volumetric propagation strain was more significant under slower unloading rates and smaller initial confining pressures, indicating that more cracks occurred in the sandstone under these conditions, which could explain the difference in the increase in permeability under different unloading paths.