Evolution of damage and permeability in sandstone subjected to direct shear loading after freeze-thaw cycles

The evolution of damage and permeability in sandstone subjected to direct shear loading after freeze-thaw cycles was investigated using a combined experimental and numerical approach. Direct shear tests were conducted on sandstone samples subjected to 0, 25, 50, and 75 freeze-thaw cycles. Normal stresses of 10 MPa, 15 MPa, 20 MPa, and 25 MPa were applied, with permeability and acoustic emission (AE) responses monitored simultaneously. A coupled Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) model was developed, incorporating a modified Kozeny-Carman (K-C) equation. This model captured freeze-thaw damage from water-ice phase transitions and simulated permeability evolution during shear. Experimental results show an S-shaped permeability of evolution. This process is characterized by an initial reduction via pore compaction, a rapid surge during microcrack coalescence, and a post-peak decline. With an increasing number of freeze-thaw cycles, the shear strength, cohesion, and stiffness decrease, whereas permeability and the internal friction angle increase. Elevated normal stress suppresses AE activity and attenuates freeze-thaw damage. And freeze-thaw cycles increase microcrack complexity, as evidenced by enhanced multifractality. Numerical analyses demonstrate a spatial gradient in freeze-thaw damage, and shear microcracks dominate the fracturing process. The proposed model precisely replicates experimental trends, clarifying the link processes between damage and permeability. These findings furnish a reliable methodology for assessing the shear failure and seepage characteristics of rock in cold regions.