Fracture evolution and failure characteristics of sandstone under freeze-thaw cycling by computed tomography

Long-term freeze-thaw (F-T) action causes the expansion of the fracture network in rock mass with ice-filled joints, triggering more serious engineering geological disasters. This study explores F-T damage effects and loading failure in double-fractured white sandstones with different saturation types (saturated and ice-filled). Saturated intact, saturated fractured, and ice-filled fractured sandstones were subjected to F-T cycle tests and uniaxial compression tests. Three-dimensional images of the specimens under various F-T cycles and stress states were obtained using computed tomography. The evolution in fracture morphology was evaluated visually, with pore network model parameters used for quantitative characterisation. Sandstone fractures evolved according to four main processes: frost-heave cracking, progressive expansion of damage, rock bridge penetration under load, and shear failure. The proportion of connected pores was used to estimate the influences of F-T cycling on pore-structure connectivity and fracture expansion in different specimen layers. During F-T cycling, pore characteristics vary according to size: micropores (equivalent radius < 100 μm) account for >60% of pores and increase in number more than pores of other sizes. Mesopores (100–500 μm) gradually decrease in number but their volume fraction remains >70%. Macropores (500–1000 μm) have the greatest increase in volume fraction. Early F-T cycling induces the connection of pore throats, while later cycling accelerates their expansion. Hydrostatic pressure increases are the leading cause of damage in ice-filled fractured sandstone, which exhibits greater variations in porosity, heterogeneity coefficient, and pore-throat parameters under F-T cycling than saturated fractured sandstone. The results provide insight into fracture evolution and failure characteristics in fractured rock masses with different saturation types (saturated and ice-filled) in cold regions.