Mechanical Behavior and Damage Evolution Properties of Sandstone with Parallel Planar Flaws Under Freeze–Thaw Cycles

Freeze–thaw cycles (FTCs) are an influential factor in deformation and damage of geotechnical engineering works within seasonal frozen ground. In this study, sandstone specimens with varying parallel planar flaw angles (θ) were subjected to different FTCs, followed by uniaxial compression tests (UCS). The impacts that FTCs and θ have upon the mechanical behaviors were investigated. Furthermore, a new damage constitutive model based on energy evolution was proposed, and the practical significance and evolutionary patterns of damage variables were comprehensively discussed. The findings reveal that flawed sandstone specimens experience a decline in peak stress and elastic modulus as the number of FTCs increases, while these properties exhibit an increasing trend with higher θ. At lower FTCs, no significant changes are observed in the failure mode. However, as the number of FTCs approaches 45, new potential crack propagation paths emerge within the specimens, leading to a change in the failure patterns. The total energy U and elastic strain energy U_e at the peak stress are negatively correlated with FTCs but positively correlated with θ. Furthermore, based on the energy evolution, a new damage constitutive model that considered the effect from θ and FTCs was developed to describe the failure behavior of the specimen. By analysing the damage variables, it is found that damage caused by parallel planar flaws (D_θ) decreases with an increase in θ. The FTC-induced damage (D_FTC) is also influenced by θ, and specimens with lower θ experience higher D_FTC, while those with higher θ exhibit lower D_FTC. Additionally, an increase in FTC numbers leads to an increase in the gap between the damage values of specimens with lower θ and those of higher θ, whereas the parallel planar flaw angles have a smaller impact on the damage growth rate. The research findings provide valuable guidance in evaluating the stabilization and optimizing the design for geotechnical engineering in cold regions.