## 3 Results and Analysis
### Freeze-Thaw Strain
At this stage, the damage inflicted on the red sandstone by the freeze-thaw cycles is essentially complete; micro-cracks have developed within the sample, and theexpansion from the pore water phase change is no longer sufficient to fill the pores and exert pressure on the rock matrix. Figure 3: Attachment method of strain gauge. Figure 2: Variation of maximum strain with F-T cycles. Figure 4a shows the frost-heave strain during the 1 st, 3 rd, 5 th, 10 th, 20 th, 30 th, and 40 th freeze-thaw cycles in the unidirectional freeze-thaw process. The strain evolution can be divided into four distinct stages:  
1. Heat transfer and phase change stage. The cooling rate of red sandstone is relatively low under unidirectional freeze-thaw conditions. At the beginning of the freezing stage, the rock temperature changes slowly due to the limited heat transfer between the environmental temperature and the rock sample; additionally, the surface of the strain gauge is encased within the thermal insulation layer and thus shielded from the cold environment. Despite this, the strain value increases rapidly at the onset of the freeze-thaw cycle. Although the temperature of the rock sample has not yet decreased to the freezing point, expansion begins at the measuring point. This occurs because the upper surface of the sample shrinks during cooling, while the strain measuring point is located in the middle of the sample and oriented perpendicular to the temperature gradient direction. Consequently, the measuring point experiences tensile deformation due to the freezing shrinkage of the upper sample region. This tensile deformation, combined with pore-water frost heaving induced by the gradual temperature decrease, results in the observed strain increase during this stage. 2. Cold shrinkage stage. After the phase transformation of pore water in red sandstone is largely completed, the sample primarily undergoes thermal deformation. In unidirectional freeze-thaw, the freezing front continues to advance along the temperature gradient direction after passing the strain measuring point. At this juncture, the frost-heave strain at the measuring point begins to decrease, marking the onset of the freezing contraction stage. 3. Melting phase transition stage. Analogous to the first stage, the sample exhibits shrinkage at the start of heating. This shrinkage occurs because unidirectional heating causes the exposed section to expand, thereby exerting compressive force on the measuring point in the middle of the sample, manifesting as shrinkage at the measurement point. 4. Thermal expansion stage. Following the complete melting of pore ice, the sample temperature continues to rise. Consequently, distinct thermal expansion deformation becomes evident in the strain curve. Figure 4b shows the frost-heave strain during the 1 st, 3 rd, 5 th, 10 th, 20 th, 30 th, and 40 th freeze-thaw cycles in the all-directional freeze-thaw process. Analogous to the process of unidirectional freeze-thaw, the phase change curve exhibits a similar division into four stages. However, distinct differences exist between the freezing phase transition and melting phase transition stages compared to unidirectional freeze-thaw:  
1. Frozen phase transition stage. The frost-heaving strain value under all-directional freeze-thaw is typically greater than that of unidirectional freeze-thaw, yet the peak occurs significantly later. This delay occurs because the measuring point is located directly on the sample surface during all-directional freeze-thaw. Additionally, the strain of dry and saturated sandstone samples is similar during the cooling process prior to freezing. Frost heave commences only when the pore water adjacent to the surface freezes. 2. Cold shrinkage stage. As the temperature decreases further, pore-water phase change ceases, and the temperature of the red sandstone sample approaches the ambient temperature. The thermal deformation of rock skeleton particles progressively diminishes, and the slope of strain curve gradually approaches zero.