## 3 Results and Analysis
### Uniaxial Compressive Strength and Failure Mode  
Figure 6 shows the stress-strain curve of rock sample after freeze-thaw cycling. As can be seen, the rock sample undergoes four stages during uniaxial compression: the initial pore compaction stage, the elastic deformation stage, the plastic deformation stage, and the failure stage. Most specimen failures show obvious distinct characteristics, meaning an obvious stress drop occurs after the peak on the stress-strain curve. The stress-strain curves of samples under different freeze-thaw cycles exhibit slight differences, with the following specific characteristics:  
1. In the pore compaction stage, the pore compaction process in samples subjected to all-directional freeze-thaw is shorter, and the samples enter the elastic deformation stage after only a small amount of axial deformation. 2. In the elastic deformation stage, the stress-strain curve of red sandstone shows pronounced linear characteristics. The sample deforms elastically under load, and the elastic modulus of the all-directional freeze-thaw samples is larger than that of the unidirectional freeze-thaw samples. Due to cooling occurring primarily on the upper surface, the unidirectional freeze-thaw specimens exhibit more pronounced vertical inhomogeneity. Consequently, weak surfaces within the specimen tend to fail first during compression. 3. In the plastic deformation stage, with the further increase in load, the frozen-thaved red sandstone enters the plastic deformation stage. During this process, the stress-strain curve exhibits multiple discrete stress drops. The plasticity of the sample increases with the number of freeze-thaw cycles. 4. In the post-peak stage, with the increasing plastic deformation of the specimen, the fluctuation in the stress-strain curve further increases. After reaching the peak stress, the specimen fails and its strength drops rapidly. Figure 6 also shows the variation in the unconfined compressive strength (UCS) of rock samples with the number of freeze-thaw cycles. The UCS decreases as the number of freeze-thaw cycles increases. For all-directional freeze-thaw samples, the UCS decreased slightly initially but rapidly later: the reduction was only about 34% after 0-20 cycles but accelerated  
Figure 5: Frost-heaving cracks. to approximately 46% between cycles 20 and 30, resulting in a total reduction of about 80% after 30 cycles (relative to the initial strength). In contrast, the UCS of unidirectional freeze-thaw samples decreased rapidly in the early stages and then stabilized: a 57% reduction occurred within the first 20 cycles, followed by a much smaller decrease of 16% between cycles 20 and 30. This behavior is significantly different from that observed in all-directional freeze-thaw. The migration of pore water during unidirectional freezing and thawing is affected by temperature gradient, and there is an uneven distribution of water content in the vertical direction. When frost heaving occurs, the samples subjected to unidirectional freeze-thaw have uneven frost-heaving damage due to the uneven distribution of water content in the vertical direction. In the process of freezing and thawing in all directions, the uneven frost-heaving damage of rock samples is uneven from the central position of the samples to the outer surface. Comparing the two types of uneven damage, vertical inhomogeneity leads to a more obvious decrease in the early compressive strength. With an increase in the number of freeze-thaw cycles, the rock samples subjected to freeze-thaw in all directions have obvious macro-failure and the frost-heaving cracks extend outward from the center and connect with the outer surface. At this time, the crushing degree of the samples subjected to all-directional freezing and thawing is greater than that of samples subjected to unidirectional freezing and thawing, and the unconfined compressive strength is significantly reduced.