## 4 Analysis and Discussion
### Discussion  
In this experiment, the results of strain variation in cracked rock under freeze-thaw cycles indicate that rock skeleton particles undergo thermal expansion and contraction during the freeze-thaw process. After each freeze-thaw cycle, residual strain and expansion deformation, i.e., plastic deformation, occur. The residual strain accumulates with each cycle, leading to a gradual increase in the overall expansion deformation of the rock as the number of freeze-thaw cycles increases. In summary, the freeze-thaw deformation evolution mechanism of the cracked red sandstone in this experiment can be concluded in the following three points:  
1. Thermal Expansion and Contraction of Skeleton Particles: The rock skeleton particles undergo thermal expansion and contraction during freeze-thaw cycles, resulting in body deformation. Since the skeleton particles are not perfectly elastic, a portion of residual strain remains after each cycle of expansion and contraction. This residual strain constitutes only part of the total residual strain for each freeze-thaw cycle.
2. Frost Heave Force Due to Water Molecules in Rock Pores: The presence of water molecules in the rock pores leads to approximately 9% volume expansion when the water freezes. If the rock constrains this expansion, a frost heave force is generated [34; 35; 36]. This frost heave force affects the contact stress between surrounding skeleton particles, altering their arrangement and causing structural deformation.
3. Damage to Cementing Materials: In red sandstone, cementing materials bond the surrounding particles together. These materials, being chemical precipitates, are prone to damage from physical and chemical actions. During freeze-thaw cycles, the cementing materials themselves undergo thermal expansion and contraction,and they are also subjected to external influences such as the compressive effect of expanding skeleton particles, the frost heave force of ice, and the dissolution effects of water molecules. As a result, some of the cementing material gradually breaks down and disappears during the freeze-thaw process. The space it originally occupied transforms into intergranular pores, leading to a significant change in the stress state of the surrounding rock skeleton particles. The contact stress between the particles changes, altering their arrangement and causing structural deformation of the rock.  
Additionally, it is worth noting that during the freeze-thaw cycles in this experiment, some of the cementing materials were damaged and disappeared. After the space they originally occupied developed into intergranular pores, the porosity of the rock increased. Since the rock specimens in this experiment were kept in a sealed environment without external water replenishment, the water content of the specimens could not fully fill these newly formed intergranular pores, causing the specimens to become unsaturated. Based on the results of this experiment, the strain variation in the cracked rock and the changes in the strain characteristic values under multiple freeze-thaw cycles did not show significant changes in the later stages of the experiment, and the patterns were largely similar to those observed in the earlier stages. Further research on this issue will be conducted.