## 3 Results and Discussion  
### Freezing Characteristics of Clayey Sand  
The freeze-thaw curves of clayey sand with different initial volumetric water contents are shown in Figure 5. It can be seen that the freezing and thawing curves of soil samples with varying initial volumetric water contents exhibit similar trends throughout the test, characterized by gentle slopes at both ends and a steep curve in the middle. According to the trends of the freeze-thaw curves, the freeze-thaw process of clayey sand is divided into three main stages. The freezing process is divided into the rapid heat release stage, rapid freezing stage, and stable freezing stage; the thawing process corresponds to the rapid heat absorption stage, rapid thawing stage, and stable thawing stage [36; 37]. Figure 5: Cont. During freezing, the free water in the soil sample first rapidly releases latent heat, causing the temperature to drop and gradually enter the rapid heat release stage. In this stage, free water begins to freeze slowly, and the curve shows a gentle decline. As freezing progresses and the temperature falls below 0 \({}^{\circ}\)C, free water quickly transforms into ice, entering the rapid freezing stage. At this point, free water content decreases sharply, accelerating the freezing process, which is reflected by a steep downward curve [38]. Once most free water has transformed into ice, the soil enters the stable freezing stage, where the temperature gradually decreases, bound water freezes slowly, and the freezing process approaches completion, causing the curve to flatten. During thawing, as the temperature rises, ice in the clayey sand absorbs heat and transitions into liquid water. In the rapid heat absorption stage, ice gradually melts and absorbs heat, causing a slow upward trend in the curve. As the temperature further increases, the thawing process accelerates, entering the rapid thawing stage. At this point, a large amount of ice converts to water, free water content rapidly increases, and the curve rises sharply [39]. Finally, as thawing nears completion, the sample enters the stable thawing stage, where thawing stabilizes, the temperature continues to rise, the curve flattens, and free water content approaches equilibrium [40; 41]. Furthermore, a comparison of Figure 5a-c reveals that freeze-thaw cycles of clayey sand with different initial volumetric water contents all exhibit hysteresis loops. With decreasing initial volumetric water content, the difference between freezing and thawing curves increases, i.e., the hysteresis loop enlarges. This enlargement is mainly caused by changes in the freezing curve, while the thawing curve varies less. This indicates that as soil saturation decreases and the internal liquid water gradually deviates from free water status, the soil matrix effects become more pronounced. Moreover, changing the water state inside the soil requires the input of more energy to alter the system's state [42; 43]. Different initial volumetric water content freezing curves of clayey sand are shown in Figure 6a. From Figure 6a, it can be observed that the freezing temperatures of clayey sand with initial volumetric water contents of 30%, 18.5%, and 10% are \(-\)2.34 \({}^{\circ}\)C, \(-\)4.69 \({}^{\circ}\)C, and \(-\)6.48 \({}^{\circ}\)C, respectively. This indicates that the higher the initial volumetric water content,  
Figure 5: Hysteresis characteristics of freeze–thaw cycles for clayey sand with different initial volumetric water contents: (**a**) 30%, (**b**) 18.5%, (**c**) 10%. the higher the freezing temperature of the clayey sand. This is because, with higher soil saturation, the content of free water within the liquid phase increases, and the influence of the soil surface and pore structure on free water decreases, making the soil sample easier to freeze, i.e., exhibiting a higher freezing temperature [44].