## 3 Results and Discussion
### Effect of F-T Cycles on the Microstructure of Soil Samples  
To analyze the mechanism of the SWCC change after the F-T cycles, the microstructure of the samples was analyzed by the SEM. Three parallel samples were prepared for electron \begin{table}
\begin{tabular}{c|c c c c|c c c c|c c c c|c c c} \hline \hline
**Density** & \multicolumn{8}{c|}{**1.73 g/cm\({}^{3}\)**} & \multicolumn{8}{c|}{**1.67 g/cm\({}^{3}\)**} & \multicolumn{8}{c}{**1.61 g/cm\({}^{3}\)**} \\ \hline
**F-T Cycles** & **0** & **1** & **3** & **5** & **7** & **0** & **1** & **3** & **5** & **7** & **0** & **1** & **3** & **5** & **7** \\ \hline \(\psi_{b}\) & 335.06 & 304.23 & 278.99 & 259.25 & 256.03 & 182.34 & 155.51 & 136.34 & 125.51 & 125.08 & 135.2 & 106.72 & 91.49 & 81.41 & 80.19 \\ \(s_{1}\) & 12.96 & 12.98 & 13 & 13.05 & 13.06 & 13.4 & 13.44 & 13.5 & 13.51 & 13.51 & 13.51 & 13.58 & 13.62 & 13.63 & 13.63 \\ \(\theta_{i}\) & 40.19 & 40.23 & 40.26 & 40.31 & 40.3 & 40.43 & 40.5 & 40.57 & 40.61 & 40.61 & 40.41 & 40.45 & 40.47 & 40.49 & 40.5 \\ \(\theta_{r}\) & 13.03 & 12.86 & 12.64 & 12.49 & 12.47 & 12.65 & 12.39 & 12.2 & 12.15 & 12.14 & 12.34 & 12.21 & 12.1 & 12.03 & 12 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: SWCC characteristic parameters table. microscopy scanning in each case to ensure the validity of the analysis results. The scanning range was selected from the most representative area of the samples after different freeze-thaw cycles, and at least five scanning images were obtained for each sample. As an example, Figure 5 shows the representative SEM images without processing after zero, one, and five F-T cycles. Analysis of the images revealed that the particles of SC specimens without freeze-thaw cycle are closely connected and present a plate-like structure as a whole. After one F-T cycle, the compact connections between the particles are broken and the sample shows cracks. With the continuation of a F-T cycle, the sample structure was significantly different from that before freezing and thawing, especially the connectivity between soil particles was significantly reduced. The microstructure of the sample becomes looser and larger pores began to form between the soil particles. In order to quantitatively analyze the characteristics of pore changes after F-T cycles, IPP (Image Pro Plus) software was used to statistically analyze the porosity and average diameter of the samples. Before this, the original image needs to be binarized to accurately identify the pores in the image and remove the small and irregular noise points by selecting an appropriate threshold. Based on the observation of the statistical data, the pore size was divided into small pores (\(d<2\) um), medium pore (\(2<d<5\) um), and large pores (\(d>16\) um). The results are shown in Table 3.  
Figure 5: SEM image of soil sample. (**a**) 0 F-T cycle; (**b**) 1 F–T cycle; (**c**) 5 F–T cycle. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{1}{c}{**F–T Cycles**} & \multicolumn{1}{c}{**Porosity (\%)**} & \multicolumn{1}{c}{**Mean Diameter (mm)**} & \multicolumn{1}{c}{**Small Pore (\%)**} & \multicolumn{1}{c}{**Medium Pore (\%)**} & \multicolumn{1}{c}{**Large Pore (\%)**} \\ \hline
0 & 0.94 & 2.04 & 83.51 & 16.49 & 0 \\
1 & 4.35 & 2.28 & 73.24 & 25.91 & 0.85 \\
3 & 5.84 & 2.49 & 72.40 & 23.04 & 4.56 \\
5 & 7.11 & 2.71 & 66.59 & 25.70 & 7.71 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Pore parameters after F–T cycles. The data in Table 3 indicate that the F-T cycles result in the large and medium pores in the soil structure increase, and the small pores become less. As the number of F-T cycles increases, the porosity gradually increases, and the average diameter of the soil pores becomes larger. After the first F-T cycle, the increase is most obvious and then gradually decreases. Figure 6 shows the comparative relationship between pore parameters and SWCC characteristic parameters. Figure 6 explains the results of the above SWCC experiment.