## 3 Results and Discussion
### Effect of F-T Cycles on Microstructure for Soil Samples  
It has been demonstrated that the significant decrease in resilient modulus after F-T cycles is mainly related to the water transfer, volume, and microstructure change of soils during the freezing and thawing process [23; 25]. To investigate the effect of F-T cycles on microstructure change for the two materials, SEM tests were conducted to scan the soil samples after F-T cycles. The original scanning images for the two materials after 0, 1, and 5 F-T cycles are shown in Figures 7 and 8. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Soil Types** & **Group** & **Fixed Factors** & \begin{tabular}{c} **Sum of** \\ **Squares** \\ \end{tabular} & **Variance** & \begin{tabular}{c} **Mean** \\ **Square** \\ \end{tabular} & **F Value** & \(\boldsymbol{p}\)**Value** \\ \hline Unmodified & 40-1 and & \
\begin{tabular}{c} Confining \\ pressure \\ \end{tabular} & 8447.723 & 1 & 8447.723 & 235.677 & \textless{}0.05(Significant) \\ SC & 50-2 & Deviator stress & 7943.085 & 2 & 3971.542 & 110.799 & \textless{}0.05(Significant) \\ F-T cycles & 22731.808 & 4 & 5682.952 & 158.545 & \textless{}0.05(Significant) \\ \hline The modified & 40-1 and
\begin{tabular}{c} Confining \\ pressure \\ \end{tabular} & 10362.438 & 1 & 10362.438 & 348.872 & \textless{}0.05(Significant) \\ SC & 50-2 & Deviator stress & 11361.213 & 2 & 5680.606 & 191.249 & \textless{}0.05(Significant) \\ F-T cycles & 9734.423 & 4 & 2433.606 & 81.932 & \textless{}0.05(Significant) \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Results of significance analysis for test soils. Figure 7: SEM images for unmodified SC after (**a**) 0, (**b**) 1, and (**c**) 5 F-T cycles. Figure 10: _Cont._  
Figure 9: Variation curves of resilient modulus and porosity versus F-T cycles for **(a)** unmodified SC and **(b)** the modified SC. As shown from the results, the significance values (2-tailed) are 0.021, 0.007, 0.010, and 0.049, respectively, which are all lower than 0.05. This means that the resilient modulus is correlated with porosity and mean diameter at the level of 0.05 (2-tailed). The Pearson correlation coefficients of both unmodified SC and the modified SC are negative. The absolute values of them are all higher than 0.95. This means that there is very strong negative correlation of resilient modulus versus porosity and mean diameter. To discuss the pore distribution change characteristics after F-T cycles, the pores of the two materials after F-T cycles were counted and classified into small pores (d \(<\) 2 \(\upmu\)m), medium pores (2 \(\upmu\)m \(<\) d \(<\) 5 \(\upmu\)m), and large pores (d \(>\) 5 \(\upmu\)m). The results are listed in Table 10.  
For unmodified SC, there are mainly small and medium pores before F-T cycles. With increasing F-T cycles, the amounts of medium and large pores increase and that of small pores decreases. The situation for the modified soil is slightly different. Compared with unmodified SC, the amounts of medium and large pores in the modified soil are larger than those in silty clay before F-T cycles. The mean diameter of pores for the modified soil is larger as well. As F-T cycles increase, the distribution of pores tends to be stable: the amounts of small pores slightly decrease, and the amounts of medium and large pores increase. From the SEM test, the effect of F-T cycles on soils is mainly to destroy the initial structure, increasing the porosity and mean diameters of pores. It can be concluded that the microstructure change during F-T cycles is the main factor that results in the decrease of the resilient modulus.