## 3 Results and Analysis
### Compaction Characteristics and Pore Size Distribution
#### 3.1.3 Freeze-Thaw Cycles' Effects on Soils with Different Compaction Degrees  
Soil in frozen areas takes up more than 50% of China's land area; the soil is repeatedly frozen and thawed due to temperature fluctuations. Previous authors have shown that the porosity increases under the effect of freeze-thaw cycles [38; 39; 40; 41]. To ensure the stability and safety of the infrastructure in seasonally frozen areas, the freeze-thaw effect should be considered in construction in such areas. Soils with different compaction have different mechanical behaviors as induced by freeze-thaw cycles [42]. The compaction degree of embankment soil is generally greater than 90%. Thus, in the present study, samples with compaction degrees of 90% and 100% as described in Section 3.1.2 were studied; the effect of freeze-thaw cycles on the pore size distributions at different compaction degrees was investigated using NMR. The \(T_{2}\) distribution curves of a loose sample (K = 90%) and dense sample (K = 100%) are presented in Figure 10. Under increasing freeze-thaw cycles, the mesopores and macropores increase, and micropores decrease. Figure 11 illustrates that the pore size distribution curve moves to the right, indicating that the freeze-thaw cycles increase the pore diameter. For loose samples, the diameter of the micropores increases from 0.0266-2.29 \(\mu\)m to 0.04-5.5 \(\mu\)m while the diameter of such increases from 0.0047-0.25 \(\mu\)m to 0.006-0.88 \(\mu\)m for dense samples. Obviously, the first freeze-thaw cycle significantly affected the pore diameter compared with the others. However, the increase in total pore volume was more significant than that in pore diameter for mesopores. The proportions of the peak area in the \(T_{2}\) curve are shown in Figure 12. Since the proportion of Sub Peak 2 is significantly small, only the variations in the mesopores and macropores are discussed here. For the loose sample, under increasing freeze-thaw cycles, the proportion of the main peak gradually declined, from 0.92 to 0.85, while the proportion of the Sub Peak 1 increased from 0.06 to 0.13. Under the number of cycles \(N_{\mathit{FT}}\) < 8, the proportion of the main peak decreased and Sub Peak 1 increased with an increase in \(N_{\mathit{FT}}\), when \(N_{\mathit{FT}}\) > 8, the proportions of the two peaks varied slowly. For the dense sample, when \(N_{\mathit{FT}}\) < 4, the main peak took up over 0.99, and with an increase in freeze-thaw cycles, the proportion of the main peak slightly decreased and Sub Peak 1 increased; when \(N_{\mathit{FT}}\) > 4, the decrease rate of the main peak and the increase rate of sub peak 1 rises sharply. The proportion of the main peak was reduced from 0.99 to 0.96, and that of the Sub Peak 1 increased nearly 4 fold from 0.007 to 0.027. This accords with the results of [PERSON] and [PERSON] [41], who reported that macropores are strongly affected by freeze-thaw cycles. In conclusion, dense soil has a small initial void ratio and small and evenly distributed pores, and it exhibits a stable structure that resists damage from the external environment, at the beginning of the freeze-thaw cycles. Accordingly, the freeze-thaw cycle slightly affects the soil structure. However, with a further increase in freeze-thaw cycles, the original structure of the soil varies significantly, the freeze-thaw cycles still significantly affect the pore structure when \(N_{\mathit{FT}}\) = 12. By contrast, for loose soil, the void ratio is large, and the effect of freeze-thaw cycles on the soil's pore structure mainly occurs in the early stage of freeze-thaw cycles (\(N_{\mathit{FT}}<8\)). The variation of the soil's pore structure caused by freeze-thaw cycles is slight when \(N_{\mathit{FT}}\) > 8. Figure 9: The proportion of each peak with different compaction degrees. The pore size distribution reflects the internal adjustment of the pore structure, and the void ratio is a parameter of the pore volume variation at the macroscale. The effects of freeze-thaw cycles on pore volume cannot be ignored.