## 4 Discussion
### Mechanistic Analysis of CBR at the Microscale  
The expansion deformation of expansive soil is the macroscopic behaviors which affect the CBR, and explains the CBR characteristics of expansive soil to a certain extent. However, the microstructure is considered a vital aspect of in the study of soil, and the macroscale mechanical behavior is affected by the performance of the microstructure. Thus, this paper tries to explain the CBR mechanism of expansive soil from the viewpoint of pore structure based on an NMR test for the first time. The compaction tests show that in the moisture content range of 10.5%-21.7%, the compaction degree can reach 90% with a change in the compaction energy. As discussed above, the CBR is related to the strength of the soil penetrated by the piston. The depth of penetration is slightly greater than 5 mm; soil is saturated and even supersaturated at this depth. The pore structure of the soil at the top of the compacted sample may reflect the actual situation. However, it is hard to determine the pore structure of soil at 5 mm; because it is damaged when removing the soil block used for the NMR test from the sample obtained in the compaction tests. Therefore, small samples (H = 40 mm, D = 39.1) with different initial moisture contents under the same degree of compaction were prepared. The immersion process for the CBR test was then simulated. Subsequently, NMR was conducted on the immersed samples. The test results verified that expansion significantly impacts the CBR value. Figure 20a presents the results obtained by NMR. At the same degree of compaction, the main peak moved upward and the Sub Peak 1 moved downward as the moisture content increased, indicating that the micropores increased and the mesopores decreased, respectively. According to the data shown in Figure 20b, the pore volume of the micropores increased and the pore diameter decreased with an increase in moisture content. The CBR values of the sample with the same compaction and moisture contents in the NMR tests were used. By fitting the experimental data, a linear relationship  
Figure 19: (**a**) Curve of expansion strain and moisture content; (**b**) CBR values and expansion strains for samples subjected to different numbers of freeze–thaw cycles. Figure 18: Curve of CBR value and expansion strain of samples obtained in compaction tests. between the CBR values and total areas of the \(T_{2}\) distribution curves as well as that between the CBR values and proportions of main peaks, is observed in Figure 21. It is indicated that the CBR value of expansive soil is closely related to the pore distribution which is the dominant factor for the bearing strength. The different expansions caused by various initial moisture contents lead to the differences in pore structure characteristics, which, in turn, affects the bearing strength. In fact, the weakly expansive soil in the non-cold regions of China, such as that in the Baise area [52] and along the Xiang-Jing and Han-Shi expressway [53], has been considered for use as an embankment filler. However, the compaction and CBR value according to the data are insufficient for soil in cold regions. For an upper embankment filler, the CBR values must be greater than 3%; for the lower embankment, they must be greater than 3% for highways and greater than 2% for low-grade highways, according to the _Specifications for Design of Highway Subgrades(JTG D30-2015)_[54]. The CBR values in this study were all measured on soaked samples. As described, the soaked samples exhibited the lowest bearing strength. The results show that the bearing strength of the soil after immersion in water compacted at the maximum dry density, could still satisfy the requirements for an embankment filler, even after 12 freeze-thaw cycles. The buried depth of the embankment was 0.8m below ground, demonstrating that the expansive soil would not be directly exposed to the environment, and complete flooding and no-load freeze-thaw cycles will be rare. All of these factors contribute to the stability of expansive soil.