## 4 Discussion  
### Mechanistic Analysis of CBR at the Macroscale  
[PERSON] et al. [1] conducted CBR tests on loess with different compaction energies and moist contents. The results showed that the water sensitivity was strongly related to the mineralogy and microstructure of the compacted loess. It was recommended to avoid compact soil with extremely  
Figure 16: The relationship of the CBR value with the number of cycles \(Nr\). Figure 15: Curve of CBR values and compaction degrees under different moisture contents. low moist content. For a given compaction energy, the samples with a lower moisture contents expanded more after immersion in water, resulting in more strength loss when soaked in water. [PERSON] et al. [47] studied the bearing mechanism of weakly and moderately expansive soils in the Jingmen area; they supposed that the CBR value depended on the water content, dry density and structural state after the soaking expansion of the compacted expansive soils. However, the mechanisms influencing the CBR of expansive soil were less analyzed. [PERSON] et al. [48] believed that the CBR characteristics of expansive soil were determined by its air and water states of expansive soil under different saturation conditions. These studies analyzed the mechanism of CBR to a certain extent, but CBR is related to the type of soil. The specificity of expansive soil means that its CBR mechanism is different from that of other soils. The increase in volume of expansive soil after absorbing water is the most intuitive aspect that affects the CBR value. Therefore, this paper attempted to explore the effect of expansibility on the CBR value. The CBR value refers to the ratio of the force per unit area required to penetrate the sample with a standard piston to that required for corresponding penetration in a standard material. In this study, the bearing ratio at 2.5 mm penetration was adopted. Expansive soil swelled upon absorbing water [49; 50; 51]; the structure of the expanded soil was loose, which would significantly reduce its strength. In the immersion process during the CBR tests, the soil swelled upon immersion in water. There were restrictions on the bottom and sides of the sample, so only the top could deform. Due to the expansion, it gradually weakened from the top to the bottom, and because of the low permeability of the soil, the soil on the top exhibited the weakest strength with the largest amount of expansion. In the penetration process, the penetration piston is pressed into the top swelled soil; a schematic of the process is shown in Figure 17. The CBR was affected by the expansion of sample, especially for the compacted samples with low moisture contents. This could be verified from the curve of the relationship between the CBR value and expansion strain (the expansion strain is the ratio of expansion deformation to the height of the unsaturated sample), as Figure 18 shows. Under an expansion strain greater than 6%, the CBR values were less than 3%. From Figures 15, 18 and 19, it can be concluded that the CBR values were affected by the compaction degree and expansion. Given a moist content of \(w\leq w_{wt}\), the samples manifested large expansions and low CBR values. In this case, the CBR values of the samples were mainly controlled by the expansion; for samples with moisture contents of \(w_{wt}<w\leq 26.4\%\), the expansion measured in the CBR tests were slight, and the compaction degree had an obvious effect on CBR. This could explain why the CBR value of the soil sample with a 26.4% moisture content (small compaction degree) was small, while the expansions were slight. This can also be verified from the expansion strain and CBR value of the compacted soil subjected to freeze-thaw cycles; the CBR value decreased linearly with an increase in expansion strain (Figure 19b). Note that the attenuation amplitude of the CBR value with an initial moisture content of 16.4% is greater than that with a moisture content of 19%.