## 3 Results  
### Cracks and Pore Volume of Expansive Soil Development  
After four freeze-thaw cycles, the development of cracks at the top of the \(S_{1}\), \(S_{2}\) and \(S_{3}\) slope models is shown in Figure 6. The maximum width of cracks on the \(S_{1}\) slope top was 4 mm, that of the \(S_{2}\) slope was 5 mm, and that of the \(S_{3}\) slope was 6 mm. [PERSON] et al. [24] divided the development of cracks in expansive soil into three stages: crack initiation, crack propagation and crack stabilization, and attributed the formation of cracks to moisture evaporation and ice sublimation. However, in this test, the slope was in a closed system, and there was no moisture exchange with the outside. Figure 7 shows that there was obvious moisture migration to the slope surface during the freeze-thaw cycle. For a closed system, moisture migration to the surface of the slope model means that the moisture content in the slope decreases, and the expansive soil particles shrink rapidly due to moisture loss. This will lead to an increase in pores in the slope model and will produce fine cracks, which will affect the structure and stability of the slope. Figure 5: Nuclear magnetic resonance analysis instrument and test samples. Figure 6: Crack development of the slope model after four freeze-thaw cycles (**a**) \(S_{1}\)(**b**) \(S_{2}\) (**c**) \(S_{3}\) (**d**) Vertical cracks. Figure 7: T2 distribution of the expansive soil after six freeze-thaw cycles. (**a**) A1 sample test results; (**b**) A3 sample test results. In the process of freeze-thaw cycles, the evolution mode of cracks in expansive soil slopes gradually changes from irregular straight lines to polygonal cracks. [PERSON] [37] called this kind of quasi-vertical slope crack network a systematic crack network. The development pattern of surface cracks in the slope is related to its initial moisture content. The surface of the slope with a higher moisture content tends to generate irregular cracks during freeze-thaw cycling. In Figure 6 the cracks of \(S_{1}\), \(S_{2}\) and \(S_{3}\) at the top and slope surface are mainly systematic cracks, and the maximum width of cracks also increased with the gradual increase in the slope ratio, which indicates that steep slopes, such as \(S_{3}\), are more likely to generate large cracks during freeze-thaw cycles. This leads to moisture penetration along the cracks, and eventually reduces the stability of the expansive soil slopes. Figure 6d shows the development of the vertical fissure depth of the \(S_{1}\), \(S_{2}\) and \(S_{3}\) slope models after freeze-thaw cycling. The vertical cracks depth of the \(S_{1}\) slope was 8.3 cm, that of the \(S_{2}\) slope was 9.1 cm, and that of the \(S_{3}\) slope was 11.3 cm. The fissure width and fissure depth were directly proportional to the slope ratio; the larger the slope ratio, the greater the fissure width and depth. The freeze-thaw cycles weathered the shallow slope of the expansive soil slope. The larger the slope ratio, the greater the weathering depth of the slope model surface, and the greater the impact on the stability of the expansive soil slope. Compared with the T2 response spectrum of freezing and thawing process, it can be seen that the signal amplitude in the thawing stage was significantly greater than that in the freezing stage. The reasons for this are mainly as follows: a. During the freezing process, the pores between expansive soil particles decrease, that is, they experience freeze shrinkage; b. part of the free moisture in the soil becomes solid ice, which further reduces the pores between soil particles. In the thawing state, the peak value of the NMR spectra decreases with increasing thawing times (Figure 7a,b), and tends to stabilize. However, in the freezing state, the peak value of T2 decreased sharply after the first freezing, and then increased slowly with increasing freezing times. The peak value of the T2 response spectrum measured at thawing was significantly greater than that measured at freezing.