## 2 Experimental Methodology  
### Slope Model Test  
#### 2.1.1 Slope Model Preparation  
The parameters of the basic physical properties of the soil used in the slope model are shown in Table 1. The remolded expansive soil used in the test is from Meicun Town, Weifang, China. It is located in the seasonal freezing area of China. The slope model test of expansive soil attempts to study the potential problems in road construction of expansive soil in cold areas. Considering that expansive soil is a kind of engineering disastrous weathered soil, this test is only applicable to a single soil to find the dangerous state. The optimal moisture content and maximum dry density of expansive soil were measured according to ASTM D698 [28]. Figure 1 shows the grain-size distribution curves and the optimum moisture content for the expansive soil. Before optimum moisture content (OMC), the water film of soil particles is thin and it is difficult to reach the maximum dry density. For soil exceeding OMC, the water film is thick enough to produce large pore pressure during compaction, which makes it difficult to compact the soil. It is very necessary to evaluate frost susceptibility in frozen soil research. Figure 1a shows that the grain-size of expansive soil used in this test is very small; clay (<2 \(\upmu\)m) accounts for about 49% and silt (2-75 \(\upmu\)m) accounts for about 36%. [PERSON] [29] believes that clay, due to its low permeability, will not cause frost heave unless freezing is very slow, but silt soil shows frost susceptibility. However, this does not take into account the large volume change during the change in moisture content of expansive soil. [PERSON] [7] also found that expansive clay showed high sensitivity to freeze, and [PERSON] [30] obtained similar results. Therefore, the expansive soil used in this paper can be considered as frost susceptible soil. The free expansion ratio of expansive soil in Table 1 is 1.51, and according to [PERSON] et al. [5], it can be classified as moderate expansive soil. The determination process of free expansion ratio is to place two 10 g dry expansive soil samples in distilled water and kerosene, respectively, and pass them through a 425 mm sieve after they are stable. The volume ratio of the two is the free expansion ratio. Its value represents the water absorption and expansion capacity of expansive soil. The model test includes three expansive soil slope models. For convenience, they are called the \(S_{1}\), \(S_{2}\) and \(S_{3}\) models, respectively. The process of preparing the expansive soil slope model is shown in Figure 2. The expansive soil slope model is prepared with layers, which guarantees the compactness of each layer. To ensure the integrity of the model, a scraping process was conducted at the junction of each layer. When the slope was prepared at the appropriate height, the mold was removed and the slope was manually cut. The four steps are as follows: (a) layered compaction; (b) surface scraping; (c) dismantling mold and (d) slope cutting. In the test, three groups of slope models were placed parallel to each other in a custom-fabricated environmental chamber. [PERSON] et al. [31] investigated the Riogordo Landslide and found that shallow rotational slides occurred at slope angles of 15-70 \({}^{\circ}\)C  
Figure 1: Grain-size distribution curves and compaction test for the soil. (**a**) Ratio and classification of different soil particles; (**b**) compaction curves of the soil. in seasonal frozen expansive soil area. In order to study the slope stability characteristics under different slope ratio conditions, the optimum moisture content corresponding to the maximum dry density was controlled and different slope ratio tests were set up. Therefore, the moisture content of the slope model was 20% (the optimal moisture content), and the slope ratios were 1:1.5, 1:1 and 1:0.5 (Figure 3). The selection of the slope ratio was to select the common slope angle in the project, which was set from gentle to steep with a certain degree of differentiation.