## 1 Introduction  
The stability of slopes in seasonal frozen areas is affected by freeze-thaw cycles [1; 2; 3]. Expansive soil is a special soil that expands and shrinks with changes in moisture content and is widely distributed worldwide [4]. Expansive soil is mainly composed of silicate minerals (montmorillonite, illite and kaolin). According to the proportion of different components, the expansion and shrinkage capacity of expansive soil is different. [PERSON] et al. [5] divided it into five types: negligible, low, moderate, high and very high. X-ray diffraction shows that the higher the content of montmorillonite, the greater the expansion and contraction capacity of expansive soil. There are many expansive soil areas in China's vast frozen soil areas. In these areas, the expansive soil is often affected by climate and subjected to long-term freeze-thaw cycles [6]. [PERSON] et al. [7] introduced the distribution points of expansive soil in Northeast China, among which the railway construction in Yanji area of the Jilin Province was seriously affected by the freeze-thaw disaster of expansive soil. Many scholars have studied the physical and mechanical properties of expansive soil during freeze-thaw cycles. [PERSON] et al. [8] shows that freeze-thaw cycles will lead to larger and more uniform pores in expansive soils, and freeze-thaw cycles will reduce the shear strength of expansive soils. The lower shear strength means that the bearing capacity of expansive soil slope in frozen soil area decreases, and the stress field in the slope will also change. [PERSON] et al. [9] think that the effective cohesion and internal friction angle of expansive soil will decrease significantly during freeze-thaw cycles. However, the conclusion of the strength changes of expansive soil after freeze-thaw cycles is not uniform. [PERSON] et al. [10] tested the dynamic characteristics of expansive soil slopes during freeze-thaw cycles. It is found that under a certain confining pressure, the freeze-thaw cycle can produce additional deformation in the expansive soil, which means that the expansive soil slope after the freeze-thaw cycle can bear a greater load. The stability of expansive soil slopes with different slope ratios is very complex. It is generally considered that the expansive soil slope in seasonally frozen areas is affected by periodic ice-water conversion, which causes structural damage to the soil [9], resulting in a significant reduction in the safety factor of the slope [11]. The effect of the freeze-thaw cycle on expansive soil is mainly to change its internal pore structure, resulting in surface cracking and reduced bearing capacity [6]. [PERSON] et al. [12] monitored that there were severe fluctuations in earth pressure and pore pressure during the freeze-thaw cycle. Moreover, the freeze-thaw cycle caused the water in the soil to collect to the slope surface, increasing the landslide risk [13]. To analyze the changes of physical properties of expansive soil slopes, many scholars have conducted field tests [14; 15], centrifuge model tests [16; 17; 18] and static model tests [19; 20; 21]. [PERSON] et al. [22] used temperature and displacement sensors to measure the frost heave of soil in seasonal freezing areas, and determined the freeze-thaw stability of coarse saline soil. However, these tests only monitor or simulate the changes of natural moisture environments and do not consider the change in environmental temperature, especially the influence of freeze-thaw cycles on slope stability. Under the freeze-thaw cycling, the expansive soil is in the complex stress state of multifield coupling. In this process, the expansive soil slope has obvious changes in moisture content and soil pressure. After freeze-thaw cycles, expansive soil slopes are characterized by multiple cracks, low resistance and destructive landslides, and large relative displacement at the macro level [23].