## 1 Introduction
The applicability of weakly expansive soil as an embankment filler in frozen areas has rarely been reported. As infrastructure has developed, roads in seasonally frozen areas have inevitably had to cross weakly expansive soil, so its compaction and bearing strength characteristics should be studied in depth. The applicability of expansive soils as the embankment fillers in frozen areas should be explored. Jiamusi in Heilongjiang Province is considered as a vital frontier city in China's opening up to Russia, as well as an important node city in the development strategy for the land-sea silk road. Jiamusi has a mid-temperate continental monsoon climate, characterized by a long and cold winter and rainy summers. In the low mountains and hilly areas in the southern part of the city, the Cretaceous volcanic rocks are widely exposed, leading to the formation of montmorillonite-based weathered materials. They exhibit a relatively high liquid-plastic limit, obvious expansion, and contraction deformations; these pertain to a weakly expansive soil. As the \"Belt and Road\" has pressed ahead, and the need for infrastructure construction is rising, a growing number of roads should cross areas of weakly expansive soil. However, there is insufficient high-quality soil to replace expansive soil. In Jiamusi, a better quality (the plastic index of the weakly expansive soil is 16.2%) is found than that of other weakly expansive soils. The weakly expansive soil in this area was used directly without special treatment, but there is no reliable research to evaluate its accuracy. Thus, in terms of economy, convenience, and sustainable development, the applicability of weakly expansive soil in Jiamusi as an embankment filler should be further considered and rigorously studied. Moreover, the freeze-thaw cycles are considered a strong weathering process in seasonally frozen areas [22], which will remold soil structure when soils are exposed [23]. The freeze-thaw cycle effect should be constantly considered in seasonal frozen areas. The negative impact of freeze-thaw cycles on the behavior of expansive soils has been the subject of recent studies [24; 25; 26; 27], whereas the CBR characteristic of weakly expansive soil in seasonally frozen areas as embankment filler and its change under repeated freeze-thaw cycles have not been reported. The soil microstructure is the significant feature regulating the soil's behavior at the macroscale. It is necessary to study the interaction of the microstructure and CBR value of compacted weakly expansive soil. As described above, although achievements have been reported in CBR research, most of them focused on the evaluation, prediction and improvement of CBR value; analyses of the CBR mechanism of expansive soil are scarce. The characteristics of expansive soil mean that their CBR mechanism is different from that of other soils. However, few studies have been performed in this regard. Embankment refers to the foundation of the road bearing the vehicle loads; the stability of the embankment is important for driving safety. Considering economy and safety, the behavior of weak expansive soil acting as embankment fillers in frozen areas must be urgently studied. The main objectives of this study were to examine the applicability of expansive soil as embankment fillers in frozen areas through compaction and CBR tests, and to analyze the influence of freeze-thaw cycles on CBR values. More importantly, the study aimed to analyze the CBR mechanism of expansive soil. First, the relationship between deformation and CBR value was analyzed from the macro scale. Then, with the help of advanced nuclear magnetic resonance (NMR) technology, the relationship between pore structure and CBR value was microscopically analyzed. Lastly, according to the test results, the applicability of weakly expansive soil as an embankment filler was discussed.