## 1 Introduction  
As a kind of high-plastic clay, expansive soil has a high bearing capacity. Due to its high content of hydrophilic clay particles (e.g., illite, montmorillonite) and strong hydrophilic and physical characteristics of water absorption expansion and water loss contraction, mud pumping and uneven settlement is caused in roads, resulting in fracture formation. Therefore, when expansive soil is used as the subgrade filler, it often needs to be improved in practical engineering. As the modified expansive soil, many scholars have conducted a lot of research showing that it is better to use line, fly ash, cement, EPS, MICP, SLGP, etc., to improve expansive soil [1; 2; 3; 4; 5; 6; 7]. As a kind of difficult construction area, seasonal frozen soil areas cause great harm in actual engineering. With the increasing number of freezing-thawing cycles, the pore water inside the soil constantly undergoes phase transformation, which causes the change of the pore structure of the soil, and the macroscopic performance of the soil is degraded and the performance of the mechanical properties are reduced [8; 9]. Expansive soil is a kind of porous clay with strong expansion and fast strength decay, and the deterioration effect of freeze-thaw cycles is particularly significant, so it is important to study the mechanical properties of expanded soil after freeze-thaw. [PERSON] [10] took Nanyang expansive soil as the research object and found that the influence of the freeze-thaw cycle on the mechanical properties of expansive soil increased along with the water content, especially the influence of the first cycle, and found the change rule of internal pores of soil with the increase of freezing-thawing times. [PERSON]'s research [11] shows that under the condition of the freeze-thaw cycle, expansive soil is under the combined influence of the drying-wetting cycle and the freezing-thawing cycle, resulting in macroscopic and microscopic cracks in the soil, and the bearing capacity is reduced. [PERSON] [12] found that the shear modulus and the volumetric strain tend to be stable after more than seven freezing-thawing cycles during the triaxial tests under consolidation and drainage conditions. Based on the experimental results, he establishes a microscopic damage-prediction model considering the freeze-thaw action. Therefore, if expansive soil is used in seasonal frozen regions, it is necessary not only to improve the expansive soil to reduce its expansibility, but also to pay attention to the effect of the improved freezing-thawing cycle on its mechanical properties. For the improvement of the expansive soil's expansibility, studies have shown that the weathered sand has a better effect on the improvement of expansive soil. Weathered sand is sand and soil formed from rocks that have undergone prolonged weathering. In geotechnical engineering, the content of weathered sand in the soil body plays an important role in the engineering properties and stability. [PERSON] [13] added different amounts of weathered sand into expansive soil, and found through the no-load expansion test that weathered sand could effectively reduce the soil expansion rate. The improvement effect was proportional to the amount of weathered sand. Through the static triaxial test, it was determined that the shear strength reached its maximum value when the amount of weathered sand was at 16%. [PERSON] [14; 15; 16] studied the mechanical properties of weathered-sand-modified expansive soils under freeze-thaw cycles and found that the compressive properties and the frost resistance of the modified soils were significantly improved. The above research shows that weathered sand can improve the swelling and frost resistance of swelling soil at the same time, but the current research on weathered sand's improvement of swelling soil is still focused on the aspect of static mechanics.