## 3 _Introduction_  
Gobi, mountains and deserts are widely distributed in northwest China [1, 2, 3], and coarse-grained soil exists as a convenient backfill. Coarse-grained soil has significant engineering characteristics, high shear strength and good compaction, which can improve the rigidity of engineering structures. However, coarse-grained soil is mostly composed of noncohesive soil, such as stone, pebbles and gravel, which belong to typical single particles with large looseness. Compared with clay, the tensile strength of coarse-grained soil is lower, which leads to poor structural stability. A geogrid is a two-dimensional mesh engineering material. It can effectively improve the overall strength of roadbeds, pavement, slopes and retaining walls [4; 5; 6; 7]. More specifically, on the one hand, under the destruction of pressure and shear force, the soil particles locked by a geogrid mesh are blocked by transverse ribs, and the reinforcement-soil interface produces greater friction than the shear surface of the unreinforced soil. On the other hand, under the action of pressure, the stress of the geogrid-reinforced interface changes. The pressure on the upper layer causes the geogrid to form a concave surface, and the geogrid produces an upward lifting force so that the stress of the lower soil of the geogrid is less than the upper stress as a net effect [8]. To further explore the interface characteristics between geogrids and soil, scholars have performed much research based on direct shear and pull-out tests. [PERSON] [9] compared the direct shear test of gravel and cohesive soil and concluded that the interface shear strength of gravel is higher. [PERSON] [10] analyzed the influence of the change in overlying load on the pullout resistance of an HDPE geogrid through a pullout test of expansive soil. [PERSON] [11] obtained the time-history function by pull-out testing of reinforced soil and monitoring the instant strain of the geogrid. [PERSON] [12] carried out geogrid pull-out tests on gravel soils with different gradations and concluded that the particle size had a great influence on the pull-out resistance of the reinforcement-soil interface. When the content of coarse-grained soil with a particle size of 10 mm was more than 30%, the pull-out peak was the largest. [PERSON] [13] carried out pull-out tests on geogrids with different cross-sectional areas and concluded that the pull-out resistance increased with increasing cross-sectional area. However, it was noted that the pull-out part of the geogrid in the middle of the fixture was slightly larger than the two ends. Therefore, if a wider geogrid is used for the pull-out test, the stress distribution at the fixture should be noted. [PERSON] [14] found that the shear strength of reinforced soil decreases with decreasing maximum particle size, and the increase in water content will reduce the interfacial shear strength by a direct shear test between a polypropylene biaxial geogrid and soil. Frozen soil area accounts for 70% of the total land area in China and is concentrated in the Northeast Plain, Xinjiang and Tibet [15]. According to the engineering application of seasonal frozen soil and permafrost areas, geogrids, as flexible reinforced structures, have strong advantages in inhibiting thaw settlements, subgrade frost heaves, and the overturning and displacement of retaining walls and have been applied in a small range [16; 17; 18; 19]. However, most reinforcement projects in cold regions are based on empirical conclusions obtained from field monitoring data and theories obtained from freeze-thaw cycle tests. Freezing and thawing are two long-term processes, and the migration of water in the soil will cause a series of changes in the soil. Due to the uneven distribution of temperature gradient potential energy, water will flow from high potential energy to low potential energy, changing the water distribution inside a structure [20; 21].