# Shear Resistance Evolution of Geogrid Reinforced Expansive Soil Under Freeze-Thaw Cycles
## 1 Introduction  
The world's permafrost accounts for 24 percent of the land area and is widely distributed, exceeding 50 percent in some countries, such as Norway, Arctic islands, Greenland, and Antarctica, where it is particularly extensive [1]. In China, the permafrost distribution area is large, accounting for about 22% of the country's land area, and the seasonal permafrost area accounts for about 48%. Under long-term F-T conditions, geotechnical engineering produces irreversible and persistent damage accumulation and leads to numerous engineering problems [2, 3, 4], for example, slope instability, severe deformation of the roadbed, and damage to retaining walls. Many studies have shown that F-T cycles change the pore structure characteristics of soils, and soil strength, modulus of elasticity, cohesion, and angle of internal friction decrease significantly with an increment of F-T cycles [5, 6, 7, 8, 9, 10, 11]. Expansive soil is a kind of high plasticity clay containing montmorillonite and ilmenite, and other strong hydrophilic clay minerals, which has multi-fracture and strong expansion and contraction, expansion by water absorption, and contraction by water loss. Repeated expansion and contraction deformations are very likely to occur under natural environmental conditions, which leads to the creation of disorganized cracks in the soil body, increasing the deformation of the soil body, and reducing the shear strength of the soil body. Expansive soils are widely distributed in many countries and cause great damage to civil engineering structures [12; 13; 14]. In the F-T cycle environment, the phase state of water in expansive soil is constantly changing between solid and liquid, and the difference in volume between equal mass ice and water can lead to deformation of the soil. In addition, expansive soils undergo expansion and contraction deformations due to expansion by water absorption and contraction by water loss during changes in the moisture phase state. It causes serious damage to the structural properties of the soil and also has a large impact on the physical and mechanical properties of expansive soils. F-T cycles have a significant effect on the static and dynamic properties of the soil, which is detrimental to engineering safety issues. Therefore, the study of soil reinforcement techniques is extremely important. In ancient times, simple materials were mixed with soil to form simple reinforced earth materials [15]. Many scholars have used hair, straw, fiber, rubber and other materials to improve the mechanical properties of soils [16; 17; 18; 19; 20; 21], which was found that the spatial confinement of the soil by the reinforcement material and the friction between the reinforcement and the soil could inhibit the pore enlargement caused by the F-T cycle and improve the F-T resistance of the soil. Geogrid is widely used to reinforce roadbed, retaining walls, slopes and other projects, because of its uniform overall structure, high nodal tensile strength, high tensile modulus, good resistance to twisting, low strain, good creep resistance, which can restrain the lateral deformation of the soil body and improve the shear strength of the soil body through Friction between the expansive soil and geogrid and geogrid embedded locking effect [22; 23; 24]. [PERSON] and [PERSON] [25] and [PERSON] et al. [26] found that geogrids were able to increase soil shear strength by unconfined compressive strength tests and triaxial tests. Some scholars investigated the interfacial properties of the reinforced soil and the reinforcement mechanism by pullout tests and direct shear tests [27; 28; 29]. According to previous studies, the reinforced soil structures exceeded unreinforced soil structures under F-T cycles in indoor modeling tests [30; 31]. In summary, expansive soils have the property of expansion by water absorption and contraction by water loss, which further produces irreversible and continuous damage accumulation under the influence of F-T cycles, increasing the risk to engineering.