# Study of the Load-Bearing Characteristics of Bolts under Asymmetric Freezing Conditions
## 1 Introduction  
With the advantages of good mechanical properties, high reinforcement efficiency and good elasticity, anchor rod reinforcements are widely used in slope reinforcement, pit support, roadway excavation and other projects [1; 2; 3]. The stability and mechanical properties of loosely packed soils are enhanced by anchor reinforcement [4]. In some cold regions, the migration and formation of ice by water in soil at low temperature causes the soil around the anchor rod to enter a freeze state under the negative temperature load. The water in the soil solidifies into ice crystals that are immobile, expand in volume and fill the pores between the soil particles. The presence of ice crystals causes soil particles to crowd and changes the mechanical properties of soil. Many scholars have studied the nature of permafrost. Some scholars evaluated the shear strength of frozen soil by direct shear tests [5; 6; 7; 8; 9; 10; 11; 12; 13; 14]. [PERSON] [15] developed model pile-testing equipment and used it to conduct several field freezing strength tests. The results showed that the freezing strength and residual freezing strength increased linearly with decreasing temperature. There is a limiting value of water content in frozen soil. When the water content in the soil is less than this value, the freezing strength and residual freezing strength increase with increasing water content and conversely decrease with increasingwater content. [PERSON] et al. [16] believed that the shear strength of permafrost increased linearly with decreasing temperature when the shear strength was shared by ice crystals and soil particles. [PERSON] et al. [17] launched a large-scale direct shear test on permafrost soil and plotted shear stress-displacement curves. They considered that the damage process consisted of five parts: elastic deformation, plastic deformation, integral sliding, strain hardening and residual strength stabilization. The peak shear strength was linearly related to temperature and normal stress and nonlinearly related to the moisture content of the soil, and the moisture content had a more significant effect than temperature on the peak strength. The shear strength of frozen soil was related to the shear rate in the direct shear test. [PERSON] et al. [18] considered the shear strength of permafrost to be influenced by the rate of damage, and the peak shear decreased with increasing shear rate. [PERSON] and [PERSON] [19] believed that the shear strength of permafrost was affected by the surrounding pressure of the soil. When the surrounding pressure was larger, the soil particles squeezed one other to increase soil compaction; however, the fracture of ice crystals was restricted, and the internal friction of the soil was enhanced. In contrast, [PERSON] [20] believe that when the surrounding pressure was small, the water between soil particles produced a greater degree of expansion during freezing, which reduced the relative compaction of the soil and soil strength. Apparently, in the perennial permafrost region, there is strength anisotropy in the soil due to the formation of ice wedges according to [PERSON] et al. [21]. The change in the underground environment temperature affects the load-bearing capacity of the soil mass and then changes the stress conditions of the underground infrastructure. Some engineering structures, such as underground pipe galleries, retaining walls and anchorage systems, are damaged [22]. The stress state and pull-out resistance of the anchor change due to the frost heaving force. Some scholars evaluate the bearing capacity of the frozen soil anchor through the shear strength of the interface between the bolt and frozen soil and analyse the influence of various factors on the load-bearing characteristics of the anchor and the safety of the anchoring project. [PERSON] and [PERSON] [23] proposed the \"m\" coefficient to evaluate the roughness of the interfaces of anchors in frozen sand. The shear strength of the interface was related to the cohesion and friction of the frozen soil.