# Experimental Study on Shear Mechanical Properties of Pile-Soil Interface Under Freezing Conditions
## 1 Introduction  
The Qinghai-Tibet Plateau is the world's highest and largest permafrost region in low- and middle-latitude regions [1]. With the implementation of national strategies, such as \"the Belt and Road\" construction and the new round of China Western Development, more and more engineering projects will be carried out in cold areas [2]. Under the background of permafrost degradation caused by climate warming and human activities, pile foundations have inestimable application prospects and will play an increasingly crucial role in permafrost engineering, due to their advantages of a high bearing capacity, small pile top settlement, and strong adaptability [3]. The vertical bearing capacity of a single pile in a permafrost area is mainly provided by the freezing strength of the frozen soil-concrete interface, which can be characterized by the interface shear strength along the pile [4]. However, the formation mechanism, the distribution, and the law of evolution of the freezing strength at the interface along the pile still need to be further studied, which limits the design and verification of pile foundations in cold regions. Therefore, the experimental and analytical investigation of the shear strength and influencing factors is a prerequisite for ensuring the security and stability of pile foundations, which has a certain guiding significance for the design, construction, and maintenance of pile foundations in cold regions. According to [PERSON]'s definition, the freezing strength of the frozen soil-concrete interface can be measured as shear strength [5]. In 1981, the freezing strength between piles and soils was successfully measured through experimental techniques [6,7]. Since then, many investigations on the shear properties of the soil-structure interface under frozen conditions have been conducted. [PERSON] et al. [8] performed a shear test on an artificially frozen sand-structure interface during the freezing process, observing that the ice crystal content at the interface and the cementation of the interface increase with decreasing temperature. [PERSON] et al. [9] performed direct shear tests on a frozen soil-concrete interface and reported that the freezing strength increases slightly with decreasing temperature. [PERSON] et al. [10] investigated the development of shear strength, ice-cementation, and shear displacement/stress-shear of sand-concrete interfaces through a serious of frozen sand-concrete interface direct shear tests. [PERSON] et al. [11] investigated the freeze-thaw cycling impact on the shear behavior of a frozen soil-concrete interface by direct shear tests. [PERSON] et al. [12] presented an experimental study of the shear stress response, shear strength, and shear strength indexes of a soil-structure interface under frozen conditions by utilizing an improved roughness algorithm. [PERSON] et al. [13] analyzed the shear strength characteristics of the frozen soil-structure interface during the thawing process. [PERSON] et al. [14] conducted the orthogonal direct shear test considering the water content, freezing temperature, and freezing time. [PERSON] et al. [15] performed a series of the shear tests for concrete piles, steel piles, and wood piles embedded in the silty clay at different freezing temperatures by using the static pressed-pile method. [PERSON] et al. [16] performed direct shear tests of the contact surface between frozen silt and concrete under various temperatures and moisture contents by using a large stress-control shear apparatus. [PERSON] et al. [17] performed a series of laboratory direct shear tests to evaluate the shear characteristics of wood-soil, concrete-soil, and steel-soil interfaces. Most of the existing studies have focused on the strength mechanism of the frozen soil-structure interface at different temperatures, and the characteristics of the temperature and changes in the interface freezing strength have been obtained.