1 INTRODUCTION
Frozen soil is a complex geo-material composed of solid particles, liquid water, gas inclusions and ice crystals. Currently, the widely accepted definition of frozen soil is soil and rock that have a temperature equal to or below 0℃ and contain ice. The area of frozen soil in the world accounts for about 25% of the total land area of the earth1-2. The physical and mechanical properties of frozen soil are closely related to changes in temperature, moisture, and loading. Under different conditions, complex physical processes occur within the soil mass, resulting in changes such as water and heat movement, phase transition, and stress redistribution, which further lead to frost heave and thaw settlement deformation of the soil3-8. Therefore, when conducting engineering construction in frozen soil regions, it is necessary to conduct a thorough analysis of the changes in the mechanical properties of frozen soil during construction and usage, as well as the patterns of interaction between frozen soil and structures. Concrete is currently one of the main materials used in building structures, and its application is indispensable in large-scale infrastructure projects such as high-speed railways, canal slopes, high-rise buildings, and transmission pile foundations constructed in frozen soil regions. Due to the differences in the physical and mechanical properties of soil and concrete materials, the soil-concrete interface has become one of the key factors that affect the stability of structures9-14. Especially in frozen soil regions, the reciprocating changes in water, heat, and force conditions alter the physical and mechanical properties of the soil mass, and the frost heave and thaw settlement deformations of the soil can significantly affect the stress state and stability of the interface between the soil and structures. In severe cases, this can even lead to instability and failure of engineering structures. Therefore, the study of the interaction mechanism between frozen soil and concrete interfaces has received widespread attention.
The study of soil-concrete interface shear characteristics is one of the important topics in the field of geotechnical engineering. Conducting a thorough investigation into the shear mechanical behavior of the interface, as well as the evolution mechanism and influencing factors of the shear zone, holds significant theoretical and practical guiding significance for addressing practical engineering problems such as foundation settlement and slope failure15-17. Direct shear test, as an economical and efficient experimental method for studying interface shear behavior, can conveniently obtain the stress-displacement curve relationship, shear strength, and strength parameters of the interface. It has been widely used in the study of soil-concrete interface shear characteristics18-20. A series of direct shear test studies have shown that factors such as the grading of soil particles, the roughness of the structural surface, environmental conditions, and boundary conditions can all influence the shear mechanical behavior of the interface21-23. Under negative temperature conditions, the dynamic changes between ice and water, as well as the formation of cemented ice particles within the soil at the interface, complicate the interfacial shear mechanism. The interfacial shear strength not only comprises the cohesion between soil particles and the friction between soil and concrete, but also includes the cementing force generated by the cemented ice. Liu et al. by combining the methods of nuclear magnetic resonance (NMR) layered testing, direct shear testing, and fractal theory analysis, the degradation mechanism of the soil-rock mixture-concrete interface strength in cold regions under freeze-thaw cycles is revealed24-27. Sun et al carried out a series of direct shear tests on frozen soil-concrete interfaces to investigate the characteristics and formation mechanisms of the freezing strength at the frozen soil-concrete interface. By decomposing the peak strength, the formation mechanism of the freezing strength at the frozen soil-concrete interface was explained28. Wang et al. conducted a series of indoor direct shear tests on silty clay-concrete binary system under freeze-thaw cycles. Based on the indoor test results, they combined macroscopic and microscopic observations to obtain the changing patterns of interfacial shear strength, shear strength parameters, and shear strength damage degree29-30. Wan et al. explored the relationship between shear stress and displacement at the contact surface as well as the variation of shear strength through a combination of indoor experiments and mathematical modeling. They revealed the mechanical properties of the contact interface between saline frozen soil and concrete under the influence of multiple factors. Furthermore, through grey correlation analysis, they determined the significance ranking of various influencing factors31. Xie et al. conducted dynamic shear tests on the interface between frozen clay and concrete pile foundation under different influencing factors using a temperature-controlled dynamic direct shear system32.
The aforementioned studies have conducted an analysis of the interfacial strength characteristics, providing a thorough elucidation of the features and formation mechanisms of interfacial freezing strength. For structures such as pile foundations buried underground at a certain depth, the normal boundary condition at the interface between the soil and the structure is a condition of constant normal stiffness33. Based on the characteristic changes in the volume of soil surrounding the structure during the shearing process (shear dilation or shear contraction), the restraining effect of the surrounding soil on the structure can be simplified as a series of springs with certain stiffness, which is referred to as the constant normal stiffness boundary condition34. In tests with constant normal stiffness, the effect of normal stiffness on the interfacial shear strength depends on the volume response of the interface during the test. Research indicates that under shear dilation conditions, an increase in normal stress during tests with constant normal stiffness leads to an increase in shear stress. On the other hand, the change in shear stress under shear contraction conditions is opposite. Some studies have shown that under conditions of constant normal stiffness, the friction angle remains unchanged during the variation of interfacial shear strength35-37.
However, for structures such as pile foundations in frozen soil regions, research on the shear characteristics of the soil-structure interface under constant normal stiffness conditions is still not systematic. Therefore, this paper utilizes a two-stage temperature-controlled direct shear test to investigate the effects of different normal stiffnesses, temperatures, and moisture contents on the shear characteristics of the frozen soil-concrete interface. It analyzes the variation patterns of stress-displacement curves, shear strength, cohesion, and internal friction angle under different conditions. The research results can provide scientific references for the design, operation, and maintenance of structures such as pile foundations in frozen soil engineering.