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.