## 1 Introduction  
Freeze-thaw (_FT_) cycles are recognized as one of the primary natural factors contributing to slope instability in open-pit rock slopes [1]. Investigating the damage characteristics of rock masses in _FT_ regions is crucial for understanding _FT_-related geohazards in both alpine and seasonally frozen areas. Natural rock masses, shaped by long-term tectonic activity, commonly contain structural planes such as faults, joints, and infill layers. Among these, structural planes play a key role in geohazards like landslides, with the stability often controlled by the properties of the infill materials within them. Consequently, many researchers have conducted shear creep tests and developed corresponding models of structural planes, yielding substantial insights. [PERSON] et al. [2] conducted direct shear tests on weak interlayer rock joints with varying filling ratios and moisture contents under multi-level normal stresses. The results showed that the peak shear strength of the weak interlayer joints first increased and then decreased with increasing water content, while it initially decreased and then stabilized with increasing filling ratio. [PERSON] et al. [3] investigated the influence of infilling condition between weak layers on the shear strength and failure mode of rock masses through direct shear tests and predicted the effect of infill thickness on jointed rock mass shear strength by analyzing changes in apparent cohesion and internal friction angle. [PERSON] [4] studied the shear behavior of natural rock joints filled with clay, sand, and sandy clay, and proposed a new model to predict the shear strength of filled joints. [PERSON] and [PERSON] [5] examined the shear strength under forward and reverse shearing of artificially prepared rough joints with varying surface roughness and infill characteristics under constant normal load. [PERSON] et al. [6] carried out direct shear tests under different saturation levels to quantitatively assess the effect of saturation on the shear behavior of rock joints, finding that both peak and residual shear strengths decreased with increasing joint saturation. [PERSON] et al. [7] employed the Fast Lagrangian Analysis of Continua (FLAC) with two-phase flow modeling to conduct a series of constant-moisture direct shear tests on unsaturated filled joints, emphasizing the changes in effective stress and average permeability of the infill layer induced by shearing. [PERSON] et al. [8] categorized rock mass failure modes based on the influence of filling layers and summarized the key factors affecting rock mass strength, including joint strength and toughness, interface angle, interlayer effects, and dynamic loads. [PERSON] and [PERSON] [9] introduced a new parameter, the filling ratio, defined as the thickness of the weak layer (t) to the amplitude of joint surface undulation (a). [PERSON] et al. [10] found that even a thin layer of infill material can significantly reduce the shear strength of structural planes. Their results also indicated that when the filling ratio (t/a) was between 1.4 and 1.8, roughness had a marked impact on shear strength, whereas beyond this critical ratio, the shear behavior was governed solely by the infill material. Once the infill material controlled the joint shearing behavior, the displacement corresponding to the peak shear stress decreased significantly. [PERSON] et al. [11] investigated the strength and failure modes of rocks with filling interlayers, analyzing their impact on slope failure, and revealed the influences of interlayer angle and confining pressure on the strength and failure modes of strongly weathered mudstone, clay interlayers, and weak interbedded mudstone. [PERSON] et al. [12], [PERSON] et al. [13], and [PERSON] et al. [14] studied the deformation and failure characteristics of slopes from aspects including shear behavior, failure mechanisms, creep properties of filling layers, and the deformation law of dual weak interlayers, providing theoretical support for slope stability and reinforcement analyses.