## 3 Shear Strength Evolution of Structural Plane Infill Specimens
### Evolution of Mechanical Parameters of Infilled Fractured Rock Masses Under Freeze-Thaw Action
#### 4.1.2 Determination of Representative Elementary Volume (REV) Under Triaxial Compression Conditions  
To investigate the mechanical response of fractured rock masses with infilled joints under freeze-thaw conditions, a series of triaxial compression numerical simulations was performed on models with side lengths increasing from 1 m to 9 m under a confining pressure of 1 MPa. The rock material parameters were selected with reference to [PERSON] et al. [30] and are presented in Table 2. This approach aimed to evaluate the influence of model size on mechanical behavior.  
Figure 6: Triaxial compression numerical model of a 1 \(\times\) 1 m jointed rock mass: (**a**) geometric model; (**b**) mesh discretization.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Freeze-Thaw Cycles (\(n\))** & **Corresponding** & **Compressing** & **Tensile Strength** & **Rock Elastic** & **Rock Mass Elastic** & **Internal Friction** & **Cohesion (kPa)** \\
**Codes (\(n\))** & **Depth (\(m\))** & **Strength (MPa)** & **(MPa)** & **Modulus (GPa)** & **Modulus (MPa)** & **Angle (\({}^{\circ}\))** & **Cubesion (kPa)** \\ \hline
0 & \(\backslash\) & 98.44 & 7.06 & 17.84 & 2.96 & 4  
As shown in Figure 7, the peak compressive strength of the rock mass exhibits a decreasing trend with increasing model size. This phenomenon is attributed to the limited number of joints in smaller models, which results in more intact rock and consequently higher strength. As the model size increases, the number of joints grows, enhancing structural complexity and intensifying shear weakening effects, thereby reducing overall strength. A comparative analysis of strength variation across different sizes reveals that, when the model side length reaches 9 m, the peak strength stabilizes, with fluctuations controlled within 10%. Based on this convergence criterion, the 9 \(\times\) 9 m model is identified as the minimum representative elementary volume (REV) in this study. At this scale, the joint distribution is sufficiently representative, and the mechanical response of the rock mass achieves statistical stability, effectively capturing the average macroscopic mechanical properties. This determination provides both geometric and computational justification for simulating the evolution of mechanical parameters in subsequent analyses.  
The 9 \(\times\) 9 m representative elementary volume (REV) adopted in this study is determined based on the stabilization of peak compressive strength under a confining pressure of 1 MPa and is considered appropriate for representing the average mechanical response scale of the simulated slope (the peak compressive strengths under different angles and grid densities are shown in Appendix A). However, REV is inherently influenced by various factors such as loading mode, fracture scale, and lithological heterogeneity. Under shear-dominated or fracture-penetration-controlled failure mechanisms, the REV may require redefinition. Future studies may evaluate its general applicability and boundary conditions through multi-scale numerical simulations or discrete element modeling.