## 4 Results Analysis  
### Failure Mode and Curve Characteristics  
Figure 6a,b display the experimental and numerical shear stress-shear displacement and normal displacement-shear displacement curves for the \"L\" and \"S\" samples, respectively. The results from both the laboratory and simulation tests exhibit a strong correlation, indicating good agreement between the two sets of data. The shear stress curves reveal several distinct stages in the shear behaviour. The initial stage, marked by contact gap closure and micro-crack compaction, is unique to the experimental shear stress curves. The second stage corresponds to elastic behaviour, followed by the pre-peak elastic-plastic stage in the third stage. A noticeable stress drop is observed after reaching the peak (first peak) stress point, which is the fourth stage. In the post-peak stage, the shear stress initially increases slightly, and then gradually decreases, ultimately reaching a residual value. Notably, for samples \"L-4\" and \"S-4\", the post-peak shear stress value surpasses the first peak value, indicating high residual strength. This phenomenon can be explained by the heightened frictional force caused by the increased normal stress. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Scheme Name** & **Description** & **Scheme Name** & **Description** \\ \hline \multirow{4}{*}{L-1} & normal stress 1 MPa & & normal stress 2 MPa \\  & model size 200 mm & & model size 200 mm \\  & step height 10 mm & & step height 10 mm \\  & step width 22 mm & & step width 22 mm \\ \hline \multirow{4}{*}{L-4} & normal stress 4 MPa & & normal stress 1 MPa \\  & model size 200 mm & & model size 200 mm \\  & step height 10 mm & & step height 10 mm \\  & step width 22 mm & & step width 22 mm \\ \hline \multirow{4}{*}{S-2} & normal stress 2 MPa & & normal stress 4 MPa \\  & model size 200 mm & & model size 200 mm \\  & step height 10 mm & & step height 10 mm \\  & step width 22 mm & & step width 22 mm \\ \hline \multirow{2}{*}{SS-100} & model size 100 mm & \multirow{2}{*}{SS-300} & model size 300 mm \\  & normal stress 1, 2, 4 MPa & & normal stress 1, 2, 4 MPa \\ \hline \multirow{2}{*}{SS-350} & model size 350 mm & \multirow{2}{*}{SS-400} & model size 400 mm \\  & normal stress 1, 2, 4 MPa & & normal stress 1, 2, 4 MPa \\ \hline \multirow{2}{*}{SH-3} & Step height 3 mm & \multirow{2}{*}{SH-5} & Step height 5 mm \\  & normal stress 2 MPa & & normal stress 2 MPa \\ \hline \multirow{2}{*}{SH-7} & Step height 7 mm & \multirow{2}{*}{SH-13} & Step height 13 mm \\  & normal stress 2 MPa & & normal stress 2 MPa \\ \hline \multirow{2}{*}{SH-15} & Step height 15 mm & \multirow{2}{*}{SW-16} & Step width 16 mm \\  & normal stress 2 MPa & & normal stress 2 MPa \\ \hline \multirow{2}{*}{SW-18} & Step width 18 mm & \multirow{2}{*}{SW-20} & Step width 20 mm \\  & normal stress 2 MPa & & normal stress 2 MPa \\ \hline \multirow{2}{*}{SW-24} & Step width 24 mm & \multirow{2}{*}{SW-26} & Step width 26 mm \\  & normal stress 2 MPa & & normal stress 2 MPa \\ \hline \multirow{2}{*}{SW-28} & Step width 28 mm & \multirow{2}{*}{SW-29} & \multirow{2}{*}{SW-27} \\  & normal stress 2 MPa & & normal stress 2 MPa \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Test scheme designs in this paper. Figure 6: _Cont._  
The normal displacement-shear displacement curves highlight the samples' dilatation effects. The experimental curves typically exhibit no growth during the compaction stage, stable growth during the failure stage, and gradual growth as they approach the residual value. Due to limitations in particle radius and the accuracy of stress servo, the residual part of the simulation curves exhibits some fluctuation, with the normal displacement value in the residual region being marginally larger than that observed in the experimental results. Nevertheless, the overall trend of the simulation curves closely mirrors that of the experimental curves. Figure 6c,d illustrate the failure modes observed in the tests.