## 3 Laboratory and DEM Modelling Establishment  
### Sample Preparation  
The fundamental materials for laboratory experiments were sourced from a location adjacent to the BDRM, ensuring that the stratigraphic lithology matched that of the BDRM, with the native material being limestone, as illustrated in Figure 2a. Through meticulous procedures of cutting, grinding, and other preparatory steps, the rock samples were fashioned into standard cylindrical specimens with precise geometric dimensions of 100 mm in height and 50 mm in diameter, as depicted in Figure 2b. These prepared samples were then subjected to a series of uniaxial compression tests to ascertain the fundamental mechanical properties of the rock, including, but not limited to, elastic modulus and compressive strength, as outlined in Figure 2c. The macroscopic mechanical parameters of the rock, as detailed in Table 1, served as a crucial foundation for all subsequent analyses and simulations.  
Combining geological data with on-site survey results (Figure 3a), it can be observed that the shear band and the bedrock exhibit the characteristics of layered rock masses. The interface is stepped, and the stability is maintained by the interlocking of more regular rock slabs within the shear band and the recessed parts in the bedrock. Based on the  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Limestone** & \begin{tabular}{c} **Density** \\ **(kg/m\({}^{3}\))** \\ \end{tabular} & \begin{tabular}{c} **Elastic Modulus** \\ **(GPa)** \\ \end{tabular} & \
\begin{tabular}{c} **Uniaxial Compressive** \\ **Strength (MPa)** \\ \end{tabular} &
\begin{tabular}{c} **Poisson’s** \\ **Ratio** \\ \end{tabular} \\ \hline Value & 2580 & 13.99 & 131.74 & 0.24 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Basic physical and mechanical parameters of limestone.  
Figure 2: Field sampling and parameter measurement: (**a**) Field sampling; (**b**) Sample processing; (**c**) Parametric measurement.  
characteristics of the layered rock mass, considering the size limitations of the servo direct shear testing machine, the precision of the experimental processing, and the proportion of the on-site block size within the shear band, the shear band was generalised into rock slabs of uniform thickness but varying heights, and the bedrock was a complete rock block with protruding steps. The step height was set at 10 mm, and the width at 22 mm. The rock slabs and the bedrock accurately formed a cube with dimensions of 200 mm. The generalised model and dimensions of the entire specimen are shown in Figure 3a. It is worth noting that regular stratified rock masses are generally isotropic in the horizontal plane; thus, the three-dimensional stratified rock mechanics problem can be treated as a planar problem perpendicular to the stratification. This is the reason why the interface roughness is considered a simple two-dimensional geometric problem. However, there are certain limitations to this kind of model generalisation, as the step size at the interface actually has some spatial variability, making it difficult to assess the mechanical properties of the interface. Despite this limitation, in this study, the mechanical shear behaviour of the interface with a stepped shape was the focus, and the non-uniform step size within the same specimen was not considered a harsh initial condition. This was essentially a preliminary exploratory study, hence the step size being unified. This model generalisation method can be applied to shear zones formed by the lateral unloading of layered slopes in steep gorge areas due to river incision or open-pit mining excavation. This is because the fracturing of layered rock masses will inevitably lead to the formation of interlayer fractures, resulting in this stepped interlocking structure.