## 2 Materials and Methods
### The Impact of Strata Orientation and Water Presence on Slope Stability  
The modelling process in FLACSlope, as described on the Itasca Consulting Group website, follows a structured approach. The procedure begins by defining the project, which includes configuring the project file, slope parameters, and model layout. After setting up the project, the model is built by defining the slope's layers, with the material properties inputted for each layer (see Figure 5 for an example of the built model). These details are crucial for simulating slope behaviour under varying conditions. In this analysis, the layers were adjusted based on the specific properties of the slopes, as outlined in Table 2. To ensure accurate results for the Factor of Safety (FoS) calculations, a fine-grid model was selected. The water table was raised incrementally, based on regional rainfall data, to model various hydrological conditions. While the water table values fluctuated, the material properties of the layers remained constant throughout the simulation. The model then produced key outputs, including FoS, strain, and pore pressure contours. For normal (sunny) conditions, a water table volume of 1000 m\({}^{3}\) was used, while rainy conditions were modelled with a water table of 2500 m\({}^{3}\) (as documented in Table 2, which includes the input parameters applied for the model). This allowed for a comparison of slope stability under different environmental scenarios. In terms of boundary conditions, Flacslope follows the upper bound theorem, which states that \"_Collapse must occur if, for any compatible plastic deformation, the rate of working of the external forces on the body equals or exceeds the rate of internal energy dissipation\"_ (after [PERSON] and [PERSON] [31]). The previous statement would mean that the compatible plastic deformation, which clearly indicates that any deformation that can satisfy all displacement boundary conditions, is possible kinematically based on the flow rule which governs admissible dilation.  
\begin{table}
\begin{tabular}{l l l l l} \hline \hline
**Parameters** & **Lower Layers** & \begin{tabular}{l} **Upper Layers** \\ **(Slope 1)** \\ \end{tabular} & \begin{tabular}{l} **Upper Layers** \\ **(Slope 2)** \\ \end{tabular} & \begin{tabular}{l} **Upper Layers** \\ **(Slope 3)** \\ \end{tabular} \\ \hline \begin{tabular}{l} Unsaturated density \\ (kg/m\({}^{3}\)) \\ \end{tabular} & 1900 & 1600 & 1400 & 1300 \\ \hline \begin{tabular}{l} Saturated density \\ (kg/m\({}^{3}\)) \\ \end{tabular} & 2200 & 1900 & 1700 & 1600 \\ \hline \begin{tabular}{l} Porosity \\ \end{tabular} & 0.2 & 0.3 & 0.5 & 0.4 \\ \hline \begin{tabular}{l} Cohesion (Pa) \\ \end{tabular} & 10,000 & 5000 & 8000 & 6000 \\ \hline \
\begin{tabular}{l} Frictional angle (\({}^{\circ}\)) \\ \end{tabular} & 30 & 20 & 25 & 27 \\ \hline
\begin{tabular}{l} Particle density (g/cm\({}^{3}\)) \\ \end{tabular} & 2.8 & 2.65 & 2.63 & 2.66 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Material properties of the layers used in the model.  
Figure 5: Example of built model with strata orientation and groundwater level.  
In terms of the type of mesh used, the study used fine mesh to ensure the accuracy of the results; there are options for coarse and medium mesh, but these were not appropriate for this study. Following that, various rock mass parameters were used in order to perform the simulation; Mohr-Coulomb was used. The Mohr-Coulomb parameters used include the Geological Strength Index (GSI), Mi, D, and Sigci, which are Hoek-Brown rock mass properties estimation charts. Therefore, based on the geological mapping and measurements, the rock mass properties were estimated from the Hoek-Brown criteria. The Hoek-Brown criteria are available within the model; hence, the selection of material properties can be made easily.