## 4 Slope Stability Evaluation During the Water Compoundment Process  
The slope stability simulation results under the current condition and water levels of \(-50\) m, \(0\) m, and \(+50\) m are shown in Figure 7. Due to the northern slope being mainly composed of green mudstone and brown shale, which have relatively low mechanical strength parameters, its stability remains lower than that of the southern slope throughout the entire water impoundment process. As shown in Figure 7a, the minimum stability coefficient is \(1.15\), located in the central-eastern section of the northern slope, below \(-150\) m elevation. In regions above \(-50\) m, the slope stability coefficients are all greater than \(1.3\). This indicates that under current conditions, the slope is generally in a relatively stable state. It also suggests that prior to water impoundment, the existing slope has not been significantly affected by pore water pressure, resulting in higher effective stress and stronger resistance to sliding. At the water level of \(-50\) m (Figure 7b), the stability coefficients of the northern slope from the middle-western to the eastern end of the open-pit mine, below the pit bottom and the pit lake water level, are all below \(1\), indicating a high likelihood of landslides. The western part of the northern slope above the water level, as well as the southern slope, remains generally stable. On the eastern northern slope, the stability coefficient gradually increases with elevation; however, some areas still have stability coefficients below \(1.1\), highlighting potential landslide risks. These unstable zones are confined to the shallow surface layers of the slope and have not yet affected the ground surface. This indicates that during the impoundment process, increasing pore water pressure reduces the effective stress, thereby weakening the slope's shear resistance. At the water level of \(0\) m (Figure 7c), the stability coefficients of the northern slope below the water table remain below \(1\), while the unstable zones above the water level have significantly expanded. A marginally stable zone with a stability coefficient below \(1.1\) appears in the eastern part of the southern slope. A large-scale instability area emerges from the eastern part of the northern slope to the eastern end wall, where the stability coefficients above the water table are generally less than \(1\). A wide surface area with stability coefficients between \(1.1\) and \(1.2\) is observed, extending up to approximately \(200\) m from the pit edge. These results indicate that as the water level continues to rise, pore water pressure increases further, leading to a significant reduction in effective stress and a marked decline in the slope's shear resistance. Moreover, the instability in the submerged zones \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Lithology** & **Density** & **Elastic** & **Poisson’s** & **Cohesion** & **Tensile** & **Friction** & **Permeability** \\
**[g/cm\({}^{3}\)]** & **Modulus [GPa]** & **Ratio** & **[MPa]** & **Strength [MPa]** & **Angle [\({}^{\circ}\)]** & **Coefficient [m/s]** \\ \hline Backfill Material & 1.8 & 0.8 & 0.3 & 0.3 & 0.1 & 30 & \(3\times 10^{-2}\) \\ Green Mudstone & 2.25 & 2 & 0.24 & 0.7 & 0.25 & 28 & \(2.1\times 10^{-7}\) \\ Brown Shale & 2.2 & 1.2 & 0.3 & 1 & 0.3 & 25 & \(9\times 10^{-7}\) \\ Coal & 1.5 & 1.2 & 0.24 & 1.4 & 0.6 & 40 & \(1.04\times 10^{-6}\) \\ Oil Shale & 2.1 & 3.4 & 0.26 & 1 & 0.4 & 40 & \(3.47\times 10^{-6}\) \\ Sandstone & 2.3 & 4 & 0.25 & 2 & 1 & 45 & \(8.68\times 10^{-7}\) \\ Tuff & 2.0 & 0.4 & 0.2 & 9.5 & 3 & 25 & \(9\times 10^{-8}\) \\ Basalt & 2.8 & 7 & 0.14 & 10.3 & 5 & 45 & \(5\times 10^{-7}\) \\ Granite & 2.6 & 8 & 0.2 & 10.3 & 5 & 45 & \(7.6\times 10^{-7}\) \\ Fault Zones (F1, F1A) & 2.0 & 2 & 0.3 & 0.7 & 0.3 & 20 & \(1.18\times 10^{-6}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The mechanical parameters for numerical calculations.