## 5 Case Study
### Identification of Slope Slip Pathways  
Since the main body of the earth core rockfill dam in the case study is constructed with fresh rockfill, which has good drainage performance, rapid drawdown of the water level has a minor impact on the dam slope stability. The earthquakes are inactive near the dam site, and thus seismic loading conditions are not critical for the dam slope stability. Because the dam operates most of the service time under normal reservoir water level conditions for power generation, this working condition is adopted for illustrating the proposed method. The dam construction and reservoir impoundment processes are simulated, and seepage calculation is performed to obtain the seepage force and uplift force acting on the units. These forces are then applied as external loads on the corresponding nodes for stress analysis, resulting in the stress distribution of the dam under normal reservoir water level steady-seepage conditions, which serves as the initial stress state for strength reduction. In this study, the FEM-SRM method is utilized to identify the critical slip surfaces in the dam slope. The FEM-SRM method has been widely applied to reveal the evolution of slope failure and the critical slip modes. By gradually reducing the shear strength parameters of the dam body materials and foundation cover layer, the main failure zones of the dam slope are identified. When the strength reduction factor K = 1, both the dam body and foundation materials remain within the elastic range, with only a few localized failure elements appearing in the weaker overburden layer. At K = 1.2, the failure zone within the foundation overburden expands and extends in both the upstream and downstream directions. Scattered failure elements also begin to emerge at the interface between the upstream core wall and the filter layer. When K = 1.4, additional failure elements appear in the overburden, as well as at the upstream core-filter interface. Meanwhile, failures begin to occur near the contact surface between the main and secondary rockfill zones in the downstream shell. At K = 1.6, failure elements continue to increase in the foundation overburden and upstream core-filter interface. In the downstream shell, the failure zone further propagates along the interface between the main and secondary rockfill zones, and failure begins to appear in the upstream shell. When K = 1.8, significant failure is observed in both upstream and downstream overburden layers. Damage intensities near the slope surface of the upstream shell, with internal failure elements becoming nearly continuous. In the downstream shell, a potential slip pathway emerges along the contact between the main and secondary rockfill zones and within the foundation overburden layer. When the strength reduction factor K = 2.0, the elastic-plastic finite element iteration does not converge, and the dam and foundation lose their bearing capacity. The final failure state of the dam body and foundation is shown in Figure 2. The shaded part is the position of the element where shear failure occurs, when the elastic-plastic finite element iterative calculation does not converge and there are three main slip channels, as shown in Figure 2.  
Figure 2: Three slip zones of the dam slope and corresponding reliability indices. From the figure, it can be seen that the dam body and foundation are severely damaged.