## 4 Results and Discussion  
### Groundwater Inflow into Underground Powerhouse  
The underground powerhouse consists of three major caverns like the main and auxiliary powerhouse, main transformed cavern, and tailrace gate chamber. The geometry of these underground caverns is simplified as a circle on the vertical section for the application of analytical methods. Then, the selected parameters and calculated results of analytical methods are shown in Tables 4 and 5, respectively. It is worth noting that in the big well method, the unconfined well formula is selected to calculate the water inflow, and \(t\) is assumed to be twenty days, referring to [PERSON] et al. [44]. Obviously, the water inflow rates calculated using the Karlsrud method [10] are the largest, while the big well method [12] leads to the smallest values. Compared to the analytical solutions considering the drainage effect, the values obtained using the analytical methods assuming a stable groundwater level are apparently larger. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Model**} & \multicolumn{4}{c}{**Formations**} & \multicolumn{4}{c}{**Faults**} & \multirow{2}{*}{
\begin{tabular}{c} **Waterproof** \\ **Curtain** \\ \end{tabular} } \\ \cline{2-2} \cline{4-10}  & **CW Rock** & & & & & & & & & & \\ \hline Permeability \(K\) (m/d) & 4.97 \(\times\) 10\({}^{-2}\) & 1.42 \(\times\) 10\({}^{-2}\) & 8.36 \(\times\) 10\({}^{-3}\) & 7.41 \(\times\) 10\({}^{-3}\) & 1.0 & 1.0 & 0.1 & 0.1 & 0.1 & 8.64 \(\times\) 10\({}^{-5}\) \\ Thickness \(d\) (m) & 0–17.3 & 6.5–58.8 & 22.5–109.9 & 156.4–668.9 & 0.7 & 1.0 & 0.5 & 0.5 & 4.0 & 1.0 \\ Specific yield \(\mu\) & 0.1 & 0.02 & 0.01 & 0.001 & 0.05 & 0.05 & 0.01 & 0.01 & 0.05 & 0.0001 \\ \hline \hline \end{tabular} Note: CW = completely weathered, SW = strong weathered, WW = weakly weathered, LW = lightly weathered.  
\end{table} 
Table 2: Model parameters used in the numerical study. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multicolumn{2}{c}{**Water Head (m)**} & \multicolumn{2}{c}{**Seepage Control Structures**} & \multirow{2}{*}{**Working Conditions**} \\ \cline{2-2} \cline{4-6}  & **Left Boundary** & **Right Boundary** & & & & & \\ \hline Case 1 & 745 & 210 & None & None & None & Excavation condition \\ Case 2 & 745 & 255 & Effective & Effective & Effective & Operation condition \\ Case 3 & 745 & 255 & Failure & Failure & Effective & Risk condition \\ Case 4 & 745 & 255 & Failure & Failure & Failure & Risk condition \\ \hline \hline \end{tabular} Note: “Failure” means that the anti-seepage function of structures has failed in risk conditions, whereas “Effective” means that the anti-seepage function of structures remains normal in operation conditions.  
\end{table} 
Table 3: Summary of the model design for four synthetic cases. The SGR coefficients of three caverns were determined by taking account joint frequency and aperture, crashed zone width, and groundwater level, as listed in Table 6. Due to the absence of sparsification phenomena in the study site, \(S_{4}\) and \(S_{5}\) are assumed to be zero. Additionally, \(S_{2}\) is ranged from 1 to 5 regarding the degree of schistosity. \(S_{3}\) related to crashed zones is scored by the fault distributions, and \(S_{7}\) is assumed as unit because of the cavern excavation in saturated zone. Overall, the underground powerhouse is in \"_Low Risk_\" class with the SGR coefficient of 292.86. Considering the low permeable rocks, it can be claimed that the low risk in excavation is subject to the middle crash zones caused by faults (F\({}_{318}\) and F\({}_{317}\)) and joints. Table 7 lists the numerical results of four synthetic cases in various working conditions. Clearly, the value of water inflow rate is approximately 1230 m\({}^{3}\)/d in Case 1. Because of the function of drainage holes and tunnels, the water inflow rate of underground powerhouse in operation condition is significantly reduced to 104.47 m\({}^{3}\)/d.