## 4 Results and Discussion  
### Groundwater Drawdown  
Three-dimensional finite element analyses are performed to investigate the groundwater drawdown due to subsurface drainage of horizontal drains. The variation of groundwater drawdown with elapsed times along the \(A\)-\(A^{\prime}\) profile (refer to Figures 4 and 12) with two drainage wells for \(K\)s=\(1.0\times 10^{-4}\) cm/sec case is illustrated in Figure 13a. The \(A\)-\(A^{\prime}\) profile approximates along the middle of the fan-shape array horizontal drains. As shown in the figure, for both two drainage wells, the final groundwater level (at Day 20) after drawdown descends to the elevation of the bottom horizontal drains (4 th level in Table 2). After 20 days of the subsurface drainage, the groundwater level drops from the initial groundwater level at the end of horizontal drains of _Drainage Well-1_ (50 m away from _Drainage Well-1_) to the elevation of 4 th level horizontal drains at approximately 32 m (=\(L_{t1}\) in Figure 13a) from the end of the horizontal drains. The length \(L_{t1}\) is defined as the transition length of effective groundwater drawdown (or transition length) of _Drainage Well-1_. Meanwhile, the corresponding maximum groundwater drawdown is equal to 10.19 m (=\(L_{t\text{s}/\text{max}}\)). These calculated \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Material** & **Thickness** & **Unit weight** & **Poisson’s Ratio** & **Young’s Modulus** \\
**Type** & \(t\) **(m)** & \(\gamma_{\text{s}}\) **(kN/m3)** & \(\gamma_{\text{s}}\) & \(E_{\text{s}}\) **(GPa)** \\ \hline Steel liner & \(3.2\times 10^{-3}\) & 77.0 & 0.30 & 200.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Input of linear isotropic material model parameters of corrugated steel ring liner for slope stability analysis. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Soil Type** & **Yunsat (Ysat)** & **Young’s Modulus** & **Poisson’s Ratio’** & **Cohesion** & **Friction Angle** & **Dilatancy Angle** \\
**(kN/m3)** & \(E^{\prime}\) **(kN/m2)** & **Ratio’** & \(c^{\prime}\) **(kPa)** & \(\phi^{\prime}\) **(\({}^{\circ}\))** & \(\psi\) **(\({}^{\circ}\))** \\ \hline \multirow{2}{*}{Colluvium} & 20.89 & \multirow{2}{*}{\(2.0\times 10^{5}\)} & \multirow{2}{*}{0.30} & \multirow{2}{*}{10.0} & \multirow{2}{*}{28} & \multirow{2}{*}{0} \\  & (23.37) & & & & & \\ \hline \multirow{2}{*}{Shale} & 21.88 & \multirow{2}{*}{\(7.0\times 10^{5}\)} & \multirow{2}{*}{0.35} & \multirow{2}{*}{39.0} & \multirow{2}{*}{29} & \multirow{2}{*}{0} \\  & (23.82) & & & & & \\ \hline \multirow{2}{*}{Sandstone} & 23.44 & \multirow{2}{*}{\(7.0\times 10^{6}\)} & \multirow{2}{*}{0.30} & \multirow{2}{*}{0.0} & \multirow{2}{*}{35} & \multirow{2}{*}{0} \\  & (23.84) & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Input of M-C material model parameters of soil strata for slope stability analysis. drawdown behaviours of groundwater levels are identical with those observed in engineering practice presented by [PERSON] and [PERSON] [27]. Theoretically, the design length of the horizontal drains \(L_{4}\) (=50 m in this study) should be extended beyond the potential sliding surface (PSS) to achieve the groundwater drawdown within the PSS. As shown in Figure 13a, the transition length \(L_{t1}\) of _Drainage Well-1_ is located far beyond the PSS and the drawdown area (=the area between original and drawdown groundwater level) is only partially located inside the PSS. Meanwhile, the groundwater level at the downslope of _Drainage Well-1_ tends to recover gradually to the original level and only a slight drawdown of the groundwater level achieved by _Drainage Well-2_ after 10 days of the subsurface drainage. Moreover, in Figure 13a, a length of \(L_{t12}\) for _Drainage Well-2_ is defined as the drain length at the transition zone of effective groundwater drawdown (or drain length of transition zone) of _Drainage Well-2_.