## 4 Results and Discussion
### Considerations to the Location Adjustment of Drainage Wells  
As shown in Figure 18, the circular points represent the contact point of the horizontal drain. According to the calculated groundwater drawdown, it is obvious that both drainage wells (_Drainage Well-1_ and _Drainage Well-2_) can be moved down simultaneously to improve the drainage efficiency of horizontal drains within the main sliding body or within the potential sliding surface (PSS). Considering  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{5}{c}{_Drainage Well-1_} \\ \hline \multirow{3}{*}{\
\begin{tabular}{c} **Drainage** \\ **Duration** \\ **\(t\) (day)** \\ \end{tabular} } & \multicolumn{3}{c}{**Discharge Rate of Horizontal Drains at Four Elevations**} & \multicolumn{2}{c}{**Total Discharge Rate of**} \\  & & **Qi (m3/day)** & & **Drainage Well-1** \\  & & **(\(i\) = \(w\)1\(d\)1\(-\)\(w\)1\(d\)4)** & & **Qw1 (m3/day)** \\ \cline{2-6}  & \(w\)1\(d\)1 & \(w\)1\(d\)2 & \(w\)1\(d\)3 & \(w\)1\(d\)4 & **=\(\Sigma\)Qi** \\ \hline \(1\) & 0.648 & 5.808 & 59.016 & 163.104 & 228.576 \\ \hline \(2\) & 0.456 & 3.864 & 44.904 & 144.072 & 193.296 \\ \hline \(3\) & 0.456 & 2.664 & 37.128 & 129.576 & 169.824 \\ \hline \(4\) & 0.456 & 2.160 & 32.616 & 117.120 & 152.352 \\ \hline \(5\) & 0.480 & 1.896 & 29.832 & 106.728 & 138.936 \\ \hline \(10\) & 0.624 & 1.032 & 21.312 & 78.648 & 101.616 \\ \hline \(20\) & 0.048 & 0.480 & 16.008 & 58.848 & 75.384 \\ \hline \multicolumn{6}{c}{_Drainage Well-2_} \\ \hline \multirow{3}{*}{
\begin{tabular}{c} **Drainage** \\ **Duration** \\ **\(t\) (day)** \\ \end{tabular} } & \multicolumn{3}{c}{**Discharge Rate of Horizontal Drains at Four Elevations**} & \multicolumn{2}{c}{**Total Discharge Rate of**} \\  & & **Qi (m3/day)** & & **Drainage Well-2** \\  & & **(\(i\) = \(w\)2\(d\)1\(-\)\(w\)2\(d\)4)** & & **Qw2 (m3/day)** \\ \cline{2-6}  & \(w\)2\(d\)1** & \(w\)2\(d\)2 & \(w\)2\(d\)3 & \(w\)2\(d\)4 & **=\(\Sigma\)Qi** \\ \hline \(1\) & 7.848 & 21.696 & 101.760 & 236.664 & 367.968 \\ \hline \(2\) & 2.808 & 14.592 & 76.704 & 214.440 & 308.544 \\ \hline \(3\) & 2.424 & 10.368 & 60.432 & 195.696 & 268.920 \\ \hline \(4\) & 2.544 & 8.208 & 51.456 & 182.112 & 244.320 \\ \hline \(5\) & 1.488 & 6.984 & 45.816 & 171.552 & 225.840 \\ \hline \(10\) & 0.384 & 3.408 & 32.832 & 140.736 & 177.360 \\ \hline \(20\) & 0.240 & 1.296 & 24.264 & 119.112 & 144.912 \\ \hline \hline \end{tabular}
\end{table}
Table 7: Discharge rate of horizontal drains of _Ks_=\(1.0\times 10^{-4}\) cm/sec case.  
Figure 17: Discharge rate of horizontal drains installed at different elevations of well shaft during 20-day subsurface drainage in typhoon season. (**a**) Drainage Well-1 (**b**) Drainage Well-2 (_Ks_=\(1.0\times 10^{-4}\) cm/sec).  
(1) the effective zone of groundwater drawdown with a range of 24 m at the upslope of _Drainage Well-1_ (2) the ineffective zone of groundwater drawdown with a range of 13.5 m at the downslope of _Drainage Well-1_, _Drainage Well-1_ and _Drainage Well-2_ can be moved a horizontal distance of 24 m and 10.5 m (=24 m-13.5 m) respectively to _Adjusted Well-1_ and _Adjusted Well-2_ as illustrated in Figure 18.  
If _Drainage Well-1_ were substituted by _Adjusted Well-1_, the effective zone of groundwater drawdown at the upslope will be situated within the main sliding body of the landslide and compensates the ineffective zone of groundwater drawdown at the downslope. On the other hand, if _Drainage Well-2_ were substituted by _Adjusted Well-2_, the effective zone of groundwater drawdown at the upslope can shelter more unstable area within the main sliding body of the landslide. As a consequence, the overall drainage efficiency of the subsurface drainage system in the landslide can be greatly promoted.