## 4 Results and Discussion
### Groundwater Drawdown
Based on the definitions of \(L_{t1}\) and \(L_{t12}\), it can be seen that the \(L_{t1}\) approximates the \(L_{t1}\) for _Drainage Well-1_ (\(L_{t1}\)= \(L_{t11}\)=32 m) whereas the \(L_{t2}\) (=24 m) interrupted by the groundwater drawdown at the downslope of _Drainage Well-1_ is not coincident with \(L_{t12}\) (=17 m) for _Drainage Well-2_. Further on, the transition length \(L_{t1}\) or \(L_{t2}\) is dependent on the elevation difference between the initial groundwater table and the top drains, which is coincident with those presented by [PERSON] and [PERSON] (1984). Figure 13: For \(K\)s=1.0 \(\times\) 10\({}^{-4}\) cm/sec case (**a**) variation of groundwater level along \(A\)-\(A^{\prime}\) profile with the subsurface drainage, (**b**) groundwater level and groundwater head contour of \(C\)1-\(C\)1’ profile after a 20-day subsurface drainage and (**c**) before the subsurface drainage. According to the field condition, the 1 st level horizontal drains of _Drainage Well-1_ are entirely situated above the initial groundwater table under dry condition whereas the 1 st level horizontal drains of _Drainage Well-2_ are fully submerged prior to the function of subsurface drainage. As listed in Table 5, the maximum groundwater drawdowns of _Drainage Well-1_ (\(h_{\text{w1,max}}\)) and _Drainage Well-2_ (\(h_{\text{w2,max}}\)) are 10.19 and 13.00 m, respectively, after 20 days of the subsurface drainage. Nevertheless, it should be noted that the groundwater flow remains at transient state after a 20-day drainage (\(t\) = 0\(\rightarrow\)20 days) and the drawdown is continued with the increasing time till the steady-state (\(t\) = 0\(\rightarrow\)\(\infty\) days) is reached. The drawdown rate and drainage efficiency of _Drainage Well-2_ is higher than those of _Drainage Well-1_ due to the fact that the horizontal drains of _Drainage Well-2_ are mainly placed at the middle-zone of PSS and extended across a large range of soil mass within PSS allowing more groundwater flowing into the drains. Figure 13b displays the transverse variation of groundwater level and colourful groundwater head (or total pore pressure head=steady-state pore pressures head+excess pore pressures head) contour along the _C1-C1\({}^{\prime}\)_ profile (Figure 12) which is perpendicular to the _A-A\({}^{\prime}\)_ profile and adjacent to residential area _RA-1_. In contrast, Figure 13c illustrates the groundwater level before the subsurface drainage starts. It can be found that after 20 days of the subsurface drainage, the groundwater level is dropped to the elevation of bottom drains. The groundwater head at the middle of the groundwater table along the _C1-C1\({}^{\prime}\)_ profile approximate 81.0 m (light yellow-green colour) and 71.0 m (light grey-blue colour) before and after the subsurface drainage and the difference of 10 m is very close to the groundwater drawdown of 10.19 m in Table 5.  
Figure 14 presents the calculated groundwater depth (\(H_{w}\)) of observation well _B12-W_ (monitoring point _B12-W_ in Figure 4) for a 20-day subsurface drainage in the You-Ye-Lin landslide. In Figure 14, the initial groundwater depth (\(-\)13.0 m for \(t\) = 0) of _B12-W_ without the subsurface drainage is determined by a steady-state groundwater flow calculation under constant head boundary conditions assigned to represent the rainfall effect during Typhoon Megi. The initial groundwater depth of simulation (\(-\)13.0 m for \(t\) = 0 in Figure 14) is in excellent agreement with that of observation (\(-\)12.97 m in Table 1) and the validity of numerical model for hydraulic calculation can be verified. Subsequently, the groundwater depth of _B12-W_ without the drainage remediation is monitored for 20 days (\(t\) = 0\(\rightarrow\)20 days) and compared with those with the subsurface drainage with different hydraulic conductivities of the colluvium (\(K_{S}\) =1.0 \(\times\) 10\({}^{-5}\), 1.0 \(\times\) 10\({}^{-4}\), and 1.0 \(\times\) 10\({}^{-3}\) cm/sec).