## 5 Conclusions  
Based on the field monitoring and 3D fully coupled flow-deformation numerical analyses, the appropriateness of the two deep drainage wells on the groundwater drawdown and slope stabilization are inspected. The slope stability is evaluated by the evolution of potential sliding area and the corresponding factor of safety using SRM. The two deep drainage wells with multi-level horizontal drains are effective to increase the slope stability and reduce the landslide potential. According to the numerical results, the following conclusions can be drawn:  
1. The effectiveness of the fully coupled flow-deformation numerical model for steady-state and transient groundwater flow calculations and slope stability analyses using _SRM_ is verified thru (a) the calibration of the initial groundwater depth of _B12-W_ monitoring point (\(-\)13.0 m from simulation and \(-\)12.97 m from observation); (b) the ground movement direction of the landslide (toward southeast from simulation and observation); (c) the depth of potential sliding surface (PSS) along _B-B'_ profile (\(-\)11.4 m for _B12-W_ and \(-\)35.6 m for _B09-W_ from simulation) and (\(-\)12.0 m for _B12-W_ and \(-\)36.0 m for _B09-W_ from observation).
2. In general, the design of the drain length \(L_{d}\) should pass through the PSS for an additional length and the transition length \(L_{tI}\) should be situated at the main part of the sliding body to achieve the maximum groundwater drawdown result within the PSS. However, in this study, the \(L_{tI}\) length locates far beyond the PSS which lowers the effectiveness of the groundwater drawdown on promoting the stability of the PSS.  
Figure 18: Location adjustment of drainage well (circular point=contact point of horizontal drain, adjusted well with hatched line=drainage well after location adjustment).  
3. _Drainage Well-1_ and _Drainage Well-2_ can be moved toward downslope by a horizontal distance of 24 and 10.5 m, respectively, to improve the drainage efficiency. After such an adjustment, the groundwater drawdown and slope stabilization becomes more effective on the PSS due to the fact the \(L_{t1}\) length is configured within the main sliding body. In addition, the groundwater level at the downslope of _Drainage Well-1_ can be effectively dropped by _Drainage Well-2_ because of the reduction of the distance between the two drainage wells.
4. Conclusively, the drainage well may exhibit higher drainage efficiency and groundwater drawdown rate if the corresponding horizontal drains are mainly placed at the middle-zone of PSS and extend across a large area of soil mass within PSS to allow for groundwater flow into the drains.
5. For the horizontal drains situated above the groundwater table, the subsurface drainage becomes ineffective to the groundwater drawdown. For both drainage wells, the bottom horizontal drains have the highest discharge rate. In addition, the groundwater drawdown rate (or drainage efficiency) of a subsurface drainage system can be promoted with an increasing hydraulic conductivity of the colluvium, however, it may turn into uneconomical for the colluvium with a lower permeability (\(K_{S}<1.0\times 10^{-5}\) cm/sec).
6. Eventually, it should be pointed out that the subsurface drainage remediation with high engineering cost remains necessary because of the unfeasibility of the relocation of the existing three residential communities. In addition, although the locations of the two drainage wells are not optimum, still it is considered to be appropriate because of the limitation of the land availability.  
[PERSON] developed the conceptualization, and wrote the manuscript; [PERSON] worked on the mathematical development, performed the numerical analysis; [PERSON] revised the manuscript; [PERSON] analyzed the data and the data curation. All authors have read and agreed to the published version of the manuscript.  
This research was supported by the Soil and Water Conservation Bureau, Taiwan, the Republic of China.  
The authors declare no conflict of interest.