## 2 You-Ye-Lin landslide
### Drainage Well for Long-Term Remediation  
#### 2.5.1 Design of Drainage Well  
In Taiwan, the subsurface drainage is by far the most commonly used method for slope stabilization, either alone or in conjunction with other methods. Attempts have been made to provide a design procedure to optimize the number and spacing of horizontal drains [1; 2; 15]. The design length of the horizontal drain should not only extend to the sliding body but also penetrate through the potential sliding surface (PSS) for an additional 3-5 m [4; 5; 6; 16]. Due to lack of the consistence of design standards, in general, the design spacing of horizontal drains shows large variation within a range of 3-30 m from the literature [1; 17; 18; 19; 20]. [PERSON] and [PERSON] suggested that the drain spacing can be determined according to the in-situ drainage condition first and adjusted during the installation process [21].  
[PERSON] et al. indicated that natural slopes are rarely homogeneous enough to allow reliable subsurface drainage design according to simple principles of dewatering [22]. In addition, [PERSON] suggested that for a successful dewatering system, the designer must choose a drainage system layout that increases the probability of intersecting the major water-bearing stratum [23]. [PERSON] et al. (presented that the horizontal drain is most effective when it is installed at the bottom of a slope [7; 8]. It was also indicated in Cornforth that the horizontal drain drilled toward the upslope can maximize the intersection area with the PSS and can effectively lower the groundwater level [20]. Meanwhile, [PERSON] et al. indicated that a horizontal drain can function effectively if the horizontal drain can catch the groundwater flow before it seeps into the unstable area of the landslide [24]. [PERSON] pointed out that to design the location of drainage well rationally, it is necessary to grasp the PSS, the main veins of groundwater flow and their flow mode within the sliding body in advance [11; 12]. Conclusively, for the design of the subsurface drainage system in the You-Ye-Lin landslide, the locations and elevations of drainage wells accompanied with well-configured drainage boreholes (or horizontal drains) are crucial to the drainage efficiency.  
The drainage well (well shaft with drainage boreholes) in the You-Ye-Lin Landslide was designed to remove the groundwater in the deep soil strata followed the configuration in Figure 9. A large amount of groundwater can be drained out from the slope through drainage wells lowering the groundwater level. For You-Ye-Lin landslide, two drainage wells (_Drainage Well-1_ and _Drainage Well-2_) were designed and the construction was completed in 2018.  
The drainage wells have a diameter of 4.0 m and the well shaft was assembled by galvanized corrugated steel ring liners. The drainage wells penetrated into the colluvium to a depth of 20 m and 16 m respectively and are close to the PSS. Along the well shaft, an array of 5-6 uncased drainage boreholes (with a diameter of 76.2 mm and length of 50 m radiating out of the well shaft) were drilled with an upward elevation angle of 8\({}^{\circ}\)-10\({}^{\circ}\) at 4 different elevations of the well shaft (multi-level horizontal drains with vertical spacing of 4.0 and 5.0 m). The horizontal drains are arrayed in a  
Figure 8: Shallow subsurface drainage of the You-Ye-Lin landslide.  
fan-shape configuration with a 30\({}^{\circ}\) interval central angle and a horizontal spacing of 1.0-2.5 m. Table 2 summarizes the design parameters of the drainage wells.