## 1 Introduction  
The You-Ye-Lin landslide, a large scale landslide in a mountainous area of Taiwan, has a long history of intermittent large downslope ground movements during rainfall periods since Tyhoon Morakot in 2008. To prevent the expansion and deterioration of the landslide, systematic field investigations, engineering designs, and emergency countermeasures for the slope stabilization have been implemented by public agencies and remain underway to date. The main remediation work for the You-Ye-Lin landslide is to lower the groundwater level through a deep subsurface drainage system composed of two drainage wells (a well shaft equipped with multi-level horizontal drains).  
For subsurface drainage, the effectiveness of horizontal drains on lowering the groundwater level and improving the slope stability is closely related to the drainage length, spacing, number, and installation location [1; 2; 3; 4; 5]. The function and the effect of horizontal drains on the groundwater drawdown and the slope stabilization mechanism have also been investigated by several researchers using numerical methods [6; 7; 8; 9; 10; 11; 12]. [PERSON] et al. concluded that lengthening a group of horizontal drains is more effective than shortening the spacing and increasing the number of a group of horizontal drains [6]. [PERSON] et al. found that the main benefit of using horizontal drains is lowering the groundwater table and the drains located at the bottom of the slope contribute the most [7; 8; 9]. [PERSON] et al. strongly suggested that a deep drainage system (a drainage gallery with sub-vertical drains) is the key measure to successfully stabilize large landslides [10]. Furthermore, the potential effect of sub-vertical drains drilled from drainage gallerys on slope stabilization has also been studied in large landslidesusing 3D numerical models [11; 12]. [PERSON] et al. demonstrated that a subsurface drainage system in a large landslide is capable of accelerating the drainage of the infiltrated rainwater induced from a high intensity and long duration rainfall during the typhoon season and protects the slope from a fast deterioration [13; 14].  
However, the parallel configuration of horizontal drains in previous studies was shallow and simple compared to the configuration of horizontal drains in the You-Ye-Lin landslide where the horizontal drains are installed on the drainage wells in a multi-level and fan-shape configuration. In particular, although a large number of drainage wells have been constructed to improve the stability of large scale landslides in Taiwan, none of their effects on the groundwater drawdown and the slope stabilization mechanism have ever been cautiously inspected up-to-date. In this study, the efficiency of the subsurface drainage system installed in the You-Ye-Lin landslide is evaluated. Using monitoring data and numerical techniques, this study aims at proposing a practical and operative three-dimensional (3D) evaluation method to quantify the drainage efficiency of horizontal drains in response to the torrential rainfall during the typhoon season. In this method, the groundwater level variation evaluated using the complicated rainfall induced seepage analysis in the unsaturated soil is replaced by the monitoring groundwater level data.