## 3 Methodology
### Numerical Model with Boundary and Initial Conditions  
According to the design parameters of drainage wells (see Table 2), topographic, geological and hydrological data from long-term field surveys and tests of soil/rock samples taken from boreholes at the study area, the 3D numerical model of the You-Ye-Lin landslide is established and shown in Figure 10(a). The soil strata are modeled by 10-node tetrahedral soil elements (or 10-node volumetric soil element, Plaxis 3D [26]) with four integration points and the elements have three degrees of freedom per node (or three displacement components per node) and provide a second-order interpolation of displacements (Figure 10(b)). The drainage well consists of the well shaft and multi-level horizontal drains. The well shaft is assembled by a continuous galvanized corrugated steel ring liner and  
Figure 10: Typical construction sequences of well shaft and multi-level horizontal drains in Taiwan.  
is simulated by 6-node triangular plate elements (or 6-node structural area elements) with three integration points provide a second-order interpolation of displacements (Figure 11c). As the plate elements cannot sustain torsional moments, the plate elements have only five degrees of freedom per node (three translational and two rotational degrees of freedom). Plate elements are structural elements used to model thin two-dimensional shaft structure in ground with a significant flexural rigidity (bending stiffness) and based on [PERSON]'s plate theory [26] which allows for plate deflections due to shearing as well as bending. Structural forces are evaluated at the plate element integration points and extrapolated to the element nodes. At last, the horizontal drains in the numerical model are simulated by a series of 3-node line drain elements with hydraulic condition or called line drains (Figure 11c), which is compatible with the side of a 6-node triangle plate elements and a 10-node tetrahedral soil elements since these elements also have three nodes on a side. 3-node line drain elements provide a second-order (quadratic) interpolation of groundwater discharge rate for horizontal drains.  
Ideally, the model boundaries are placed along natural hydrologic boundaries such as flow divides (or watershed boundaries), water bodies (creeks), and impermeable bedrock. The numerical model shown in Figure 12 places the model domain at the watershed boundary to reduce the amount of elements needed to define the model. Locations, thicknesses and hydraulic properties of each soil stratum are required for a groundwater flow calculation model. The initial groundwater level for the landslide is determined and set up by a steady-state groundwater flow calculation which is calibrated with the measured groundwater levels of observation well _B08-W_, _B09-W_, _B10-W_, and _B12-W_ (Figure 4). For a steady-state groundwater flow calculation and fully coupled flow-deformation  
Figure 11: Numerical model of You-Ye-Lin landslide with subsurface drainage system (**a**) peel-off view inside the numerical model (**b**) finite element mesh (**c**) well shaft with horizontal drains.  
analysis, except the bottom boundary (x-y plane) which is specified as a close boundary (zero flux boundary), all the surrounding boundaries (x-z and y-z planes) are defined as constant head open boundaries. In addition, as shown in Figure 12, the \(A\)-\(A^{\prime}\) profile along which two drainage wells installed for subsurface drainage is selected to monitor the calculation results.