## 3 Methodology
### Material Model Parameters  
A series of direct shear tests were carried out for soil and rock samples to determine the strength parameters. The soil samples collected from colluviums are classified as clayey or silt soils with low plasticity (_CL_, _ML_ and _CL\(\sim\)ML_). The residual cohesions \(c^{\prime}_{\text{res}}\) and frictional angles \(\varphi_{\text{res}}\)\({}^{\prime}\) are in a range of 15-35 kPa and 26\({}^{\circ}\)-30\({}^{\circ}\) respectively. In addition, the rock samples collected from the bedrock underlain the colluvium are classified as shale (_Sh_) and the residual strength parameters are \(c^{\prime}_{\text{res}}\)=0 and \(\varphi^{\prime}_{\text{res}}\)=23\({}^{\circ}\)-32\({}^{\circ}\) (uniaxial compressive strength \(q_{u}\)=3060 kPa). For sandstone (_Ss_), the strength parameters are \(c^{\prime}_{\text{peak}}\)=0 and \(\varphi^{\prime}_{\text{peak}}\)=32\({}^{\circ}\)\(\sim\)35\({}^{\circ}\) at peak state and \(c^{\prime}_{\text{res}}\)=0 and \(\varphi^{\prime}_{\text{res}}\)=25\({}^{\circ}\)-33\({}^{\circ}\) at residual state (uniaxial compressive strength \(q_{u}\)=4,000 kPa). The shear strength of the interface  
Figure 12: Dimension of numerical model and locations of protective residential areas (_RA1_, _RA2_), drainage well-1 & well-2, and \(A\)-\(A^{\prime}\), _C1-C1\({}^{\prime}\)_, _C2-C2\({}^{\prime}\)_ profiles for numerical calculation monitoring.  
between sandstone and shale was not tested in this study. The residual strengths of the colluvium and the underlain shale are used in the analysis to include the strength reduction from the large ground movement in the field. On the contrary, the sandstone was considered as the bedrock only undertaking slight shearing and assigned by peak shear strength. The linear-elastic perfectly-plastic model with Mohr-Coulomb failure criteria (Mohr-Coulomb model or \(M\)-C model) is used to simulate the mechanical behaviours of soil strata in You-Ye-Lin landslide. The input parameters of \(M\)-C model are summarized in Table 3. The linear isotropic material model parameters of corrugated steel ring liner used for numerical simulation are listed in Table 4.  
The saturated conductivity of the colluvium \(K_{S}\) were measured by in-situ borehole constant head permeability tests. \(K_{S}\) varies between \(2.08\times 10^{-5}\)-\(1.04\times 10^{-4}\) cm/sec for tests performed in a depth of 3.0-4.0 m. In addition, in-situ Lugeon tests (or Packer tests) were carried out to determine the saturated conductivity of intact sandstone \(K_{S\text{s}}\)=\(1.50\times 10^{-6}\) -\(3.14\times 10^{-5}\) cm/sec and for sandstone with intercalary strata (or inter-bed) of thin shale \(K_{S\text{s}/sl}\)=\(6.22\times 10^{-5}\)-\(1.06\times 10^{-4}\) cm/sec. In the You-Ye-Lin landslide, the conductivity of the colluvium is crucial to the design (location of drainage well, drain length and drain spacing) of the subsurface drainage because the drainage mainly occurs in the colluvium. In numerical analyses, three conductivities of the colluvium, \(K_{S}\)=\(1.0\times 10^{-3}\), \(1.0\times 10^{-4}\), and \(1.0\times 10^{-5}\) cm/sec are adopted to investigate their effects on the efficiency of the subsurface drainage whereas \(K_{S\text{s}}\)=\(1.0\times 10^{-4}\) cm/sec and \(K_{S\text{s}}\)=\(1.0\times 10^{-5}\) cm/sec are fixed.