## 6 Numerical Modeling
### Stability Analyses  
Stability analyses were performed by using the Bishop limit equilibrium method in the code SLOPE/W [61]. Three different types of stability analyses were carried out: dry period; wet period; and rainfall infiltration analyses. Dry and wet period stability analyses were conducted by assigning average matric suctions and groundwater levels presented in Table 5 as the initial boundary condition to the model. Rainfall infiltration stability analyses were performed by generating new pore-pressure distributions developing in the soil profile after various possible rainfall intensities and durations by means of seepage analyses. The initial pore-pressure distributions before rainfall were formed regarding the average dry- and wet-period suctions and groundwater levels, given in Table 5, in the seepage analyses. The main purpose of these analyses was to observe the effects of the average pore-pressure distributions recorded in dry and wet periods for the two-year monitoring period in the field on the stability of the hillslope and to examine how the stability will be affected by various possible rainfall intensities and durations. Unsaturated shear strength was applied in the slope stability analyses to include the contribution of matric suction, as suggested by [PERSON] et al. [62]. \[\tau=c^{\prime}+(\ \sigma-u_{a})\ \tan\ \phi\ ^{\prime}+(u_{a}-u_{w})\ \tan\ \phi\ ^{ \prime}\bigg{[}\bigg{(}\frac{\Theta_{w}-\ \Theta_{r}}{\Theta_{s}-\Theta_{r}}\bigg{)}\bigg{]} \tag{2}\] \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Depth (m)**} & \multicolumn{2}{c}{**Dry Period**} & \multicolumn{2}{c}{**Wet Period**} \\ \cline{2-5}  & **Matric Suction (kPa)** & **Groundwater Level (m)** & **Matric Suction (kPa)** & **Groundwater Level (m)** \\ \hline
0.58 & 60 & & 0 & \\
1.10 & 60 & 3.49 & 0 & 2.21 \\
1.63 & 15 & & 0 & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Periodic matric suctions and groundwater levels used in numerical analyses. where \(\tau\) is the shear strength; \(c^{\prime}\) is the effective cohesion; \(\phi^{\prime}\) is the effective friction angle; \(\sigma\) is the total normal stress; \(u_{\rm a}\) is the pore air pressure; ( \(\sigma-u_{\rm a}\)) is the net normal stress; \(u_{\rm w}\) is the pore water pressure; (\(u_{\rm a}-u_{\rm w}\)) is the matric suction; \(\Theta_{\rm w}\) is the volumetric water content; \(\Theta_{\rm r}\) is the residual volumetric water content; \(\Theta_{\rm s}\) is the saturated volumetric water content. In addition to the effective shear strength parameters, SWCC, which shows the relationship between matric suction and water content, should be defined to calculate the shear strength. In this study, the average SWCC in Figure 6 was used as the soil water characteristic curve. The soil properties used in the stability analyses are summarised in Table 6.  
The strength of the rock mass was determined using the [PERSON] and Brown [63] failure criterion. The rock masses in the Hisar landslide area comprise intercalations of siltstones and claystones, with interbedded sandstones. The surface condition of their discontinuities is poor. The GSI value was considered to be an average of 30. The disturbance factor D was regarded as 0 since no treatment was applied to the rock mass. The cores obtained from drillings contained approximately 10% sandstone, 45% siltstone and 45% claystone. \(m_{\rm i}\) was calculated as 6.65 by taking the weighted average values introduced by [PERSON] [64], based on the rock type of the study area. Similarly, uniaxial compressive strength (\(\sigma_{\rm ci}\)) was computed as 9.81 MPa by taking the weighted average of laboratory uniaxial compressive test results. By using the determined Hoek-Brown parameters (GSI = 30, D = 0, \(m_{\rm i}\) = 6.65, \(\sigma_{\rm ci}\) = 9.81 MPa), the normal stress-shear stress relationship of the rock mass was obtained. This normal stress-shear stress function was used as the material model of the rock mass.