## 4 Laboratory Tests  
Sieve analysis-hydrometer [46], liquid limit-plastic limit [47], falling head permeability [48], consolidated undrained (CU) triaxial [49], pressure plate soil water characteristic curve (SWCC) tests [50] were carried out in the laboratory to determine geotechnical prop  
Figure 2: Hisar landslide area ground profile A–A section. erties of soil samples taken from the field. Uniaxial compression tests [51] were performed on rock core samples. When the grain size distributions are examined in Figure 3, it can be observed that they presented scattering. Grain size ranges: 2.3-34.0% for the gravel; 8.4-23.8% for sand; 20.9-38.4% for silt; and 18.4-54.7% for clay. The ranges of Atterberg limits are given in Table 1. The distribution of the soils in the plasticity chart is presented in Figure 4. As shown in Table 1 and Figure 4, most soils were classified as CL and distributed close to the A line in the plasticity chart. The saturated hydraulic conductivity coefficients of the soils (k\({}_{\text{sat}}\)) were determined in the laboratory using the falling head permeability method in a rigid-walled permeability test setup on the undisturbed soil samples taken directly from landslide areas with permeability. Saturated hydraulic conductivity coefficients of the soils were calculated as an average of 1.76 \(\times\) 10\({}^{-7}\) m/s. The effective friction angle (\(\sigma^{\prime}\)) and effective cohesion (\(c^{\prime}\)) \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Borehole No** & **Depth (m)** & **Liquid Limit (\%)** & **Plastic Limit (\%)** & **Plasticity Index** & **Soil Classification (USCS)** \\ \hline BH1 & 1.50–6.50 & 36–42 & 22–25 & 13–18 & CL \\ BH2 & 1.50–3.95 & 33–41 & 19–26 & 13–15 & CL, ML \\ BH3 & 1.50–3.95 & 40–54 & 24–29 & 15–30 & CL, CH, ML \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Atterberg limits and soil classes of Hisar landslide area. Figure 4: Representation of the soils in plasticity chart. Figure 3: Grain size distributions of the soils. of the soils were obtained from the consolidated undrained (CU) triaxial tests. Since the undisturbed soil samples taken from the Shelby tubes contained the largest particle size, bigger than 1/6 of the specimen diameter, it was not possible to prepare a sufficient number of undisturbed test samples. Test specimens were prepared by compacting at in-situ dry density and in-situ water content. The graphs obtained as a result of the consolidated undrained (CU) triaxial tests are presented in Figure 5. The effective friction angle and the effective cohesion were 21\({}^{\circ}\) and 8 kPa, respectively. SWCC, which provides the relationship between matric suction and water content, is an important parameter in the behavior of unsaturated soils. Studies show that there is a close relationship between the soil water characteristic curve and the engineering properties of unsaturated soils [32; 52]. Depending on the precipitation and evaporation cycle, various changes occur in the water content, pore pressure, permeability, and shear strength in unsaturated soils. Previous studies showed that these changes can be calculated based on the parameters obtained from the shape of the SWCC. SWCC tests were performed using a pressure plate, according to ASTM D6836 [50], for drying. The tests were carried out on undisturbed soil specimens. The following formula proposed by [PERSON] [53] was used to plot the best-fit curve of the SWCC, which provides the relationship between matric suction and volumetric water content. \[\Theta\ =\ \Theta_{\mathrm{r}}+\frac{(\Theta_{\mathrm{s}}\ -\ \Theta_{\mathrm{r}})}{[1+( \alpha(\mathrm{u}_{\mathrm{a}}-\mathrm{u}_{\mathrm{w}})^{n})^{\mathrm{m}}} \tag{1}\]  
where \(\Theta\) is the volumetric water content; \(\Theta_{\mathrm{r}}\) is the residual volumetric water content; \(\Theta_{\mathrm{s}}\) is the saturated volumetric water content; \(\alpha\), n and m are the fitting parameters; \((\mathrm{u}_{\mathrm{a}}-\mathrm{u}_{\mathrm{w}})\) is the matric suction.