## 5 Instrumental Monitoring  
A field instrumental monitoring program was conducted for a period of two years to investigate the seasonal behavior of the representative landslide area. Instrumentation consisted of volumetric water content sensors (EC-5), matric water potential sensors (MPS-6), groundwater level sensors (TD-Diver/BaroDiver), double-spoon tipping bucket rain gauge (ECRN-100) and data loggers (EM-60). Some detailed properties of each instrument are presented in Table 4. These instrumentations were installed in the area according to the \begin{table}
\begin{tabular}{c c c c c c} \hline  & \(\alpha\) & \(\mathbf{n}\) & \(\mathbf{m}\) & \(\Theta_{\mathbf{s}}\) (\%) & \(\mathbf{R^{2}}\) \\ \hline BH1-UD-(1.50–2.00) & 0.016 & 2.997 & 0.059 & 38.2 & 0.996 \\ BH1-UD-(3.00–3.50) & 0.015 & 3.0 & 0.141 & 39.4 & 0.996 \\ BH1-UD-(6.00–6.50) & 0.012 & 2.995 & 0.244 & 41.3 & 0.998 \\ BH2-UD-(1.50–2.00) & 0.012 & 3.0 & 0.217 & 45.2 & 0.998 \\ BH2-UD-(3.00–3.50) & 0.014 & 2.982 & 0.063 & 46.6 & 0.998 \\ BH3-UD-(1.50–2.00) & 0.001 & 0.844 & 1.370 & 37.6 & 0.995 \\ BH3-UD-(3.00–3.50) & 0.014 & 2.956 & 0.194 & 42.0 & 0.997 \\ BSK1-UD-(0.80) & 0.013 & 3.0 & 0.240 & 40.9 & 0.997 \\ BSK2-UD-(0.60) & 0.014 & 3.0 & 0.187 & 41.6 & 0.997 \\ BSK2-UD-(1.05) & 0.014 & 2.962 & 0.113 & 35.5 & 0.998 \\ BSK2-UD-(1.63) & 0.013 & 3.0 & 0.206 & 40.5 & 0.998 \\ Average SWCC & 0.015 & 3.045 & 0.134 & 40.8 & - \\ \hline \end{tabular}
\end{table} 
Table 2: Fitting parameters, \(\Theta_{\mathbf{s}}\) and coefficient of determination \(\left(\mathrm{R^{2}}\right)\). Figure 6: Van [PERSON] curve fittings to test points of soil water characteristic curves. \begin{table}
\begin{tabular}{c c c c} \hline
**Borehole No** & **Rock Type** & **Depth (m)** & **Natural Unit Weight** & **Uniaxial Compression Strength (MPa)** \\ \hline BH3 & Silstone/Claystone & 15.50 & 22.58 & 4.03 \\ BH2 & Sandstone & 12.50 & 25.40 & 57.97 \\ BH2 & Silstone/Claystone & 6.50 & 22.73 & 4.88 \\ \hline \end{tabular}
\end{table} 
Table 3: Lab test results carried out on rock core samples. plan in Figures 1c, 2 and 7. The data measurement interval of each instrument was set to 60 min. Two volumetric water content sensors and three matric suction sensors were installed behind the main scarp at depths of 1.10 m, 1.63 m, and 0.58 m, 1.10 m and 1.63 m, respectively, as shown in Figure 7a. Five separate boreholes of 100 mm diameter were drilled using a hand auger to install the sensors. The boreholes were drilled to depths of sensor installation. After the sensors were installed at the bottom of the boreholes, the boreholes were backfilled with the in-situ cut soil, and sensor cables were then connected to the data logger for taking values. Rainfall and its intensity were measured by a double-spoon tipping bucket rain gauge. The rain gauge was mounted by its bracket to an installation pole at 1.80 m height (Figure 7b). A datalogger was also placed onto the pole, and a sensor cable was connected to the datalogger for data processing. In order to monitor changes in groundwater level, TD-Diver and Baro-Diver submersible sensors were installed in piezometers in the boreholes of BH1 and BH3. The Divers measure the equivalent hydrostatic pressure of the water above the sensor to calculate the total water depth and record them in its internal memory. The absolute pressure is measured by the Divers. This indicates that in addition to measuring water pressure, the pressure sensor also monitors atmospheric pressure acting on the water's surface. The Baro-Diver only measures the atmospheric pressure, and the TD-Diver measures both the atmospheric pressure and the pressure exerted by the water column above the Diver. In this study, one Baro-Diver was installed above the water level in BH3 to measure the atmospheric pressure of the landslide area. The TD-Divers were installed below the water levels in BH1 and BH3. The Divers were read using a laptop and USB Reading Unit in the field (Figure 7c).