# Practical Approach for Assessing Wetting-Induced Slope Failure
## 3 Materials and Methods  
In order to study the rainfall-induced slope failure at Cimanggung, conventional site investigations were carried out, while unsaturated soil parameters were estimated. For the conventional site investigations, nine cone penetration tests (CPT) and three boreholes (BH) along with the standard penetration test (SPT) and undisturbed soil samplings were conducted. Piezometer was also installed at a 15 m depth. According to the site inspection, soils located around the slope consisted of clayey silt and gravels. Soils at the site were residual soils from Gunungapi Muda Rock which consist of tuff, lapilli, breccia, lava, and agglomerate. Ground-water levels were located approximately 20.5 m near the slope locations. Location of the boreholes, piezometer, and CPT are shown in Figure 5 and are summarized in Table 1, while laboratory test data are summarized in Table 2. Figure 6 shows the soil stratification based on the borehole data. In the simulation, three soil layers were modelled. The upper layer was clayey silt where the sliding plane was located. All samples for laboratory tests were located in this layer, and it is shown that the soil properties were quite variable. However, it is important to note that the sliding plane was located at the first layer and hence, more emphasize was given to the first layer. Three triaxial consolidated undrained (TX-CU) tests were conducted on the three samples obtained from this layer. The range of effective cohesion was from 10 to 12 kPa, while the friction angle (\(\phi^{\prime}\)) ranged from 35\({}^{\circ}\) to 37\({}^{\circ}\). The value of \(\phi\) appeared to be relatively high. One of the possible reasons was the soil, which is above the sliding plane, might have been col  
Figure 4: Waterflow and overtopping at the crown of the slope. Figure 3: Household drainage. have had lower \(\phi\). In the simulation, \(\phi\) was back analysis such that the slip surface from the simulation and the actual slip surface matched, while \(\phi^{b}\) was always taken as \(1/2\phi\). The middle layer was very stiff clayey silt where only standard penetration test (SPT) N values were available. The SPT N value ranged from 25 to 32. \(S_{u}\) = 6 N is commonly adopted in Indonesia. By averaging the SPT N value, the total stress analysis with undrained shear strength equaled 170 kPa, which was adopted for the middle layer. Total stress analysis for the second layer was sufficient as the sliding plane was located at the first layer. Table 3 summarizes the parameters adopted in the simulation. \begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**BH**} & \multicolumn{3}{c}{**Coordinate: UTM 48 M**} & \multirow{2}{*}{**Elevation (m)**} & \multirow{2}{*}{**Depth (m)**} & \multirow{2}{*}{**GWL**} \\ \cline{2-2} \cline{5-6}  & **Easting** & & & & **Northing** \\ \hline BH-1 & 811,380 & 9,230,325 & 732.29 & 18 & * \\ \hline BH-2 & 811,427 & 9,230,270 & 728.95 & 24 & 20.5 \\ \hline BH-3 & 8  
Mapping of the slope contour after the landslide was conducted by using an unmanned aerial vehicle (UAV) or drone, as shown in Figure 7. The post-landslide slope contour through the photogrammetry method is shown in Figure 7. Figure 8 also shows the position of the first landslide, second landslide, and third landslide. The post-landslide slope contour could then be compared with the original slope contour in order to study the landslide-triggering mechanism. Based on the site investigations and the contours, the geometry of the slopes and soil stratifications are shown in Figure 9. Rainfall infiltration was modelled by using the flux boundary while overtopping was modelled by applying the flux boundary. The post-landslide slope contour is shown in Figure 9. The post-landslide slope contour is shown in Figure 9.  
pressure head equal to 0 at the location where the overtopping was located, as shown in Figure 9a. Figure 9b shows the hydraulic boundary when there was no overtopping where only the flux boundary was applied. Figure 9c shows the post-failure slope geometry.