## 4 Results and Discussions
### Determination of Shear Strength of Landslide Materials  
Shear strengths obtained from direct shear tests and ring shear tests can be useful to determine slope stability. The intact undisturbed soil sample taken from the landslide source area is applied for the direct shear test, but a reconstituted soil sample is applied for the ring shear test. For an infinite slope failure analysis, the shear resistance (i.e., combination of cohesion and internal friction angle) can be determined using a direct shear test. Figure 9 presents the determination of cohesion and internal friction angle of landslide materials in shallow landslides. The shear stress obtained from the in situ undisturbed soil condition (i.e., unsaturated condition) is approximately 23 kPa. This value may decrease with an increase in the degree of saturation. For example, the shear stress can reach much less than 10 kPa when the soil samples are fully saturated. Consequently, the cohesion will be very small (\(\sim\)0 kPa), and the internal friction angle is similar to the angle of repose, such as that observed for coarse-grained sediments (Figure 9a). The solid line is the experimental result, and the dashed line is the estimate. In slope stability, the factor of safety (FOS), the ratio of shear resistance (\(\tau_{\text{f}}\)) to the driving force (\(\tau_{\text{d}}\)), is crucial to estimate whether the slope is stable. For an infinite slope failure assumption (Figure 9b), the factor of safety is simply determined using the ratio of the internal friction angle to the slope angle for the drained condition (i.e., dry condition and c = 0 kPa); otherwise, the factor of safety is half of the factor of safety obtained from the undrained condition (i.e., wet condition and c = 0 kPa). The FOS is in the range 1.2-2.2 for c = 0 kPa when the drained condition is considered for a slope angle of \(\alpha\) = 20-35\({}^{\circ}\). For the same slopes, FOS is in the range 0.6-1.1 for c = 0 kPa when wet and seepage conditions are considered. In reality, the slope is usually in an unsaturated state. They have a specific value of cohesion and internal friction angle. According to [PERSON] et al. (2018), the factor of safety can be determined by considering the cohesion, internal friction angle, and suction stress function. Occasionally, the matrix suction and volumetric water content cannot be easily measured under in situ conditions. In this case, the residual shear stress may be one of the good alternatives. The residual shear strength is defined as the shear stress mobilized along the pre-existing shear surface, and the soil particles are rearranged parallel to the slip surface (Figure 9c). Therefore, the slope can be easily reactivated along the slip surface. It can be naturally solidified over time. Therefore, the slope returns to the unsaturated cemented  
Figure 8: Shear stress as a function of shearing speed: (**a**) drained and (**b**) undrained conditions. condition, which has strong bonding between the particles just before reaching the failure stage. These results can be repeated again depending on the rainfall event, until the slope completely fails. As shown in Figure 8, the residual shear stresses of landslide materials are in the range 10-40 kPa for all shear speeds under the normal stress of 50 kPa in the drained and undrained conditions. The residual shear stress at a relatively low shear speed (i.e., 0.1 mm/s) is in the range 10-20 kPa for both drainage conditions. It should be noted that the degree of saturation and shear speed used in the direct shear test are different from those in the ring shear tests. However, it can be comparable with the shear resistance obtained from the direct shear tests. A shear speed of 0.1 mm/min (i.e., 0.002 mm/s) is often applied for sandy soils and is employed in the direct shear test. Actually, they differ by two orders of magnitude in terms of the shear speed applied.