## 7 Results and Discussion
### Rainfall Infiltration Stability Analyses
On the other hand, as shown in the wet period analyses, all the factor of safety curves overlapped for the same hydraulic conductivity values, so the changes in the factor of safety with duration were not affected by the rainfall intensities for a specific conductivity (Figure 19a-c). It is possible to explain these changes in the factor of safety with the relationship among initial conditions-drainage-infiltration. In the dry-period initial conditions, the soil was unsaturated above the groundwater level and had matric suctions reaching 60 kPa. Since the soil was not saturated, the hydraulic conductivities varied with the suction. In this case, according to the HCF defined for the saturated hydraulic conductivity value of 1.76 \(\times\) 10\({}^{-8}\) m/s (Figure 18a), it was relatively difficult for the rain to infiltrate the ground. Changes in pore water pressure took a long time, accordingly. There was no remarkable increase in the amount of water infiltrating the ground with the variation in rainfall intensity. Excess water that cannot infiltrate is drained by passing into runoff. Therefore, in the case of Figure 18a, where the hydraulic conductivity was lower, the factor of safety varied with a time overlap, and it took a long time to reach the lowest safety factor due to the slow progress of infiltration. For the other two cases with higher hydraulic conductivity coefficients (Figure 18b,c), infiltration occurred more easily and in shorter durations. Changes in pore water pressure were faster than those of lower hydraulic conductivity. As the rainfall intensity increased, the factor of safety curves converged. For wet-period initial conditions, the effects of matric suctions were not observed because the soil was saturated. Since there was no variation between hydraulic conductivity and matric suction, all curves overlapped and were not affected by rainfall intensities. The pore water pressure and, thus, the stability depended on the saturated hydraulic conductivities. When Figures 18 and 19 are compared, it is observed that the wet-period initial conditions reduced the factor of safety below the limit-equilibrium condition more quickly for the same rainfall intensity and saturated permeability values compared to dry-period initial conditions, thus creating a more critical situation before failure. In this study, the relationship between the rainfall intensity and duration that can cause landslides in the region was established using the graphs in Figures 18 and 19. Numerous numerical analyses were carried out considering the in-situ measurement parameters. From these graphs, rainfall intensity-duration thresholds were obtained by determining the rainfall intensities and durations which create the factor of safety, 1.50 and 1.02 (the lowest factor of safety reached in the numerical analyses) for the dry period conditions and 1.00 (limit equilibrium state) for the wet period conditions. The rainfall intensity-duration thresholds calculated for the dry and wet periods are presented in Figures 20 and 21, respectively. Figure 19: Variations in the factor of safety over time for wet-period initial conditions. (a) \(\mathrm{k_{s}}=1.76\times 10^{-8}\) m/s, (b) \(\mathrm{k_{s}}=1.76\times 10^{-7}\) m/s, (c) \(\mathrm{k_{s}}=1.76\times 10^{-6}\) m/s. Intensity-duration (I-D) thresholds are frequently used in the literature. Inspection of the literature reveals that empirical I-D thresholds have a general power function as \(I=\alpha\times\mathrm{D}^{\beta}\) (where I is the rainfall intensity; D is the rainfall duration; \(\alpha\) and \(\beta\) are parameters). The suggested I-D thresholds encompassed a significant variety of rainfall durations and intensities. However, the majority of the thresholds included durations ranging from 1 to 100 h and intensities ranging from 1 to 200 mm/h. The value of \(\beta\) ranged from \(-\)0.19 to \(-\)2.00, and \(\alpha\) ranged from 4.00 to 176.40, as reported in the literature [14].