## 7 Results and Discussion
### Rainfall Infiltration Stability Analyses  
In order to obtain possible stability conditions in the field, considering various possible rainfall intensities (1 mm/h, 2.5 mm/h, 5 mm/h, 10 mm/h, 20 mm/h, 40 mm/h, 80 mm/h, 160 mm/h), durations and hydraulic conductivity variations (1.76 \(\times\) 10\({}^{-6}\) m/s, 1.76 \(\times\) 10\({}^{-7}\) m/s, 1.76 \(\times\) 10\({}^{-8}\) m/s) for the field, many numerical analyses were made and changes in pore pressure distribution were obtained. Stability analyses were then conducted with the calculated pore pressures and variations in the factor of safety were investigated. Pore pressure distributions and changes in the factor of safety after 2 h and 0.3833 h with a rainfall intensity of 20 mm/h for dry- and wet-period conditions are presented as an example for a saturated hydraulic conductivity of 1.76 \(\times\) 10\({}^{-6}\) m/s in Figures 16 and 17. As observed in Figure 16a, at the beginning of the dry period, the groundwater level was at a depth of 3.49 m from the ground surface. Negative pore water pressures had a minimum value of \(-\)60 kPa. According to the analysis, it can be observed that there were some negative pore water pressures (matric suctions) in certain parts of the critical slip surface of the slope for the initial dry period conditions. In this case, the factor of safety was calculated as 1.74. As a result of the rainfall lasting for 2 h, it is observed that the groundwater level rose to the ground surface due to inflow into the ground and the matric suctions disappeared completely (Figure 16b). The FoS decreased to 1.02 as the groundwater level rose to the ground surface. Figure 16: _Cont._  
Figure 16: Factor of safety and pore pressure distributions. (**a**) Dry period initial conditions, (**b**) the situation that develops after 20 mm/h rainfall intensity lasting 2 h.  
Figure 17: Factor of safety and pore pressure distributions. (**a**) Wet period initial conditions, (**b**) the situation that develops after 20 mm/h rainfall intensity lasting 0.3833 h.  
On the other hand, as observed in Figure 17a, the groundwater level was at a depth of 2.21 m at initial wet-period conditions. There were higher positive pore water pressures on the critical slip surface. The effect of matric suctions was not observed. As a result of rainfall infiltration performed at the same rainfall intensity (20 mm/h), the factor of safety decreased below the limit equilibrium state (FS = 0.99) after a shorter duration (0.3833 h) (Figure 17b). As rainfall infiltrated the unsaturated soil, it caused an increase in water content and soil saturation, while leading to a decrease in matric suction. Instability can occur when the matric suctions at the critical slip surface of the slope decrease or disappear or when positive pore water pressures develop [70; 71]. Results obtained herein are in agreement with studies such as References [6; 72; 73] that failure is based on the development of positive pore pressure. Variation in the FoS for various probable rainfall intensities and saturated hydraulic conductivities was examined separately according to the dry and wet periods (Figures 18 and 19). As observed in the figures, the FoS decreased as the rainfall continued and eventually reached its lowest value. For a specific rainfall intensity, higher hydraulic conductivity enabled the lowest factor of safety to be achieved in a shorter duration, as compared to the lower hydraulic conductivity. From the analyses made for dry period conditions, it is observed that the variations in the factor of safety with rainfall intensities are quite close to each other in the case where the saturated hydraulic conductivity is the lowest (Figure 18a). As the saturated hydraulic conductivity increased, scatterings in the factor of safety curves increased, depending on the rainfall intensities (Figure 18b,c).