# Practical Approach for Assessing Wetting-Induced Slope Failure
## 4 Results and Discussion  
Figure 16 shows the slope failure contour after 4.5 h of rainfall with a flux of 144 mm/day with overtopping while Figure 17 shows the change in safety factor with time under different flux. The analysis shows that rainfall with flux of 144 mm/day will cause the slope to be under critical condition when the duration is approximately 4 h to 5 h. When the flux is 100 mm/day, 50 mm/day, and 30 mm/day, the duration required to reach SF equal to one will be longer. The slope contour matched with the post-failure contour. The triggering mechanism is indicated to be due to the decrease in soil suction and also due to an increase in the ground-water table. It is interesting to note that the phreatic line indicated by the blue dashed line raised up to the top of the slope. This phenomenon is not according to the site condition where the first crown was located somewhere below the drainage pipe.  
Figure 16: Slope failure after 4.5 h with a flux of 144 mm/day with overtopping.  
Figure 15: Inclinometer reading IN-03.  
Figure 18 shows the slip surface contour when the flux is 144 mm/day without considering broken drainage. It is shown that, without broken drainage, the phreatic line does not reach the top of the slope but reaches below the drainage pipe. The most critical slip surface appears to be more consistent with site observation where the first landslide occurs below the drainage pipe. Based on this analysis, the triggering mechanism for the first landslide is only the heavy rainfall and not the overtopping. The first landslide, induces progressive failure (second landslide) and damage to the drainage pipe which then causes overtopping. The red zone (zone with safety factor range from 0.981 to 1.081) appears to be quite consistent with the actual slip surface from the observation and hence indicates that progressive failure occurs as the second landslide is located inside the red zone.  
Figure 19 shows the change in safety factor due to the infiltration without overtopping with time. It is shown that the change in safety factor with time does not differ much whether there is overtopping or not when the flux is high. However, as the flux, due to  
Figure 17: Change in safety factor with time under different flux.  
Figure 18: Slope failure after 5.5 h with a flux of 144 mm/day without overtopping.  
rainfall, decreases, the effect of infiltration due to overtopping becomes more prominent. This phenomenon is due to the infiltration rate, due to overtopping, being limited by the saturated permeability of the soil. For a better presentation, Figure 20 shows the critical duration (duration required to reach safety factor equals to one) and rainfall flux with and without overtopping. It is shown that with overtopping, the critical duration required under the same rainfall flux will be shorter, especially when the rainfall flux is lower.  
Post-failure slope stability is also investigated in this paper. Figure 21 shows the stability of the slope when subjected to rainfall with a flux of 144 mm/day. Change in safety factor with time under different flux is shown in Figure 22. It can be seen that the slope remains stable even with 24 h of rainfall with a flux of 144 mm/day. While the soil properties remain the same, the slope has become much steeper and, hence, ground-water level does not raise significantly. Hence, post-failure slope geometry has a sufficient safety factor. This observation is consistent with the inclinometer reading where the movement of the slope is very small.  
Figure 19: Change in safety factor with time under different flux without overtopping.  
Figure 20: Critical duration under different rainfall infiltration.