Influences of Rainfall on Shallow Slope Failures

Assessment of rainfall-induced shallow slope failures is very important to reduce damages of infrastructures and lives of people living close to hazardous areas. Although the rainfall intensity-duration thresholds for initiation of slope failure (ID thresholds) based on the historical slope failure data is commonly used to assess slope failure, critical influence factors triggering shallow slope failures are often disregarded. Three sets of parametric study were therefore performed through finite element model to investigate the effect of saturated permeability of soil, slope angle and antecedent rainfall on instability of shallow slope. It is found that the hydrological mechanisms involving the rainfall induced shallow slope failure are either (1) the rising of water table mode or (2) the rainfall infiltration mode. The hydrological mode during the failure depends on the magnitude of rainfall intensity comparing with the infiltration capacity at soil saturation state. The rate of reduction in safety factor increases with an increasing the intensity of rainfall, only in a range of lower than the infiltration capacity at soil saturated state. As such the saturated permeability of the soil, which is equal to the infiltration capacity at soil saturated state, plays an important role in the shallow slope failure. The saturated permeability was found also to govern a range of applicability of the ID thresholds. If the rainfall intensity is not greater than the infiltration capacity at soil saturated state, the rainfall duration to failure (Trf) can be read from the ID thresholds. Slope angle and antecedent rainfall were found to play role on instability of the shallow slope. They control the initial stability of slope, which results in the different linear relationship of ID thresholds. In addition, the slope angle might accelerate the rate of rain water infiltration, and hence it reflects the slope of the ID thresholds