Influence Factors Involving Rainfall-Induced Shallow Slope Failure: Numerical Study

Abstract

Assessment of rainfall-induced shallow slope failures is important for reducing damage to infrastructures as well as for the safety of people living close to hazardous areas. The rainfall intensity-duration thresholds for initiation of slope failure (ID thresholds) based on the historical slope failure data are commonly used to assess slope failure. However, in these slope-stability assessments, the critical influence factors triggering shallow slope failures are often disregarded. Three sets of parametric studies were performed through finite-element modeling to investigate the effects of saturated permeability of soil, slope angle, and antecedent rainfall on instability of a shallow slope. It was found that the hydrological mechanism involving the rainfall-induced shallow slope failure is 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 compared with the infiltration capacity at the soil saturation state. The rate of reduction of safety factor (FS) increases with an increasing intensity of rainfall, only in a range lower than the infiltration capacity at the soil saturated state. As such, the saturated permeability of the soil, which is equal to the infiltration capacity at the soil saturated state, plays an important role in the shallow slope failure. The saturated permeability was also found to govern a range of applicability of the ID thresholds. If the rainfall intensity is not greater than the infiltration capacity at the 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 significant roles in the instability of shallow slopes, because 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 rainwater infiltration; hence, it reflects the slope of the ID thresholds.