Instability of unsaturated soil slopes due to infiltration

It is widely known that rainfall induces a rise of the groundwater level and an decrease in matric suction (negative pore water pressure) that results in slope failure. However, in many situations where shallow failures are concerned, it has been noted that there is not much evidence of a rise in the groundwater level sufficient to trigger the observed slope failures. Instead, the failures have been attributed to the advancement of a wetting front into slopes until it reaches a depth where it triggers failure. This is because the shear strength provided by matric suction decreases enough to trigger the failures. This paper presents a procedure for calculating the safety factor for an unsaturated slope suffering from rainfall infiltration. The process of infiltration into a slope due to rainfall and its effect on soil slope behavior were examined using modified Mohr-Coulomb failure criterion and conjunct with rainfall-infiltration model. The results indicate the matric suction, which is closely related to slope stability, is affected by the effective degree of saturation controlled by rainfall events.