Failure mechanism of a slope with a thin soft band triggered by intensive rainfall

Rainfall is considered as one of the paramount factors for slope failures in many regions around the world, particularly in tropical and subtropical regions. To study the effect of rainfall storm and its duration on the stability of slopes with a thin soft band layer, a 2D seepage numerical analysis and experimental investigation were implemented on an unsaturated model, consisting of clayey sand soils with a thin soft layer inclined to the horizontal level by 30° at a slope angle of 50°. It was subjected to intensive rainfall 20 mm/h for 8 h. Positive pore water pressures and horizontal earth pressures were monitored during the rainstorm using sensors distributed inside the experimental model. Both the experimental and numerical simulation results showed that the stability of the slope decreased during the time of the rainfall storm. Infiltration of rainwater resulted in reduction of soil shear strength, due to the loss occurring in soil suction after 1 h of rainstorm; the tension cracks appeared at the top of the slope and a certain displacement was observed in the sliding blocks. During the time of rainstorm, the infiltrated water flowed out from the slope through the weak interlayer near to the toe causing piping and local failure, so the formed cracks at the top of slope grew and expanded due to sliding of the failed soil blocks. Moreover, the ground water table rose and the positive pore water pressures increased, resulting in a reduction of effective stress, which is considered as a main factor in soil shear strength. A surface runoff was also present following the full saturation of the slope, leading to dragging the fine particles with water flow causing erosion. The combination of piping and erosion effects led to a quick local failure at the toe, as well as sliding of the failed blocks and spreading of the cracks.