Mechanisms of rainfall-induced landslides in gently inclined red beds in the eastern Sichuan Basin, SW China

Abstract

Red beds are widespread in the eastern Sichuan Basin, SW China, and are often called “landslide prone strata”. The dip angle of these red beds is commonly less than 10°, which is much less than the angle of internal friction between the bedding planes. Thus, in theory, failure will not occur along the bedding planes in such strata. We used field methods, numerical modeling, and ring shear tests to investigate the effects of rainfall on landslides in shallow-dipping red beds, using the Qingning landslide in SW China as an example. Our results suggest that the probable mechanisms leading to failure are (1) permeation of rainfall through the cracks at the back of the landslide site that increases the hydrostatic water pressure, which adds to the sliding stress and reduces the effective stress on the failure surface, gradually destabilizing the mass. In the Qingning landslide, the maximum water pressure was as high as 21.8 m before failure. When the factor of safety decreased markedly to 1.015, the sliding mass started to slowly slide. (2) As sliding takes place, the sliding zone soil is under undrained shear and its shear strength decreases sharply to a critical value, which promotes rapid sliding. In the Qingning landslide, the shear resistance of the sliding zone soil decreased markedly to 25.9 kPa as the pore water pressure increased to 118.3 kPa. This study may provide a theoretical basis for landslide forecasting in gently inclined red beds of the eastern Sichuan Basin as well as in similar geological settings worldwide.