Effect of initial failure geometry on the progress of a retrogressive seepage-induced landslide

Natural and man-made slopes can undergo a retrogressive landslide when subjected to seepage. Studying the mechanism of retrogressive landslides contributes greatly to the employment of effective mitigation approaches. A multi-stage limit equilibrium-based study was performed to explore how the geometry of initial failure affects the progress of a retrogressive, multiple-rotational landslide caused by seepage flow. Seepage-stability analyses were carried out on two silty sand slopes which were previously found to experience successive rotational failures upon raising the water table in centrifuge tests. Analyzed in prototype dimensions were the slope models with the height of 24 cm and inclinations of 45° (1V:1H) and 63.4° (2V:1H) tested at different centrifugal accelerations. According to the tests, the initial shallow failures were assumed to be circular initiating from the face and emerging from the toe of the slopes in the stability analyses. The impact of curvature and length of the initial failure surface (referred to as IFS) as well as the height of the scarp shaped at each failure episode were investigated. The results show that the landslide continues until the slope profile finds a stable curvature. For the landslides that occur in the 45° inclined slope, when the initial failure initiates at a higher elevation on the slope face, the landslide retrogresses further. In addition, with an increase in the length or curvature of IFS, the final retrogression distance decreases. Further, it was observed that the progress of landslides depends on the height of the scarp exposed at each failure episode.