Centrifuge Study of a Retrogressive Seepage-Triggered Landslide in Silty Sand Slopes

A landslide can consist of multiple failures advancing upslope retrogressively. This paper investigates the mechanism of a seepage-induced retrogressive landslide in a silty sand slope and assesses the accuracy of the limit equilibrium analysis in predicting the stability and sequential failure surfaces at retrogressive failures. A centrifuge model test was performed on a 12-m-high silty sand slope with an inclination of 45° at 50 g using a 4.5 m radius large beam centrifuge facility available at IIT Bombay. Seepage was generated using an in-flight simulator, and the behavior of the model slope was monitored. Three retrogressive slides were observed in the slope model when the seepage flow was established. The results indicated that the time interval between initial and second failure episodes is more prolonged than between second and last failure episodes. The length of sliding surfaces decreases when the landslide retrogresses, although their depth remains roughly unchanged. In addition, the slope with an inclination of 63.4° (2 V:1H) had significantly higher values in normalized depth, depth of displaced mass, and retrogression distance than those for 45° slope inclination. The limit equilibrium approach can estimate the stability at pre-failure stages, provided that pertinent geometry and loading conditions are satisfactorily incorporated.