Discovering multiple regimes of landslide motion with critical state soil mechanics

The intermittent nature of climate forcing causes seasonal groundwater table fluctuations, subjecting slow-moving landslides to cyclic stress changes over long periods. However, the effects of cyclic loading on the landslide kinematics and potential failure remain under-explored. This study integrates a bounding surface modified Cam-Clay model with the sliding–consolidation framework to simulate landslide kinematics under seasonally fluctuating water tables. The results categorise four regimes of landslide motion based on stress state: healing, neutral, stable deterioration and unstable deterioration. It is found that landslides characterised by a representative stress path located on the right side of the critical state exhibit persistent motion, whereas those with stress paths crossing the critical state line, even within the bounding surface, enter an unstable regime and develop cyclic failure after a sufficient number of cycles. This finding shows that whether a landslide will continue to move indefinitely or develop catastrophic acceleration is likely to depend on the competition between shear zone healing and deterioration mechanisms driven by repeated hydrologic cycles. In addition, by defining suitable dimensionless velocity and displacement, the model defines a relationship between the variables that reflects the evolution of the bounding surface size, independent of the mechanical properties and the geometry of the landslide. This finding provides a theoretical basis to define kinematic-based precursors of impending critical state and vanishing safety margins.