Key Features of Rain-induced Clayey Landslide: Groundwater Level Rising Rate Highly Synchronized with Landslide Speed

The movement of rain-induced landslides is a complicated process with largely unknown mechanisms that can have devastating consequences globally every year. Aiming to unravel the hydrological-mechanical interactions that govern the dynamics of such landslides, we studied a rain-induced clayey landslide in the Greater Bay Area of China, where the sliding surface was located beneath the groundwater level, representative of many landslides in clay rich lowland slopes near human activities. Aligning with previous studies, there is a strong correlation between observed groundwater levels and landslide displacement. However, our field measurements accented an intriguing pattern: the rising rate of groundwater levels, rather than their absolute values, exhibited a remarkably synchronized relationship with landslide motion. Slope stability modeling suggests that the observed landslide behavior could be predicted by including rainfall infiltration based on initial groundwater levels, whereas modeling considering solely transient groundwater levels may fail to capture landslide movement. The changes of groundwater levels in all landslides involve rainfall infiltration processes, and the speed of groundwater level rise may actually reflect the saturation state of slopes. Our findings suggest that the saturation state of slopes likely modulates landslides movement and should be considered to improve predictions of clayey landslides initiation and mobility.