Rising speed not just value of water level in observation hole: Indicator highly synchronized with rain-induced clayey landslide movement

Largely unknown mechanisms control the movement of clay-rich, rain-induced landslides, which are widespread worldwide and cause significant losses every year. By field monitoring, laboratory testing, numerical and theoretical analysis, we studied a rain-induced clayey landslide with sliding surface beneath groundwater level (GWL) typical of thousands in the Greater Bay Area, China, to decipher the key features that regulate landslide movements. In 2023, an 11-day typhoon rainfall event from September 7 to 17 delivered a cumulative precipitation of 627 mm to the landslide area, triggering approximately 260 mm of displacement within the slip zone at depths of 11–12 m. Like previous studies, there is a correlation between monitored borehole water level (BWL) and landslide displacement. But interestingly, closer examinations demonstrate that BWL peak lags the fattest displacement period by nearly one week. Time derivative analysis suggests that BWL rising rate, rather than its absolute values, shows a high synchronous trend with displacement speed. Slope stability modeling and theoretical analysis confirm that due to slow infiltration processes, BWL rise rate serve as a proxy for GWL and saturation state in the landslide body, which strongly synchronized with landslide displacement, whereas modeling considering solely transient BWLs may not successfully forecast landslide movement. Our study demonstrates that the overall saturation state is a critical factor in rainfall-induced landslides, observable via non-invasive and more extensive spatial method like electrical resistivity tomography (ERT). However, challenges remain in in-situ monitoring, real-time data transmission, and correlating ERT data with slope stability metrics, presenting both challenges and opportunities for early warning systems.