Multi-Factor Rainfall Thresholds (MF-RT) for shallow slip prediction based on field monitoring

Flow-like landslides initiated as soil slips are commonly triggered by heavy rainfall infiltrating granular and partially saturated slopes. Field monitoring of suction and water content provides valuable insights into slope responses to hydraulic loads, serving as key preparatory factors for slip initiation. However, current prediction strategies often rely on rainfall thresholds derived from empirical and statistical approaches, while integrating field monitoring into prediction procedures remains challenging. This study uses field monitoring data to develop physically based rainfall thresholds to predict shallow slips. Specifically, the short-term hydraulic response of an unsaturated slope to rainstorms was investigated in a shallow pyroclastic cover on limestone in the Lattari Mountains, Campania Region—an area highly susceptible to flow-like landslides. These field observations i) validated a numerical model predicting the slope's hydrological behaviour during rainstorms and ii) supported the development of rainfall thresholds based on mean rainfall intensity, duration, and pre-rainstorm subsoil suction. Historical landslide data from similar geological contexts were used to validate these thresholds. The study established critical surfaces in the i–d–s0 space (mean rainfall intensity, critical duration, and antecedent suction) which can be integrated into an offline early warning procedure by combining on-site monitoring of preparatory factors with the developed thresholds.