Monitoring soil-water and displacement conditions leading to landslide occurrence in partially saturated clays

Shallow landslides frequently occur during transient rainfall infiltration and under partially saturated conditions. However, a detailed analysis of what triggers them, particularly in clayey soils, is often hindered by the lack of field measurements. It is uncommon, in fact, to capture their occurrence in an instrumented natural slope. This paper presents results from an integrated field experiment monitoring the soil-water and displacement conditions that lead to the occurrence of a shallow landslide in partially saturated clays. The integration of a variety of experimental techniques allowed for the examination of interplay between soil hydrological and mechanical properties. This research also evaluates a slope stability model based on the suction stress concept. Since the model was applied after the occurrence of the landslide, the results are interpreted as a hind-casting technique for model evaluation. Nevertheless, the detailed field measurements acquired during the monitoring activity and the occurrence of a landslide during the experiment provided significant information on model parameters and data interpretation. The station provides remote satellite monitoring of data on weather variables, soil water content and soil suction. A time domain reflectometry cable was installed vertically to detect potential soil failure. The experimental area had a high probability of landslide occurrence. Indeed, slope failure occurred during the observation period, showing the effectiveness of the station in detecting the occurrence, time and depth of landslides. The landslide was triggered in consequence of changes in suction stress. The failure plane occurred at a depth of 1.4 m, corresponding to the interface between a superficial layer of higher permeability of 1 to 1.45 m thickness, slipping over a compacted substrate having lower permeability. The analysis allowed for testing of the validity of the model and the description of the triggering mechanisms of the observed shallow landslide in unsaturated conditions, indicating that oscillations in soil matric suction were the dominant variables determining soil failure.