Study of the groundwater regime in unsaturated slopes prone to landslides by multidisciplinary investigations: Experimental study and numerical modelling

The triggering of shallow landslides in granular deposits is highly controlled by the groundwater regime, namely, the pore water pressure distribution, which directly affects soil shear strength. The slope hydraulic state is usually variable over time and space due to the high variability of atmospheric loads at the ground surface and local geologic conditions, such as stratigraphic irregularities and preferential groundwater flow paths. The unfavourable combination among critical geomorphological and topographical settings with stratigraphic and hydrogeological features are commonly recognized as predisposing factors of flowslides and debris flow occurrences. This paper proposes a multidisciplinary approach that combines geological, geophysical and geotechnical investigations to identify the role of local geological and geotechnical factors on the groundwater regime in slopes prone to flow-like landslides. The study is based on seasonally repeated electrical resistivity tomography measurements integrated with geotechnical numerical modelling of hydraulic phenomena affecting the soil cover. The latter is used to analyse the effects of the stratigraphic variability in terms of the geometry, continuity, and thickness of the soil horizons on the groundwater regime over time. The proposed approach has been applied to a test site located on the northern slope of Faito Mt. in the Lattari/Sorrento Peninsula mountain chain (southern Italy), an area historically affected by many rapid instability phenomena, such as flow-like landslides and flash floods. Both geophysical and geotechnical models obtained for the test site were cross-checked and validated, providing significant insights into the hydraulic response of the soil cover to rainfall and its hydraulic interaction with the underlying bedrock. Specifically, the integrated approach proved that i) the buried paleo-morphology of the bedrock severely affects the pore water distribution in the soil cover and ii) ashy soil fills the upper karst portion of the bedrock, providing a hydraulic connection of the water flow infiltrating from the topsoil downwards.