The role of soil features and pedogenic processes as potential factors of shallow landslides. A catchment-scale multidisciplinary approach in the frame of the Project “Soil Shades”

A wide literature and research interest focus on mapping shallow landslides, investigating their triggering factors, evaluating connected hazards and providing policies for risk mitigation, using a variety of methods. Many researchers also explored the role of weathering processes as predisposing factors of landslides, but most of them applied this approach to deep mass movements or did not consider soils from a pedological point of view, thus not taking into account their intrinsic juxtaposition of different pedogenic horizons and their spatial variability. Interesting results based on the basic concept of soil profile, well-known to soil scientists but often neglected by geologists or engineers, were mainly obtained in soils developed on pyroclastic materials, frequently affected by flow-like landslides, and more limitedly in other soil types. The ongoing project “SOIL SHADES – SOIL features and pedogenic processes as predisposing factors of SHAllow landsliDES”, funded by Next Generation EU, National Recovery and Resilience Plan (PNRR) of Italy, M4.C2.1.1., National Research Programme (PNR)–Research Projects of Significant National Interest (PRIN), aims at filling this gap. In its framework, we applied an integrated multidisciplinary, multi-analytical and multiscale approach in a pilot catchment (Turbolo Stream) of Calabria, southern Italy. For its geological-geomorphological, pedological and environmental features, this basin can be considered representative of other drainage basins in several Mediterranean and mid-latitude regions. Field surveys and aerial photo interpretation allowed us to provide an inventory of landslides in that pilot area and select some benchmark soil profiles able to catch the local pedodiversity, different lithologies and geomorphological features, where shallow movements occurred. At some of these sites, remote and proximal sensing investigations, such as electromagnetic induction (EMI), electrical resistivity tomography (ERT) and drone-based 3D topography acquisition were carried out to map the soil spatial variability from the landslide scar to the toe of its body and from the topographic surface to the depth of the potential failure surface. Results of the geophysical surveys were consistent with the soil profile depths and/or with the presence of relevant morphological changes already described along the profiles. Twenty-six soil samples collected from 6 soil profiles were morphologically described (color, pedogenic structure, skeletal rock fragments, clay coatings, nodules, etc.) and analyzed in laboratory to measure physical and chemical properties (particle size distribution, organic carbon content, pH, electrical conductivity, cation exchange capacity, soluble salts, etc.), while the micromorphological analysis was carried out only on selected horizons. Geotechnical analyses to obtain bulk density, Atterberg limits, shear strength, cohesion and internal friction angle were performed on the same samples, where applicable. Major data showed clear changes of pedological and geotechnical properties across the soil profile, thus supporting a prominent role of soil-formation processes on the modification of the original properties of the parent materials, as potential predisposing factors of shallow landslides. Nonetheless, the different soil types did not display homogenous behavior and mutual relationships from top to bottom or between specific pedological and geomechanical data, suggesting a complex interplay between parent rocks, pedogenesis and other morphodynamic processes recorded at the soil profile scale.