Integration of pedological and geotechnical analyses at soil profile scale to assess shallow landslide susceptibility in a pilot catchment of Calabria, southern Italy. Results from the Project “Soil Shades”

In this work some representative soil profiles located in a pilot river catchment in northern Calabria, Southern Italy, were studied with the aim to understand the role of soil features on the stability of slopes and trigger factors of shallow landslides. The Turbolo Stream catchment was chosen as pilot area, as representative of many other geographic areas based on its geological and environmental features. This basin has an extension of about 30 km^2 and exhibits an important lithological, geomorphological and pedological variability. The main soil types range from highly mature soils (Alfisols) to poorly differentiated soils (Inceptisols and Entisols). Previous works investigated the landslide susceptibility in this basin by analyzing geological and geomorphological predisposing factors. However, the intrinsic properties of the soils that can trigger superficial landslides were not considered. The pedogenesis of the parent material leads to its differentiation into soil horizons and a varying spatial distribution of rheological properties for each horizon. The variation of these properties along the profile can potentially generate weak layers that become detachment surfaces once the limit equilibrium of the slope is overcome. The 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), brings together numerous direct and indirect methodologies, trying to address this research question. Proximal and remote sensing techniques were coupled with field description and sampling of soil profiles, located on landslide scars or close to them, for specific laboratory analyses. The investigated soil profiles are six, developed on Paleozoic-Cretaceous crystalline rocks and Neogene deposits, and. For all profiles, individual horizons were sampled, and both pedological (chemical and physical) and geotechnical analyses were performed. In addition, the observation of soil thin sections under a polarizing optical microscope enabled to detect soil micromorphological features, especially those that may affect the physical properties of the horizons (porosity, clay coatings etc.). Although no clear relationships were detected between each pedological and geotechnical property, because of an inhomogeneous behavior of the parameters measured across each profile or between different profiles, some interesting results were obtained. Among chemical data, electrical conductivity (EC) and the sodium absorption ratio (SAR), the latter calculated from soluble salts measured through ion chromatography, enabled to classify the soil horizons of the studied profiles in terms of dispersivity, according to the classification chart proposed by Rengasamy and co-authors. Only three profiles out of six fall within the classes of potentially dispersive soils and dispersive soils, whereas the others are non-dispersive. This suggests that clay dispersivity may slightly contribute to trigger shallow landslides but is not the dominant control factor. The shear resistance, determined in situ through the vane test, showed higher values, as expected, in more mature and well-structured soil profiles, although bulk density values are not always consistent. This suggests that parent materials, degree of pedogenesis and the intrinsic soil spatial variability influence geomechanical parameters at different extents.