Swelling-shrinking properties of weathered clayey soils associated with shallow landslides

This paper outlines the geological and geotechnical characteristics of colluvial clayey soils, originating from the weathering and down-slope transportation of the Complesso Caotico and Argille Varicolori, in relation to swelling-shrinkage. Shallow earth translational slides and earth flows, affecting such colluvial soils, are a recurrent problem causing damage to infrastructures. Three zones, in the Oltrepo Pavese (Northern Apennines, Italy), representative of areas where shallow landslides occur were selected for study. Geotechnical site investigation consisted of open trial pits, the weathering profile description and the collection of disturbed and undisturbed samples. In addition to the mineralogical and geotechnical tests (index properties and matrix suction determination), the samples were submitted to the methylene blue adsorption test, swelling pressure and percentage of volume change tests. The variation of the soil suction on drying and wetting paths have been also studied. The soil profiles, showing the characteristics of the colluvium, the weathered bedrock and the bedrock, do not highlight a mineralogical evolution with depth. The samples are characterised by a very high and high swelling-shrinking potential. The depth of the ‘active zone’ ranges between 0.8 m and 1.8 m and corresponds to the depth of the shallow landslides. The soils are saturated or quasi saturated for most of the year and even for high suction. Starting from these conditions, the soils tend to follow drying paths. The shrinkage is very strong in the dry season. Due to shrinkage cracks water flow rapidly through the cracks into the subsoil. This result in a very rapid response on precipitation events and in a lubrication of clay along the fissures, thus encouraging movement downhill.