Influence of clay mineralogy on the stability of a landslide in Plio-Pleistocene clay sediments near Grassano (Southern Italy)

Clayey silts involved in a landslide have been sampled to study the role of sediment composition in developing sliding surfaces. Five drillings enabled the collection of 46 samples and to define active slide surfaces and non-active surfaces (discontinuities).Active surfaces are enriched in 16–63 μm fractions and depleted in 2–8 μm fractions, whereas discontinuities show grain-size features similar to undisturbed sediments.The organic carbon is low and the Corg/N ratio is bimodal, but the Corg% and the Corg/N ratio are not related to sliding or discontinuities.Sheet silicates are more abundant than quartz carbonates and feldspars; gypsum and hematite occur in some samples at trace levels. In the clay fractions the kaolinite/mixed-layer ratio seems to be a sensitive parameter to the presence of sliding and discontinuities.Scanning electron microscopy demonstrated that active slide surfaces have an open texture, clay minerals are impoverished and non-oriented, whereas secondary phases (hematite and gypsum) are enriched.There is a relationship between geotechnical features and mineralogical composition.