Hydrogeological structures of fault zones triggering slope instability in central Valtellina (northern Italy)

Central Valtellina (northern Italy) is characterized by a long history of frequent and extensive landslide processes. The paper is aimed at demonstrating that in this area the hydrogeological structures of the fault zones strongly affect the slope stability. For this purpose, the following steps compose the study, involving both field surveys and data analysis: 1) Geo-structural and geo-mechanical surveys; 2) Assessment of the hydrogeological characteristics of the rock mass; 3) kinematic analysis of the slope stability; 4) discussion on the correlation between the slope instabilities and the hydrogeological characteristics of the fault zones in the study area. The study demonstrated that, in Central Valtellina the hydrogeological structures of the damage zone generally involve a concentration of water flow, towards rocks characterized by a geomechanical weakness (i.e. the fault core). Results pointed out that the water flow in the fault zones can affect the slope stability in three main modes: 1) channeling, which determines the slide or topple of rock blocks, as a consequence of high intensity and short time rainfall; 2) concentrating, which can lead to local instabilities after weekly accumulative rainfall; 3) supplying, which can trigger large landslides, as a consequence of continuous and long-lasting rainfall.