A simplified approach for assessing the evolution of rainfall-induced landslides in sandy soils

Broad mountainous areas worldwide experience rainfall-induced slope movements, exacerbated by climate changes, causing heavy damages and fatalities. Often in a single geomorphological context, the same rainstorm can trigger many slope instabilities characterized by different degrees of mobility presenting reach angles varying from 10° and 50°.

This is the case of a wide area around Naples (South Italy) where shallow young pyroclastic granular covers initially in unsaturated conditions are frequently involved in fast slope movements showing a very different behaviour whose prediction, together with the consequent delimitation of the exposed areas at risk, is a fundamental step towards the individuation of mitigation strategies.

This study presents a series of long-term investigations conducted both in situ and in the laboratory to identify the parameters influencing the mobility of these  landslides. Data collection at various sample sites consisted of suction and water content  monitoring  over time, also during intense rainfall events.  Laboratory investigations involved hydro-mechanical characterization of these materials to examine soil behavior under both partially and fully saturated conditions and physical modelling to verify that a process of static liquefaction can establish in these deposits.

By synthesizing the knowledge gained from past and recent investigations on pyroclastic covers involved in catastrophic flowslides and debris avalanches during the last three decades, the main factors governing their response at the onset of failure and their subsequent mobility were identified and a physically-based flowchart has been developed. The proposed flowchart, basing on geomorphological and geotechnical data, can be used, under the simplified hypothesis, to make a preliminary prediction of the landslide's evolution and to enhance knowledge of the potential areas at risk.