Determination of shallow landslides triggering factors and susceptibility through field monitoring in an area of Oltrepo Pavese

Rainfall-induced shallow landslides affect superficial soils above bedrock, with failure surface that is, generally, located within the soil-bedrock interface. Although they involve small volumes of soil, due to their close proximity to urbanized areas, they cause significant damage to structures and infrastructures and, sometimes, human losses. To understand the effect of climatic parameters and of the soil unsaturated zone physical properties and hydrology on triggering of shallow landslides, a continuous monitoring in time of these factors is needed. A monitoring station was installed in an area of Oltrepo Pavese (Northern Italy), where several shallow landslide events occurred in the last years. The test site slope, representative of Northern Apennines, is characterized by medium-high slope angle; the land use is constituted by shrubs developed on abandoned vineyards. The bedrock is made up of gravel, sand and poorly cemented conglomerates; the soils above bedrock materials are especially silty clayey loam. The objectives of the research are: (a) to monitor the unsaturated zone response to seasonal and extreme rainfall events, determining also how antecedent precipitation affects pore pressure development; (b) to reconstruct a hydro-geotechnical model in order to identify the processes that could predispose the triggering of shallow landslides, c) to evaluate the factor of safety and its evolution in time in order to assess the shallow landslides susceptibility and hazard of the studied area. The experimental station consists in a pluviometer, a thermo-hygrometer, a barometer, a sonic anemometer and a net radiometer. Six TDR probes (Time Domain Reflectometer) are installed at different depths from ground level to measure the soil volumetric water content. Three tensiometers and three heat dissipation sensors are installed to measure soil matric suction. The data are collected by a CR1000 datalogger (Campbell Sci. Inc.) every 10 minutes. In this work the results obtained from a one year monitoring are presented. The response of unsaturated soil and weathered bedrock levels towards rainy and dry period in terms of hydrological parameters were linked to the monitored atmospheric parameters and the geotechnical properties of the investigated materials in order to reconstruct the pore-pressure response and to identify the main shallow landslides triggering factors. This model provided the basis on the evaluation of the site study safety factor towards shallow landslides under unsaturated conditions. Furthermore, some physically based models were implemented to define the susceptibility and the hazard according to the main features which can cause the trigger of these phenomena.