Preliminary results of continuous monitoring of a slope with clayey soils prone to shallow landslides

Rainfall-induced shallow landslides are one of the most common type of landslides of the entire world. Besides the limited volume of soil mobilized by these phenomena, they can provoke serious damages to cultivations, infrastructures and buildings due to their high speed of development and their high density in small areas. Shallow landslides triggering is strictly linked with the hydrological and mechanical responses of a usually unsaturated soil to rainfalls. When these phenomena occur in clayey soils, hydro-mechanical behaviors are more complex because of other different physico-chemical processes affecting soil shear strength, such as softening caused by repeated cycles of shrinking-swelling. Thus, to identify the physical and hydrological conditions leading to landslide triggering, a continuous monitoring of unsaturated soil is needed, in particular related to the change in soil hydrological properties related to different rainy or dry periods. This becomes fundamental also for correctly modeling slope safety factor. In this work, the preliminary results of the continuous monitoring of a slope prone to shallow landslides with clayey soils are presented. The test-site is located in Ardivestra catchment (central Oltrepò Pavese, northern Apennines, northwestern Italy). It was affected by several shallow failures in the years 2009-2014. The main aims of this work were: i) to characterize the slope soils by a multidisciplinary point of view; ii) to identify the main soil hydrological behaviors; iii) to recognize the processes and the mechanisms which could promote the triggering of shallow landslide. The test-site soils were characterized by a multidisciplinary point of view, for identifying the features that can influence the soil hydro-mechanical behavior. Soil geotechnical characterization allowed to measure the physical parameters (grain size distribution, Atterberg Limits, volumetric index properties), the shear strength (direct shear tests, oedometric tests), the shrinking-swelling potential. Hydrological characterization determined the water retention and hydraulic conductivity properties. Pedological and mineralogical characterizations were also carried out. Field monitoring allowed to identify the soil hydrological behaviors, linked to different meteorological conditions. The station integrated field devices for measuring soil hydrological parameters (water content, pore water pressure, water electrical conductivity) at different depths, with data of rainfall, air temperature, wind speed and direction. The monitoring period has started in November 2015. These results allowed also for recognizing the predisposing factors and the hydro-mechanical conditions that can lead to trigger shallow landslides. Moreover, these analyses provided important indications for the correct application of slope stability models in slopes with clayey soils.