An integrated approach including dendrometric and dendrogeomorphological analysis, geotechnical monitoring and numerical modeling was established on a reactivating landslide in a forested Deep-Seated Gravitational Slope Deformation (DSGSD) area in Valtellina, Italy, in order to define the distribution of surface displacements in space and time as well as to identify the triggering factors (i.e. precipitation and a recent road tunneling). By means of a normalized difference eccentricity index (EI), we dated with annual resolution the surface displacements occurred over the period 1980–2016: the instability signals spread from the upper portion of the landslide (C) in 1980s to the middle section (B) especially in the 2000s, when they also occurred in the lowest portion (A). Mean Absolute EI (MAEI index) highlighted the intensity of the displacement signals, whereas changes in the MAEI series (dMAEI index) allowed to find correspondences between precipitation input and tree-ring eccentricity. A diagnostic test applied to these correspondences showed an overall accuracy of 0.74 and allowed to determine the precipitation threshold values acting as triggering factors. Topographic monitoring networks in 2016 revealed displacements <1 mm in the upper portion of the landslide, whereas the middle and lowest portion displacements were ranging 1–10 mm. The inclinometric probes confirmed the same trend for in-depth displacements, pointing out multiple sliding surfaces. A numerical stress-strain model, integrating information both from the instrumental and the dendrochronological approaches, provided information on the slope dynamics and its controlling factors, including groundwater flow conditions and the recent tunneling, thus allowing a comprehensive assessment of the geomorphological hazard.
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