Monitoring and analysis of the exceptional displacements affecting debris at the top of a highly disaggregated rockslide

The activity of large rockslides is dependent on a combination of interplaying factors that control strain localization within the slope. In some cases, continued deformation may lead to widespread disaggregation of the slide mass (i.e., loss of cohesion and development of debris). Layers with markedly different composition and strength may eventually form, making it difficult to assess the actual nature of hazards and related risks posed to the valley bottom. This paper provides updated insights into the highly disaggregated and rapidly evolving Ruinon rockslide (Central Italian Alps) based on more than a decade of monitoring by means of a Ground-Based Interferometric Synthetic Aperture Radar (GBInSAR). The rockslide was recently affected by a prolonged period of exceptional surface velocities—consistently exceeding 1 m/day. Monitoring data are examined in order to estimate the thickness of the rapidly moving layer of upper chaotic debris by means of the balanced cross-section method as well as determine the effects of hydrological forcing on the slope displacements. Finite-element modelling is then used to derive hypotheses concerning the deformation behavior of the slide mass at greater depth and different elevations. It is suggested that the upper debris moves at rates several orders of magnitude higher than the underlying substrate, and that in relative terms the sensitivity to sliding of the two layers is similarly governed by the increase of piezometric levels in the spring/summer. On lower slopes, the activity of the upper debris appears to be also influenced by precipitation events that are not accompanied by a notable increase of the piezometric levels measured at the rear of the slide. Our findings show the importance of implementing long-term GBInSAR monitoring at challenging sites like Ruinon, where fieldwork and installation of instruments on the slide mass are not feasible.