Analysis of the landslide triggering mechanism during the storm of 20th-21st November 2000, in Northern Tuscany

A severe rainstorm of high intensity occurred on 20th– 21st November 2000, in the region of Pistoia, Tuscany, Italy, which triggered, within the entire province, over 50 landslides. These landslides can be broadly defined as complex earth slides-earth flows,originating as rotational slides that develop downslope into a flow. In this paper, two such landslides have been investigated by modelling the process of rainwater infiltration, the variations in both the positive and negative pore water pressures and their effect on slope stability during the storm. For both sites, results from morpho- metric and geotechnical analyses were used as a direct input to the numerical modelling. A modified Chu, 1978 approach was used to estimate the surface infiltration rate by adapting the original Green and Ampt, 1911 equations for unsteady rainfall intensity in conjunc- tion with the surficial water balance. For transient conditions, a finite element analysis was used to model the fluctuations in pore water pressure during the storm, with the computed surface infiltration rate as the surface boundary condition. This was then followed by the application of the limit equilibrium Morgenstern and Price, 1965 slope-stability method, using the temporal pore water pressure dis- tributions derived from the seepage analysis. From this methodology, a trend for the factor of safety was produced for both landslide sites. These results indicate that the most critical time step for failure was a few hours following the rainfall peak.