Analysis of infiltration, seepage processes and slope instability mechanisms during the November 2000 storm event in Tuscany

On the days 20–21 November 2000, a storm ofexceptional intensity triggered over 50 landslides within theprovince of Pistoia in Tuscany (Italy). These failures aremostly of complex type, originating as rotational or transla-tional landslides, and transforming into flows. Two of theselandslides were investigated in this paper by modelling theground water infiltration process, the pore water pressurevariations, both positive and negative, and the effects of thesevariations on slope stability during the rainfall event. Mor-phometric and geotechnical analyses were carried out forboth sites through a series of in-situ and laboratory tests, theresults of which were used as input for the modelling process.In a first step the surface infiltration rate was simulated us-ing a modified Chu (1978) approach for the Green and Ampt(1911) equations in case of unsteady rainfall together with asurficial water balance. A finite element seepage analysis fortransient conditions was then employed to model the changesin pore water pressure during the event, using the computedinfiltration rate as the ground surface boundary condition. Fi-nally, once again using the data from the previous step asinput, the limit equilibrium Morgenstern-Price (1965) slopestability method was applied to calculate the variations in thefactor of safety during the event and thereby determine themost critical time of instability. In both sites this methodproduced a curve for the factor of safety that indicated thatthe most critical time for failure occurred a few hours afterthe peak of rainfall.