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

摘要

On the days 20-21 November 2000, a storm of exceptional intensity triggeredover 50 landslides within the province of Pistoia in Tuscany (Italy). Thesefailures are mostly of complex type, originating as rotational ortranslational landslides, and transforming into flows. Two of theselandslides were investigated in this paper by modelling the ground waterinfiltration process, the pore water pressure variations, both positive andnegative, and the effects of these variations on slope stability during therainfall event. Morphometric and geotechnical analyses were carried out forboth sites through a series of in-situ and laboratory tests, the results ofwhich were used as input for the modelling process. In a first step thesurface infiltration rate was simulated using a modified Chu (1978) approachfor the Green and Ampt (1911) equations in case of unsteady rainfalltogether with a surficial water balance. A finite element seepage analysisfor transient conditions was then employed to model the changes in porewater pressure during the event, using the computed infiltration rate as theground surface boundary condition. Finally, once again using the data fromthe previous step as input, the limit equilibrium Morgenstern-Price (1965)slope stability method was applied to calculate the variations in the factorof safety during the event and thereby determine the most critical time ofinstability. In both sites this method produced a curve for the factor ofsafety that indicated that the most critical time for failure occurred a fewhours after the peak of rainfall.