Landslides on clay and shale hillslopes in Tuscany, Italy

This paper presents the work currently underway at the Earth Science Department of the University of Florence on the setting up of a landslide data base and information system for the Tuscan Region of Italy, with particular reference to the clay and shale slopes. Some examples of preliminary processing of the collected data are given. Landslides in Tuscany are mainly controlled by the lithological features of the soils and by the intensity and frequency of extreme rainfalls. Index properties can be used for the characterization of soils in different sedimentary basins and can be employed for the estimation of the intrinsic strength parameters, such as the residual and the critical state (fully-softened) friction angles. The combination of geomorphological and geotechnical parameters gives information on the causes and the evolution of slope movements. Two examples are given in this paper. The first deals with slope failures in marine sediments at a local scale; the comparison between the height and the inclination of the slopes that are affected by active or stabilized landslides can be used as back analysis providing that reasonable assumptions on pore water pressures are made. The second example consists of a regional analysis of mudslides. Two-dimensional and three-dimensional analyses are compared in order to explain the causes of the movements. The shorter mudslides on gentler slopes can be caused by undrained loading mechanisms. The interpretation of the longer mudslides is more difficult because they often occur at low slope angle, requiring high pore water pressure to explain the movement. In these cases, paleo-climatic factors probably need to be taken into account.