On the reactivation of a large landslide induced by rainfall in highly fissured clays

The paper discusses the field investigations, geotechnical characterization and time evolution of horizontal displacements in a wide landslide. The latter, triggered by an earthquake occurring in September 2002, is located in Sicily and involves a thick body of stiff and highly fissured clays belonging to a Varicoloured Clay formation. In order to characterise the reactivation of landslide mechanisms induced by rainfall, a three-year monitoring programme (2008-2011) was implemented to measure rain, pore water pressures, and deep and superficial displacements. The monitoring data made it possible to recognize three distinct landslides, which evolve at variable rates in different directions, within the overall landslide area. The landslides were reactivated by the increase in pore water pressures in the period autumn 2009-spring 2010. The data dearly show the mechanical role of pore water pressures in the stability of the slope and point out the dependence of the time evolution of the horizontal displacement on the pore water pressure variation induced by rainfall. Back-analysis carried out on the single landslides confirmed that they are reactivated ones since the residual shear strength is mobilised on the pre-existing sliding surfaces. The pore water pressure variation and the landslide body displacement rates can be effectively related to 5-month cumulative rainfall by simple models, characterised by three parameters. These models, calibrated on the period October 2009-September 2010, made it possible to predict the horizontal displacement rates in the ensuing time period. The displacements forecasted by the model in the period September 2010-May 2011 agree very well with the measured ones.