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 displacementsinawidelandslide.Thelatter,triggeredbyanearthquakeoccurringinSeptember2002,islocated 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 clearly show the mechanical roleofporewaterpressures inthe stabilityoftheslopeandpointout thedependenceofthetimeevolution ofthe 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. Thesemodels, calibratedonthe periodOctober 2009–September2010,madeitpossibletopredict thehorizontal 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.