Back-Analysis of an Artificially Triggered Landslide: A Case Study in Northern Italy

The paper presents a case study of a landslide event, artificially triggered by an exceptional infiltration in an otherwise stable slope. The work aims at investigating the slope failure mechanism within a simplified two-dimensional conceptual framework based on the formation of a perched water table. The landslide occurred in Northern Italy in April 2010, on a hillside with average slope angle 36 degrees-37 degrees; the event affected an area of about 200 m(2), the slip surface was located approximately 1 m below the slope profile, in the uppermost layers of a predominantly coarse, well graded soil. A series of numerical simulations were performed to back-analyze the event, using a commercial computer program. The artificial water infiltration and water content evolution were simulated with a two-dimensional finite element (FE) model of the unsaturated-saturated domain with appropriate infiltration boundary conditions. The slope stability analyses were conducted with classic limit equilibrium (LE) methods and were performed at different time instants during the infiltration process. The soil-water retention curves and conductivity functions were defined according to the van Genuchten-Mualem model, with parameters estimated by means of the software Rosetta (United States Department of Agriculture). The combined FE and LE simulations showed the gradual formation of a perched water table, whose associated localized pore pressure distribution results in the loss of the suction stabilizing effect and thus in the reduction of the safety factor. Although supported by basic soil mechanical and hydraulic characterization, the numerical simulations allowed to perform a back-analysis which effectively captured the timing of the event, the location and depth of the slip surface along the slope.