Coupling limit equilibrium analyses and real-time monitoring to refine a landslide surveillance system in Calabria (southern Italy)

Abstract. On 28 January 2009, a large debris slide was triggered by prolonged rainfalls at the southern suburbs of San Benedetto Ullano (Northern Calabria). The slope movement affected fractured and weathered migmatitic gneiss and biotitic schist, and included a pre-existing landslide. A detailed geomorphologic field survey, carried out during the whole phase of mobilization, allowed to recognize the evolution of the phenomenon. A set of datum points was located along the borders of the landslide and frequent hand-made measurements of surface displacements were performed. Since 11 February, a basic real-time monitoring system of meteoric parameters and of surface displacements, measured by means of high-precision extensometers, was also implemented.

Based on the data gained through the monitoring system, and on field surveying, a basic support system for emergency management could be defined since the first phases of activation of the phenomenon. The evolution of the landslide was monitored during the following months: as a consequence, evidence of retrogressive distribution could be recognized, with initial activation in the middle sector of the slope, where new temporary springs were observed. During early May, the activity reduced to displacements of a few millimetres per month and the geo-hydrological crisis seemed to be concluded.

Afterwards, the geological scheme of the slope was refined based on the data collected through a set of explorative boreholes, equipped with inclinometers and piezometers: according to the stratigraphic and inclinometric data, the depth of the mobilized body resulted in varying between 15 and 35 m along a longitudinal section. A parametric limit equilibrium analysis was carried out to explore the stability conditions of the slope affected by the landslide as well as to quantify the role of the water table in destabilizing the slope. The interpretation of the process based on field observations was confirmed by the limit equilibrium analysis: the first activation of the landslide was, in fact, to be expected in the middle portion of the slope, provided that the groundwater levels approximate the ground surface in the same sector.

On 1 February 2010, another remarkable phase of landslide mobilization began, following a new period of exceptional and prolonged rainfalls. On 11 February, an abrupt stage of slope acceleration was observed, after further extraordinary rainfalls. The slope movement essentially replicated the phases of mobilization observed on 28 January 2009, thus confirming the results of the limit equilibrium analysis. Based on the outcomes of the parametric analysis, the support system for emergency management could then be tentatively refined on a more physical basis.