Caratterizzazione geotecnica del comporportamentodi una grande frana in argille varicolori in Sicilia

The paper reports some results of geotechnical investigations carried out to investigate the evolution of ground displacements in a large landslide measuring 1.6 km from top to toe (0.6 km at top, 1.2 km at toe) involving a varicoloured clay formation in the district of Palermo in Sicily. In order to analyse the landslide mechanisms and devise stabilization works, an extensive programme of in situ and laboratory geotechnical investigations was carried out, and a monitoring system was installed, comprising rain gauge, piezometers, inclinometers and numerous land marks to measure movements at ground surface. Monitoring was performed from instrument installation (autumn 2008/late spring 2009) until December 2011. The reactivation of the overall landslide was triggered by an earthquake in 2002 September, while successive monitored episodes are related to rising piezometric levels induced by rainfall. Several distinct active zones can be contoured where displacements are evolving at variable rates in different directions on the landslide surface. The evolution in time of the displacements is clearly affected by rain. With reference to two of the most active zones, labelled zones A and B, it is shown that displacement rates increase or decrease as piezometric levels rise or fall depending on rainfall. In zone A two active slip surfaces can be detected, located at different depths from ground surface, whereas back analyses carried out show that operative shear strength is consistent with the laboratory measured residual shear strength of the involved tectonised clays.

Si illustrano le misure degli spostamenti e delle pressioni neutre raccolte in un paio d’anni di osservazione del comportamento di un esteso corpo di frana in una formazione di argille varicolori in Sicilia. L’analisi delle misure raccolte indica che la riattivazione riguarda zone distinte del corpo di frana, nelle quali gli spostamenti si sviluppano con velocità e direzioni differenti. L’evoluzione degli spostamenti è manifestamente dipendente dalle condizioni climatiche, dal momento che essi aumentano al crescere delle pressioni interstiziali, a loro volta regolate dalle piogge, e rallentano nella stagione secca, spesso senza annullarsi del tutto. Analisi a ritroso effettuate in una delle zone maggiormente attive confermano che la resistenza a taglio mobilitata coincide con quella residua misurata in laboratorio.