Residual mechanisms and kinematics of the relict Lemeglio coastal landslide (Liguria, northwestern Italy)

The Lemeglio landslide is a deep-seated coastal landslide of the Liguria region (northeastern Italy) involving heterogeneous rock-masses formed by Miocene turbidide series, whose prevalent lithology varies from sandstones to mudrocks. It has been recognized since the end of the nineteenth century and characterized by a relict state of activity. Besides the general state of slope stability given by the current morpho-climatic conditions, different from those that determined its original paroxysmal evolution, a residual slow kinematics of the accumulation zone still exists, posing a threat to buildings and infrastructures located across the landslide foot. Consequently, the Lemeglio landslide represents a relevant case of study for advancing knowledge on residual kinematics, mechanisms and hazard inherited by a relict slope mass-movement phenomenon. The study is based on results of several drilling campaigns and geotechnical investigations carried outmainly by the Regione Liguria in the unstable landslide accumulation zone. Moreover, in this area ground deformations were assessed by inclinometer measurements and ADInSAR data covering two partially overlapping time spans, from June 2009 to July 2011 and from January 2004 to June 2014, respectively. By these data, joined with field geological and geomorphological surveys, an integrated and consistent landslide model was reconstructed, which was used for Limit Equilibrium (LEM) and Finite Elements Modelling (FEM) analyses. Among principal outcomes is the recognition, by field observations, stratigraphic and inclinometer data as well as slope stability modelling, of a basal sheared and softened band, made chiefly of remoulded mudrocks, which constrains the current landslide failure surface. Such basal band can be considered an inherited landslide structure, formed during the original paroxysmal stage of the Lemeglio landslide, which controls the current residual kinematics of the landslide deposits throughout the foot, depending also on the coastal marine erosion acting along the landslide toe.