Geotechnical study of a landslide reactivation in clay soils (central Piedmont, NW Italy).

In March 2015, exceptional meteoric events occurred in southern Piedmont caused a remarkable rise of the rivers water level of the Tanaro Basin with flooding and many landslides. The engineering geological study reported here, concerns a triggering of a previous landslide occurred in 1982 in the hills, shaped in the marine sedimentary succession (Lugagnano Clay, lower Pliocene) of the southern Asti Reliefs, near San Martino Alfieri village. Detailed geological, morphological and topographic surveys of this landslide, with the aim to recognize the morphological features linked to the previous landslide and those connected to the triggering of it, have been carried out. The reactivation of the landslide was highlighted by the presence of a main scarp with minor escarpments and tracks. The mechanism activation of the main landslide is due to a rotational movement. To define the engineering geological model of the landslide body, standard cone penetration tests. rotary drilling borings with S.P.T. and geotechnical laboratory tests were performed. Through surveys, both main slip surface and deep-water seepage have been detected. In details, failure mechanism in OC clays was caused by: a) rapid increase in the bulk density or volume weight during rainfall events; b) rise of interstitial pressures along the sliding surface with reduction of normal stress and consequent reduction of the shear strength along this surface (progressive failure); c) reduction of cohesion and friction angle, due to clay softening. The results of the study have highlighted the need, as a primary factor of slopes consolidation, to install sub-horizontal drainage pipes in order both to avoid subsoil imbibition and to control the changes of interstitial pressure. The consolidation works focused on the construction of containment structures with deep consolidation and the execution of drainage works in the landslide area.