A new interpretation of the large-scale Alpepiana landslide (upper Aveto Valley, Northern Apennines) based on a field survey, gis applications, and integrated monitoring activities

This work aims to present the results of geomorphological studies and monitoring activities concerning the landslide of Alpepiana (Aveto Valley) at the boundary of Liguria and Emilia-Romagna. Alpepiana hamlet was built between 800 m and 1000 m a.s.l. on the foot of a relict slow-flow-type landslide which represents the evolution of a large mass movement that originated from the southern slope of Mt. Oramara. The landslide presents a lengthened shape extended over 2 km2 with direction NE-SW in the upper portion and NW-SE in the lower. Instability phenomena are well known since the Roman period, and the parish church was continually rebuilt due to the movements of the landslide. The geological setting of the area is characterized by the Ottone Unit (marly limestones etherophic to ophiolitic sandstones and breccias), the Orocco Unit (marly limestones), and the Canetolo Unit (marls and limestones). Fifteen boreholes have been drilled, and equipped with monitoring instrumentation, consisting of four inclinometers, nine piezometers and two pumping wells. In addition, more than 600 m of seismic refraction have been acquired. Hydrogeological and geotechnical field tests were carried out, and some laboratory tests were conducted in order to improve our knowledge of the landslide material. The phenomenon can be classified as a complex-flow type landslide, with a maximum depth of the landslide mass exceeding 40 m in the central portion. According to geological and geomorphological features, the Alpepiana landslide seems to be a deep-seated gravitational slope deformation. The observations during the field survey show evident signs of movement, as creep forms in soil at the surface level, and structural damage can be seen in historical and recent buildings. Inclinometric monitoring activity, carried on from 1999 to 2000 and from 2001 to 2002, showed an average velocity of 10 cm/y, highlighting the slip surface’s depth, ranging between 15 and 20 m. The piezometric measurements identified an unconfined aquifer inside the landslide body, whose level doesn’t seem to be directly related to rainfall events. Back analysis carried out on the landslide cross section indicated a residual strength angle of 14°, apparently related to the finer clayey component of the soil; therefore mineralogical XRD analysis was executed, showing a good relation between clay minerals and geotechnical properties. The government has intervened with drainage and forest management measures: nevertheless the results are not exhaustive, because of the size of the landslide. This study represents a contribution with the aim of developing the most effective interventions to increase the stability of the slopes involved through a slow kinematic landslide.