Lanslide Phenomena in Flysch Deposits: the case of Ca' Staccolo slope (Urbino, Northern Apennines, Italy).

This paper discusses the findings of a study on the unstable slope of Ca' Staccolo, near Urbino in central Italy, which is characterised by landslides that affect the f1ysch deposits of the Marnoso-arenacea Formation. This formation (Tortonian-lower Messinian) belongs to the upper part of the Umbria-Marche Sequence and outcrops extensively in the Northern Apennines. In the study area it consists of interbedded arenaceous and arenaceous-shaly lithofacies. The Ca' Staccolo slope was affected by a landslide in March 1976, following a rainy periodo Though the quantity of precipitation was not exceptional, the rainfall continued over an extended period, and hence was one of the factors that triggered the landslide. The slip surface formed at a depth of 7 m in a marly bed with weak geomechanical properties. Geomorphological evidence points to a case of superficial reactivation of a deeper landslide (up to 30 m deep). The study was conducted in three stages, which allowed us to construct: i) a preliminary geological model of the landslide area; ii) a reconstructive model of the conditions that triggered the landslide; and finally, iii) a predictive model of slope instability. The obtained results show that the landslide susceptibility in the study area is related to the local structural and hydrogeological conditions. Superficial landslide reactivations that affect the f1ysch deposits can be related to rainfall events with a return period of ? to 4 years and to a water table close to the ground surface. Taking into account the variability of strength parameters, the calculated annual probability of reactivation is 0.03, obtained by the intersection probability of the relation between the rainfall event and the instability conditions (factor of safety < 1). Reactivation of deeper movements has more than 100 years return peri od as revealed by available historical data on landslides affecting the Marnoso-arenacea Formation in the Northern Apennines and could be related to very exceptional hydrological conditions.