Influence of recent morphological evolution on the stability of two slopes in the Euganean Hills

The Euganean Hills rise isolated in the Venetian alluvial plain, covering an area of about 120 km2 and reaching an altitude of 600 above m.s.l. They are composed of sedimentary and eruptive rocks, the former composed by limestone and marl and the latter by basalts, ryolithes, trachytes and latites. The marl and basalt formations are normally covered with layers of weathered clay materials. These colluvial materials, having sometimes thickness of several metres, show precarious equilibrium also on gently conformed slopes. Recent census of landslide movements singled out about 140 unstable areas, corresponding to about 4% of the total surface of the hills. The landslides are due to anthropic as well as natural causes. Since about 2000 years, the morphology of the Euganean Hills has been intensively modified by several types of human activities, among which the most important is probably represented by open quarries of marls and trachytes. The former are generally excavated from the toe of the slopes whereas the latter from the volcanic outcrops. These excavations involve sharp variations of the profiles of the hills. Terracing for agricultural purposes, construction of new roads, changes of river paths or of the surface drainage system also create alterations of the original profiles of the hills, but their effects on slope stability are probably of less importance with respect to intensive quarrying activity performed during the last decades. The influence of human activity on the stability conditions of two landslides is presented and analyzed in the paper using both limit equilibrium and finite element method. These movements, due to sliding along slightly dipping surfaces (8°-10°), are located in the Southeastern Sector of the Euganean Hills and involve the colluvial covers resting above the marly deposits.