REGIONAL ASSESSMENT OF MASS FAILURE EVENTS IN THE BAIE-DES-ANGES, MEDITERRANEAN-SEA

Pliocene and Quaternary surficial sediments of the Baie des Anges are commonly affected by instability. Laboratory analyses of sediment samples provide information about the physical properties of these sediments including consolidation state and mechanical behaviour. Some of these physical properties, specifically undrained shear strength values (S(u)), have been calibrated by a few in-situ measurements. The results show that sediments deposited on the slope and on the shelf are mainly homogeneous silty clays, whereas those deposited on the floor of canyons are coarser (possible grain-flow deposits). By using S(u) measurements we have classified the silty clays into four geotechnical types and constructed a geotechnical map of undrained slope stability analysis. Infinite slope analysis provides an assessment of the possible instabilities under both drained and undrained conditions. A regional hazard offshore map is presented including areas sensitive to earthquake-induced sliding. Two potential types of failure are likely to occur in the bay: shallow failures, which are low-volume slides involving underconsolidated near-surface sediments, and deeper failures, which may occur on the flanks of the ridges, and may be preferentially triggered by earthquakes. These sliding processes involve a large volume of normally consolidated or overconsolidated sediments and may evolve into turbidity currents (such as the 1979 Nice slide, even if it was not an earthquake-induced slide) channelled by the Var or the Paillon canyons for more than 100 km. Our study indicates that future development projects within the Baie des Anges must address offshore hazards.