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 samp les provide information about the physical properties of these sedimen ts including consolidation state and mechanical behaviour. Some of the se 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 mainl y homogeneous silty clays, whereas those deposited on the floor of can yons are coarser (possible grain-flow deposits). By using S(u) measure ments we have classified the silty clays into four geotechnical types and constructed a geotechnical map of undrained slope stability analys is. Infinite slope analysis provides an assessment of the possible ins tabilities under both drained and undrained conditions. A regional haz ard 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 unde rconsolidated near-surface sediments, and deeper failures, which may o ccur on the flanks of the ridges, and may be preferentially triggered by earthquakes. These sliding processes involve a large volume of norm ally consolidated or overconsolidated sediments and may evolve into tu rbidity currents (such as the 1979 Nice slide, even if it was not an e arthquake-induced slide) channelled by the Var or the Paillon canyons for more than 100 km. Our study indicates that future development proj ects within the Baie des Anges must address offshore hazards.