GEOTECHNICAL CHARACTERISTICS AND INSTABILITY OF SUBMARINE SLOPE SEDIMENTS, THE NICE SLOPE (N-W MEDITERRANEAN-SEA)

The slope off Nice (French Riviera) is characterized by the progradati on of a large deltaic sedimentary body fed by the Var River. It princi pally consists of fine-grained plio-quaternary deposits interbedded wi th conglomerates. These deposits are cut by deep valleys and canyons a nd have been submitted to degradation through active mass-wasting proc esses at water depth of 20-500 m. Several sediment facies are differen tiated and classified by laboratory-determined mass physical and geote chnical properties of nineteen gravity core samples. This classificati on is used in order to produce a geotechnical map of the surficial sed iment of the area. The study of consolidation states of these sediment s reveals a sedimentary hiatus appearing under Holocene deposits on th e eastern wall of the major canyon (Var Canyon) between 900 and 1500 m water depth. This sedimentary hiatus, which extends for several kilom eters, corresponds to an erosion surface rather than an omission surfa ce according to the overconsolidated status of the underlying layers. An ancient mass movement corresponding to a translational slide has be en detected with an average estimated thickness of 7 m of sediment inv olved in this mass movement. This approximation is confirmed by the re sults of the pseudostatic infinite slope stability analysis used to st udy the regional slope stability, under undrained conditions.