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 progradation of a large deltaic sedimentary body fed by the Var River. It principally consists of fine-grained plio-quaternary deposits interbedded with conglomerates. These deposits are cut by deep valleys and canyons and have been submitted to degradation through active mass-wasting processes at water depth of 20-500 m. Several sediment facies are differentiated and classified by laboratory-determined mass physical and geotechnical properties of nineteen gravity core samples. This classification is used in order to produce a geotechnical map of the surficial sediment of the area. The study of consolidation states of these sediments reveals a sedimentary hiatus appearing under Holocene deposits on the eastern wall of the major canyon (Var Canyon) between 900 and 1500 m water depth. This sedimentary hiatus, which extends for several kilometers, corresponds to an erosion surface rather than an omission surface according to the overconsolidated status of the underlying layers. An ancient mass movement corresponding to a translational slide has been detected with an average estimated thickness of 7 m of sediment involved in this mass movement. This approximation is confirmed by the results of the pseudostatic infinite slope stability analysis used to study the regional slope stability, under undrained conditions.