The Var prodelta progrades across a straight, narrow shelf (less than 2 km wide) with a very steep continental slope reaching locally more than 30°. Historically, the Var delta front is sadly famous for the 1979 catastrophic submarine landslide that resulted in several casualties and infrastructural damage. Geotechnical and geophysical investigations carried out in late 2007 to the east of the 1979 landslide scar provide evidence for the possible occurrence of a new important sedimentary collapse and submarine landslide. Geophysical data acquired in the area show the presence of several seafloor morphological steps rooted to shallow subsurface seismic reflections. Moreover, in situ piezocone measurements demonstrate the presence of several shear zones at the border of the shelf break at different depths below the seafloor. The aim of this technical note is to present and discuss acquired geotechnical and geophysical data in terms of failure mechanisms and submarine landslides. Both geophysical and geotechnical data suggest the start-up of a progressive failure mechanism and reveal the possible occurrence of a submarine landslide and the urgent need for mitigation procedures.

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Despite the gently dipping slopes (ca 1°), large-scale submarine slope failures have occurred on the mid-Norwegian continental margin (Storegga, Sklinnadjupet, Traenadjupet), suggesting the presence of special conditions predisposing to failure in this formerly glaciated margin. With a volume estimated between 2,400 and 3,200 km 3 and an affected area of approximately 95,000 km 2 , the Storegga slide represents one of the largest and best-studied submarine slides of Holocene age known worldwide. Finite element modeling of slope failure indicates that a large (6.5< Ms<7.0) seismic triggering mechanism would not be sufficient to cause failure at more than 110 m below the seabed as observed for the slip planes at Storegga (northern sidewall). This implies that other factors (e.g., liquefaction, strain softening, gas charging, rapid burial) are needed to explain the occurrence of the Storegga slide with a deep surface of failure. In this paper, we discuss the importance of the compaction effect of rapidly accumulated sediments in the slide area. During compaction, sediment grains reorganize themselves, thereby, expelling pore water. Consequently, depending on sedimentation rate and permeability, excess pore pressures might result beneath less permeable sediments. Our modeling and cross-checking illustrate how excess pore pressure generation due to high sedimentation rate could explain the development of layers of weakness, and thus, how such a large slide might have been initiated in deep sediments. Using the highest sedimentation rate estimated in the area (36 and 27 m/kyr between 16.2 and 15 kyr BP), 1D modeling shows excess pore pressure values of around 200 kPa at a depth of 100 m below the seafloor 15 kyr BP and 60 kPa at a depth of 100 m at the time of the slide (8 kyr BP). Excess pore pressure apparently drastically reduced the resistance of the sediment (incomplete consolidation). In addition, 2D modeling shows that permeability anisotropies can significantly affect the lateral extent of excess pore pressure dissipation, affecting, that way, normally consolidated sediments far from the excess pore pressure initiation area.Landslides 4 • (2007) 309