Hydrogeology and its effect on slope stability along the coastal aquifer of Nice, France

Along the Ligurian continental margin off Nice (Western Mediterranean) a considerable portion of the sediments is affected by pore pressure transients resulting from groundwater charging by the nearby river Var, and also from leaching clay-mineral bearing deposits, which results in sensitive, weak clays. In 1979 the area suffered a tsunamigenic landslide that generated several casualties and material damage to the coastal zone. The seafloor presently shows evidence for soft sediment deformation and pore water freshening. In this paper, we present the first comprehensive in situ pore pressure data set collected over space and time between 2006 and 2009 in a landslide-prone area. Using piezometers (IFREMER Brest, France) and dynamic cone penetrometers (FF-CPTu, MARUM Bremen, Germany) as well as a simple modelling approach, we illuminate what governing factors control strength and pore pressure in marine slope sediments affected by the Var aquifer. Our results attest (i) a strong link between pore pressure transients and local precipitation over a period of 11 months; (ii) overpressure at levels where groundwater flow occurs (coarse intervals containing sand and gravel); and (iii) fluidised fractures and channels that indicate high pore pressures at least temporarily. In essence, seepage and repeated pore pressure fluctuations weaken the sediment effectively, so that it cannot be ruled out that the currently stable portion adjacent to Nice Airport may fail in the near future.