Identification of the gravitational boundary in weathered gneiss by geophysical survey: La Clapière landslide (France)

Geophysical surveys were conducted on the unstable upper part of the La Clapière landslide in the French Alps (Alpes Maritimes). Electrical resistivity and seismic measurements were carried out over a 2-year period to obtain, for the first time on this landslide, general information on the weathered zones, slipping surface and the network of water drainage. This geological information is derived from two different surveyed areas presented in this paper. For the characterisation and quantification of the weathering, the data showed a very good correlation between the electrical resistivity and the velocity of the direct waves which is dependant on the quality of the rock, and put into context by the survey of geological and structural outcrops. This comparison made it possible to differentiate the weathered zones from unweathered zones. The electrical resistivity profiles also allowed mapping of the weathering zones at depth, and provided information on channeling of the water within the slope and on the depth of the slip surface. Thus, the origin of the instability of the upper part of the La Clapière landslide seems to be strongly associated to the water circulation. The maximum depth of the slipping surface in the uppermost part of the landslide is around 30 m. Moreover, for the first time we have also identified from electrical resistivity tomography, (1) a boundary, at a depth of about 90 ± 10 m, which could be the depth of the slipping surface of the La Clapière landslide and (2) a possible perched aquifer.