Contribution of multi-instrumental geophysics to determine the hydromechanical functioning of a deep-seated coastal landslide (Cirque des Graves, Normandy, France)

International audience; At regional scale, various hydromechanical phenomena are affecting Normandy coast: deep-seated and surficiallandslides, rockslides, cliffs collapse or mudflows. These phenomena are controlled by pore water pressurevariations and are depending on the underlying lithostratigraphy. Coastal areas of the Bessin, of the Pays de Cauxbut also coastal and hinterland fringes of the Pays d’Auge are particularly concerned [Maquaire, 1990; Costa,1996; Lissak, 2012; Fressard, 2013; Letortu, 2012].The RICOCHET project (ANR-16-CE03-0008) aims to identify the influence of variables controlling thehazard in terms of extension, temporality and intensity. This requires to get an important knowledge about thelandslides structures and groundwater flows, which requires the use of various geophysical methods.A focus is made on the study site of the ‘Cirque des Graves’, which is an active deep-seated coastal landslidein Villerville (Calvados). This site, located at the junction between the Paris Basin and the Armorican Massif,was subject to sediment inflows from both geological entities due to alternating between marine deposits andintense erosion periods. This context led to a complex stratigraphy, composed by numerous layers with dissimilargeotechnical characteristics. We suppose that this landslide originally triggered during the Würm glaciation(approx. 10,000 yrs. B.P.). Since this period, variation of the groundwater level in the landslide body is settingCenomanian chalk panels in motion, sinking on Albian sands and sliding on Oxfordian clays.Three different geophysical methods were used to accurately understand the internal morpho-structure andits dynamics:1 - Spontaneous Polarization (SP) to map the main drainage paths, quantifying the natural electric potential (withno particular information about the depth). Around 1,200 measurement points were acquired to realize a 2D mapof this potential over this great landslide of 47 ha.2 - Two missions of Electrical Resistivity Tomography (ERT), conducted in 2008 and 2018, for giving informationon shallow geological structures and water saturation.3 - Seismic investigations to complete the acquaintances on deepest structures wave propagation until substratum(Oxfordian ‘Hennequeville’ gritty limestone). At this end, two kinds of devices were used on field : autonomousseismic 3D sensors and seismic 2D profiles equipped with geophones along two of the ERT cross-sections.As first results, we note a correlation between the observed SP gradients and the actual knowledge of conceptualhydrogeological functioning based on field observations. This is also in agreement with other geophysical(seismic and electrical) data. We also computed hydromecanical models with FLAC® software, trying to showhow hydrology affect the displacement rates in the different compartments of the landslide. These models are setfrom geophysical observations and geotechnical values measured in laboratory. Thanks to these data, we are ableto propose an integrated conceptual model describing the hydro-mechanical functioning of the landslide.