3D geological modelling at urban scale and mapping of ground movement susceptibility from gypsum dissolution: The Paris example (France)

Gypsum dissolution can be the cause of damage associated with subsidence and collapse. In Paris, the authorities want to refine the current maps of the areas likely to be affected by ground movement due to gypsum dissolution in the Bartonian Saint Ouen Limestone and Beauchamp Sands, and in the Lutetian marl and limestone formation. The proposed method involves assessing separately the probabilities relative to factors that control the ground subsidence, i.e.: thickness of gypsum strata, sulphate-unsaturated water flows, groundwater level, thickness and nature of the overburden. The total susceptibility is represented in a map form by the product of these three values plotted on a grid map with a pixel size of 20 m. For this, we developed a multi-layer 3D geological model using the same 20 m mesh grid, integrating 21 lithostratigraphic units. The hazard prone areas were calculated by geostatistical methods from the archived descriptions of more than 3900 boreholes and from existing geological maps. At the same time, the analysis of some 300 piezometers made it possible to construct the High Water (HW) and Low Water (LW) levels of the groundwater likely to have an impact on the dissolution of the gypsum. The obtained results correlate satisfactorily with the Parisian authorities' field knowledge and to the location of the known events. The mapping also revealed some sensitive areas that had not been previously identified; these will be checked by drilling. The constructed geological model and the hydrogeological analysis have already been used for studying other phenomena (e.g. sand flow and clay creep).