Investigating physical processes leading to sinkhole occurrence in Val d'Orléans (France)

Sinkholes in Val d'Orléans occur regularly and can have significant socioeconomic impacts. They are due to the presence of a karstified limestone under 5-15 m thick alluvial deposits. Intense karstification within the area is caused by infiltration of large amount of Loire River water in the karst aquifer through swallow holes. Two mechanisms for the formation of sinkholes are proposed: 1) suffosion of alluvial deposits towards the karstic conduits leading to the occurrence of a void at the base of the alluvium that will progressively enlarge up to reach the surface; 2) karst conduit ceiling breakdown as a consequence of conduit size evolution and/or hydrostatic pressure changes within the karst aquifer. In order to better understand the mechanisms leading to sinkhole occurrence, an experimental site comprising groundwater and surface deformation monitoring has been setup in an area known to be regularly impacted by new sinkholes. First results show similar groundwater level variations in the alluvial and kar-stic aquifers with however a small difference at the beginning of recession limbs when the alluvial aquifer shows higher water levels compared to the karstic aqui-fer. This situation may favour suffosion and/or sloughing. The comparison of water chemistry between Loire River and karst groundwater seems to indicate very active dissolution processes directly downgradient of the swallow holes and a potential rapid evolution of karst conduit sizes. Such an evolution can contribute to conduit ceiling breakdown resulting in collapse sinkholes. Obtained results bring new insights that will be useful for adequate sinkhole risk management (e.g., hazard mapping, surveillance methodology, etc.)