Three dimensional characterization of complex mantled karst structures. Decision making and engineering solutions applied to a road overlying evaporite rocks in the Ebro Basin (Spain)

At mantle/cover evaporite karst settings, evidences of surficial subsidence are usually strongly dependent on geomechanical properties and spatial inhomogeneities within the cover series. In infrastructure assets, affected by karst processes, the evaluation of mitigation or engineering solutions requires a 3D reconstruction of the underground, karst-affected materials. Analysis and surveillance are also required to avoid deterioration of natural conditions and the migration or the increase in rate of karst features. In the central Ebro Basin, fluvial deposits cover soluble salts (mainly gypsum) located below the regional water table. Variations of geomechanical parameters of the cover, mainly related to carbonate crusts, and the contemporaneous sedimentation of fluvial deposits during karst activity can complicate the characterization of affected volumes. In this work, an integrated analysis including the available information, 3D characterization by means of GPR and three boreholes in a peri-urban road was carried out to finally define the geometry and evolution of karst phenomena. An evaluation of metastable situations is also considered in order to define the zoning for hazard mitigation. This evaluation shows that the volumetric affection to the cover series is in the range of tens of thousands of cubic meters. A proposed list of factors related to social aspects, infrastructure relevance and budget is analyzed.