Hydro-mechanical modeling of sinkhole occurrence processes in covered karst terrains during a flood

Abstract

Abstract Groundwater is an aggravating factor in karstic sinkhole activity as it exacerbates infiltration, percolation, soil saturation and drainage. The exceptionally high number of ground collapses triggered during the major flood of spring 2016 in the Orleans region (France) clearly supports this assertion. In this article, we examine the role of flooding in sinkhole occurrence in cohesive soil layers covering karstified limestone rock. An innovative hydro-mechanical model is applied to simulated field scenarios. Our numerical simulations combine the Discrete Element Method (DEM) to model the solid phase with the Lattice Boltzmann Method (LBM) for the fluid phase. This coupled numerical method allows us to explore the micromechanical features of internal soil erosion in a flood situation. Three processes, consistent with field observations, are simulated and studied through phase diagrams: the formation of a stable cavity within the cover material, the upward propagation of a cavity leading to a dropout sinkhole, and the downward discharge of the granular media, called the subsidence sinkhole. In particular, we perform a parametric analysis of the dropout sinkhole that gives an estimate of the collapse width. In the first approximation, the characteristic length is shown to increase linearly with cover thickness, regardless of the other main parameters (soil cohesion, hydraulic head, system geometry). Finally, we present an exploratory experimental study, using sand with artificial cohesion that successfully reproduces different erosion regimes predicted by our numerical simulations.