Abstract
Abstract. The 2-D distinct element method (DEM) code (PFC2D_V5) is used here to simulate the evolution of subsidence-related karst landforms, such as single and clustered sinkholes, and associated larger-scale depressions. Subsurface material in the DEM model is removed progressively to produce an array of cavities; this simulates a network of subsurface groundwater conduits growing by chemical/mechanical erosion. The growth of the cavity array is coupled mechanically to the gravitationally loaded surroundings, such that cavities can grow also in part by material failure at their margins, which in the limit can produce individual collapse sinkholes. Two end-member growth scenarios of the cavity array and their impact on surface subsidence were examined in the models: (1) cavity growth at the same depth level and growth rate; (2) cavity growth at progressively deepening levels with varying growth rates. These growth scenarios are characterised by differing stress patterns across the cavity array and its overburden, which are in turn an important factor for the formation of sinkholes and uvala-like depressions. For growth scenario (1), a stable compression arch is established around the entire cavity array, hindering sinkhole collapse into individual cavities and favouring block-wise, relatively even subsidence across the whole cavity array. In contrast, for growth scenario (2), the stress system is more heterogeneous, such that local stress concentrations exist around individual cavities, leading to stress interactions and local wall/overburden fractures. Consequently, sinkhole collapses occur in individual cavities, which results in uneven, differential subsidence within a larger-scale depression. Depending on material properties of the cavity-hosting material and the overburden, the larger-scale depression forms either by sinkhole coalescence or by widespread subsidence linked geometrically to the entire cavity array. The results from models with growth scenario (2) are in close agreement with surface morphological and subsurface geophysical observations from an evaporite karst area on the eastern shore of the Dead Sea.
Highlights
Karstification occurs worldwide in rocks like limestone, dolomite, gypsum, anhydrite and salt primarily by chemical dissolution (BGR et al, 2017)
While subsurface-solutionbased drainage networks and connected void spaces resulting from karstification are hydrologically important for groundwater provision (Chen et al, 2017), such features reduce the mechanical stability of the geologic material and so may Published by Copernicus Publications on behalf of the European Geosciences Union
We examine two end-member growth scenarios of model cavity arrays, and we look at the surface morphologies, subsurface structure and stress patterns developed by subsidence of the overburden as those cavity arrays grow
Summary
Karstification occurs worldwide in rocks like limestone, dolomite, gypsum, anhydrite and salt primarily by chemical dissolution (BGR et al, 2017). Systems develop into agglomerations of closely spaced or coalesced dolines and elongated valley-like depressions, potentially revealing linear patterns of drainage (Waltham et al, 2005) Such sinkhole cluster development can be highly dynamic and partly accelerating, and may affect large areas in short times Such clusters commonly lie within gentler, larger-scale (uvala-like) depressions of up to several hundreds of metres in diameter, as depicted in Fig. 1b and c These karst landforms develop as small localised subsidence zones, with single sinkholes that form in heterogeneous material made of Dead Sea mud, alluvial fan sediments and salt (Watson et al, 2019). We show that our more complex end-member modelling scenario is able to explain complementary observations from surface morphology to subsurface hydrology and subsurface geophysics
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