Genesis and shapes of salt and gypsum solution cavities created by density-driven groundwater flow : a laboratory experimental approach

Density-driven groundwater flow in evaporite bearing sedimentary rocks and resulting karstification frequently cause geomechanical problems, such as land subsidence and collapses. These processes are also of major concern in the Muttenz-Pratteln region, located in the Tabular Jura, southeast of Basel, where a tabular interstratal karst was detected at the top of the Triassic rock salt layer. In this area, a section of the European North-South railway connection is also affected by land subsidence. To overcome the complexity of a field experiment, this thesis scaled down the analysis of salt and gypsum karstification to two different types of newly developed laboratory experiments. In rock salt dissolution experiments, freshwater was pumped from the side or from above into rock salt cores through an axial borehole. In the other setup, solute transport experiments were conducted in a 2D flow tank containing a water-saturated porous medium. The main objectives of these experimental investigations were to understand (1) the genesis of a solution cavity created by concentration-driven buoyant flow coupled with mineral dissolution within salt and gypsum rocks (by a horizontallyoriented borehole), (2) the genesis of an interstratal salt karst at the top of a salt layer (by an almost vertically or inclined-oriented borehole) and (3) to develop a suitable tank (benchmark) to test densitydependent flow computer codes. In the rock salt dissolution experiments, halite dissolution took place only in the inflow of the rock salt cylinders. In the case of a horizontally-oriented borehole, the created solution cavities are approximately shaped like a half cone with a horizontal base facing upward. The halite dissolution took place at the horizontal ceiling and upper end of the facet so that the cavities enlarged under confined conditions by upward growth of the horizontal ceiling and also simultaneously by lateral growth at the upper end of the facet. A conceptual model was developed inspired by these results and based on theoretical, hydraulic-geochemical considerations, as well as on field observations in natural caves and/or man-made caverns. It proposes that triangular prism or conical shaped solution cavities develop in salt and gypsum rock under confined conditions with an artesian flow traveling from an underlying, practically insoluble aquifer into the soluble stratum. In the case of an almost vertical or inclined borehole, a tabular solution cavity developed at the upper end of the core cylinders. At first, these cavities generally enlarged radially outward from the borehole at the inlet side, subsequently they enlarged mainly in up dip direction. If NaCl-undersaturated water came into contact with a laterally lying insoluble material or upward lying salt, the cavity enlarged along the insoluble boundary laterally and downward or upward, respectively. The findings were scaled up and integrated into a conceptual model for interstratal salt karst development in the context of horst and graben structures as observed in the Muttenz-Pratteln region. For the latter region, this subsurface conceptual model may explain the detected land subsidence pattern, clearly influenced by the normal faults trend. A resistance measuring cell was successfully developed for the solute transport experiments. It can measure in situ and indirectly solute concentrations over a wide range (up to halite saturation) and is suitable for 1D and 2D flow tank experiments. First solute transport experiments reveled that the build up benchmark has to be developed further. Overall, the investigations reveal the necessity of small-scale dissolution experiments (in combination with hydrogeological, hydraulic and geochemical knowledge) when trying to understand the complex processes underlying natural karstification created by density-driven groundwater flow. Regarding the realized flow tank setup, this forms a good basis to finally obtain an excellent benchmark. This thesis will provide the base for new and additional discussions on subsurface salt dissolution and solute transport.