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

摘要

Density-driven groundwater flow in evaporite bearing sedimentary rocks and resulting karstification frequentlycausegeomechanicalproblems,suchaslandsubsidenceandcollapses.Theseprocessesare alsoofmajorconcernintheMuttenz-Prattelnregion,locatedintheTabularJura,southeastofBasel, whereatabularinterstratalkarstwasdetectedatthetopoftheTriassicrocksaltlayer.Inthisarea,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 gypsumkarstificationtotwodifferenttypesofnewlydevelopedlaboratoryexperiments.Inrocksalt 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 investigationsweretounderstand(1)thegenesisofasolutioncavitycreatedbyconcentration-driven buoyant flow coupled with mineral dissolution within salt and gypsum rocks (by a horizontallyorientedborehole),(2)thegenesisofaninterstratalsaltkarstatthetopofasaltlayer(byanalmost verticallyorinclined-orientedborehole)and(3)todevelopasuitabletank(benchmark)totestdensitydependentflowcomputercodes. Intherocksaltdissolutionexperiments,halitedissolutiontookplaceonlyintheinflowoftherocksalt 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 growthattheupperendofthefacet.Aconceptualmodelwasdevelopedinspiredbytheseresultsand basedontheoretical,hydraulic-geochemicalconsiderations,aswellasonfieldobservationsinnatural cavesand/orman-madecaverns.Itproposesthat triangularprismorconicalshapedsolutioncavities develop in salt and gypsum rock under confined conditions with an artesian flow traveling from an underlying,practicallyinsolubleaquiferintothesolublestratum.Inthecaseofanalmostverticalor inclinedborehole,atabularsolutioncavitydevelopedattheupperendofthecorecylinders.Atfirst, 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 boundarylaterallyanddownwardorupward,respectively.Thefindingswerescaledupandintegrated into a conceptual model for interstratal salt karst development in the context of horst and graben structuresasobservedintheMuttenz-Prattelnregion.Forthelatterregion,thissubsurfaceconceptual modelmayexplainthedetectedlandsubsidencepattern,clearlyinfluencedbythenormalfaultstrend. A resistance measuring cell was successfully developed for the solute transport experiments. It can measureinsituandindirectlysoluteconcentrationsoverawiderange(uptohalitesaturation)andis suitablefor1Dand2Dflowtankexperiments.Firstsolutetransportexperimentsreveledthatthebuild upbenchmarkhastobedevelopedfurther. Overall,theinvestigationsrevealthenecessityofsmall-scaledissolutionexperiments(incombination withhydrogeological,hydraulicandgeochemicalknowledge)whentryingtounderstandthecomplex processesunderlyingnaturalkarstificationcreatedbydensity-drivengroundwaterflow.Regardingthe realizedflowtanksetup,thisformsagoodbasistofinallyobtainanexcellentbenchmark.Thisthesis will provide the base for new and additional discussions on subsurface salt dissolution and solute transport.