Quantification of the dissolution kinetics of natural gypsum and particle transport processes in the evolution of dissolution cavities

The north-eastern suburbs of Paris are most prone to sinkhole development due to the natural dissolution of gypsum rocks in contact with groundwater flow. This dissolution induces a loss of solid material creating underground voids with different shapes and sizes that can lead to large underground collapse or subsidence. Until now, there is still a high uncertainty regarding the dissolution mechanisms of natural gypsum and the hydrodynamic, chemical and mechanical conditions involved in this process.

This work has two broad aims: a) to evaluate the variability of gypsum dissolution rate as function of the surface roughness and heterogeneity; b) to identify the respective role of particle transport and dissolution processes in the formation of cavities in gypsum horizons. In fact, for gypsum with interstitial porosity, the release of grains and their transport by the flow (suffusion phenomenon) could very strongly increase the growth of the cavity compared to taking into account only the dissolution.

A variety of experimental protocols have been developed to quantify the parameters controlling the studied phenomena. Rotating disk and batch experiments are employed to determine the kinetic rate model parameters of different varieties of natural gypsum with different porosity and insoluble contents  following the empirical rate expression derived from mixed kinetic theory. To get results more representative of in-situ conditions, they are adjusted according to the specific roughness and texture of each sample. The impact of erosion and particle transport related to gypsum dissolution is determined by controlled leaching tests on external surfaces. It consists of immersing entirely a block of gypsum in a horizontal canal filled with water circulating at a low velocity (≃ 10^-4 to 10^-3 m/s) so that the grains detached during dissolution are not carried out by the flow but collected in a container placed under the block. These grains are then observed microscopically and analyzed by X-ray diffraction to better determine their mineralogy. For each gypsum block tested, the particlar flux is found low composed mostly of insoluble grains with only few gypsum grains released. The distribution of insoluble at the interface has a large influence on the dissolution. When they are present as a form of thin layers, they create local reliefs, depending on their cohesion, which disturbs the flow and locally enhance the gypsum dissolution. When they are distributed at the boundary of gypsum grains, they serve as a coating which protects them and drastically slows down the dissolution kinetics.