On the Time-Development of Sulphate Hydration in Anhydritic Swelling Rocks

Anhydritic claystones are among the most problematic rocks in tunnelling. Their swelling has caused serious damage and high repair costs in a number of tunnels, especially in Switzerland and southwest Germany. The swelling is usually attributed to the transformation of anhydrite into gypsum. It is a markedly time-dependent process which might take several decades to complete in nature. The present paper focusses on simultaneous anhydrite dissolution and gypsum precipitation in a closed system, i.e. disregarding the transport processes that may also be important for the evolution of the swelling process. The paper begins with a presentation of the governing equations and continues with parametric studies in order to investigate the role of the initial volumetric fractions of the constituents and the specific surface areas of the minerals involved. A simplified model for the hydration of anhydrite is also proposed, which identifies the governing process and the duration of the swelling process. Finally, parametric studies are performed in order to investigate the effect of the anhydrite surface being sealed by the formation of gypsum. The latter slows down the swelling process considerabl

On the crystallisation pressure of gypsum

We estimate the crystallisation pressure of gypsum quantitatively, with reference to the geological context of the Gypsum Keuper formation. The formation contains sulphatic claystones which have the property of swelling in the presence of water and have caused substantial structural damage to the linings of several tunnels in Switzerland and Germany. The swelling of these rocks is attributed to the transformation of anhydrite into gypsum, which occurs via the dissolution of anhydrite in pore water and the precipitation of gypsum from the solution. This simultaneous dissolution-precipitation process happens because the solubility of gypsum is lower than that of anhydrite under the conditions prevailing after tunnelling, and it does not cease until all of the anhydrite has been transformed. The elementary mechanism behind the development of the macroscopically observed swelling pressure is the growth of gypsum crystals inside the rock matrix: If a crystal is in contact with a supersaturated solution, but its growth is prevented by the surrounding matrix, it then exerts a so-called crystallisation pressure upon the pore walls. In the present paper, the crystallisation pressure is calculated by means of a thermodynamic model that takes coherent account of all relevant parameters, including the chemical composition of the pore water and pore size. Variations in these parameters lead to a very wide range of crystallisation pressures (from zero to several tens of megapascals). By using the results of mercury intrusion porosimetry and chemical analyses of samples from three Swiss tunnels, however, we show that the range of predicted values can be reduced significantly with the help of standard, project-specific investigations

The Solubilities and Thermodynamic Equilibrium of Anhydrite and Gypsum

Anhydritic claystones consist of a clay matrix with finely distributed anhydrite. Their swelling has led to severe damage and high repair costs in several tunnels. Gypsum growth combined with water uptake by the clay minerals is the main cause of the swelling process. Identifying the conditions under which gypsum rather than anhydrite represents the stable phase is crucial for understanding rock swelling. As existing studies on the anhydrite-gypsum-water equilibrium appear to be contradictory and do not provide all of the information required, we revisit this classic problem here by formulating and studying a thermodynamic model. In contrast to earlier research, our model is not limited to the anhydrite-gypsum equilibrium, but allows for the determination of the equilibrium concentrations of the individual anhydrite dissolution and gypsum precipitation reactions that underlie the sulphate transformation. The results of the paper are, therefore, also valuable for the formulation of comprehensive sulphate-water interaction models that consider diffusive and advective ion transport simultaneously with the chemical dissolution and precipitation reactions. Furthermore, in addition to the influencing factors that have been considered by previous studies (i.e., fluid and solid pressures, concentration of foreign ions, temperature), we consistently incorporate the effect of the surface energy of the sulphate crystals into the thermodynamic equations and discuss the effect of the clay minerals on the equilibrium conditions. The surface energy effects, which are important particularly in the case of claystones with extremely small pores, increase the solubility of gypsum, thus shifting the thermodynamic equilibrium in favour of anhydrite. Clay minerals also favour anhydrite because they lower the activity of the water. The predictions from the model are compared with experimental results and with predictions from other models in the literature. Finally, a comprehensive equilibrium diagram is presented in terms of pore water pressure, solid pressure, temperature, water activity and pore size

On the Occurrence of Anhydrite in the Sulphatic Claystones of the Gypsum Keuper

We investigate why the sulphatic claystones of the Gypsum Keuper contain anhydrite rather than gypsum even at small depths of cover. This question is relevant due to the phenomenon of swelling of anhydritic claystones, which is attributed to the transformation of anhydrite into gypsum and has caused serious damage to a number of tunnels. In tunnelling, the Gypsum Keuper formation is crossed at rather small depths, where simplified thermodynamic considerations indicate that the calcium sulphate should be encountered in its hydrated form, i.e. as gypsum rather than as anhydrite. Understanding why anhydrite can be found at small depths is not only interesting from a fundamental point of view, but also necessary in order to formulate adequate initial conditions for the continuum-mechanical models that simulate the chemo-mechanical and transport processes in swelling anhydritic claystones. The paper quantitatively examines three reasons which, alone or in combination, might explain the occurrence of anhydrite: the small size of the pores in argillaceous rocks; locally high stresses in the vicinity of the sulphate crystals; and the thermodynamic state of the pore water. The computations of the paper take account of the results of porosimetry experiments on samples from two Swiss tunnels in Gypsum Keuper and show that the most probable reason is the thermodynamic state of the pore water, i.e. its ability to participate in chemical reactions. More specifically, the clay minerals reduce the chemical potential of the pore water, thus increasing the solubility of the gypsum and shifting the thermodynamic equilibrium in favour of anhydrite

A Comprehensive Framework Approach using Content, Context, Process Views to Combine Methods from Operations Research for IT Assessments

Motivated by IT evaluation problems identified in a large public sector organization, we propose how evaluation requirements can be supported by a framework combining different models and methods from IS evaluation theory. The article extends the content, context, process (CCP) perspectives of organizational change with operations research techniques and demonstrates the approach in practice for an Enterprise Resource Planning evaluation