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 considerably.

Activity of SO 4 2c ± Mean activity coefficient c A Surface free energy of the anhydrite-water interface c Ca 2þ Activity coefficient of Ca 2? c G Surface free energy of the gypsum-water interface c i Surface free energy of the interface of constituent i with water c i Activity coefficient of constituent i c SO 2À 4 Activity coefficient of SO 4 2-D f G 0 Ca 2þ Standard Gibbs energy of formation of Ca 2? D f G A 0

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Sulphatic claystones exhibit a heavily swelling behaviour and are among the most problematic rocks for tunnelling. Their swelling is usually attributed to the transformation of anhydrite to gypsum. The paper questions this simplistic hypothesis through a qualitative discussion of the processes underlying the phenomena that are observed macroscopically, and by identifying a series of fundamental issues that are important from the point of view of tunnel design. At the same time, it provides an overview of ongoing or recent research dealing with the swelling of sulphatic claystones and, more specifically, with the effects of chemical reactions and transport processes, the role of the clay fraction, the pressure dependence of swelling deformations and the possible effects of the seepage flow regime, including evaporation in the unsaturated zone.Sulfathaltige Tonsteine weisen ein besonders starkes Quellverhalten auf und gehören zu den problematischsten Gesteinen beim Tunnelbau. Ihr Quellen wird üblicherweise auf die Umwandlung von Anhydrit zu Gips zurückgeführt. Der vorliegende Aufsatz diskutiert diese vereinfachende Hypothese, indem er die dem makroskopisch beobachtbaren Quellvorgang zugrundeliegenden Prozesse qualitativ behandelt und eine Übersicht über aktuelle Forschungsprojekte vermittelt. Dabei wird eine Reihe von Themen der Grundlagenforschung identifiziert, die auch von praktischer Bedeutung sind. Zu diesen gehören die Fragen nach dem Einfluss der chemischen Reaktions-und Transportvorgänge, der Rolle der Tonfraktion, der Druckabhängigkeit der Quellverformungen und der Bedeutung der Sickerströmung. IntroductionRocks that swell when interacting with water are widely distributed in Switzerland and south-west Germany and have caused serious damage, lengthy operational disruptions and very costly repairs in a number of tunnels. This is particularly true for the anhydritic rocks of the Gypsum Keuper formation. As illustrated by setbacks experienced in two recently constructed tunnels (the Adler tunnel of SBB [1] and the Chienberg tunnel of the Sissach by-pass [2]), claystones containing anhydrite are still among the most problematic rocks in tunnelling today [3] [4]. In purely argillaceous rocks, the swelling behaviour can be traced back to osmosis-driven water uptake, while the dominant mechanism in anhydritic rocks is probably gypsum growth from sulphate solutions (Ca ++ + SO 4 = + 2 H 2 O → CaSO 4 · 2H 2 O).Research on the problem of swelling was triggered in the early 1970's by difficulties encountered in two road tunnels -the Wagenburg tunnel in Germany and the Belchen tunnel in Switzerland. Since then a series of research projects have been carried out which differ with respect to the questions addressed and thus also the methods employed, the scale of the investigation and the scientific disciplines involved. At the microscale, mineralogists have carried out theoretical and experimental studies into the interactions between clay particles, anhydrite and gypsum crystals [5] [6]. The scale of the geological fo...

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 continuummechanical 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.