Swelling of sulphatic claystones – some fundamental questions and their practical relevance /

Abstract

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...

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