ABSTRACT:

The swelling behaviour of clay-containing sulfate rocks is, in addition to the influence of the overburden pressure, mainly dependent on the mineralogical composition of the rock. Rocks containing only anhydrite do not swell, whereas, rocks containing 5 percent clay may develop a swelling stress of more than 1MPa. The maximum swelling potential was found in rocks containing 10 to 15 percent clay and 70 to 75 percent anhydrite. With this latter composition, a swelling stress greater than 4MPa was determined. Samples containing more than 15 percent clay tended to show a smaller swelling potential. The texture determines the time-swelling behaviour and the development of the mineralogical swelling potential.

RESUME:

La relation entre la composition mineralogique et le gonflement des roches sulfatees est plus compliquee, car latexture influence le potentiel de gonflement. La texture determine I"evolution temporelle et I"utilisation du potentiel de gonflement des mineralogies en presence. Les roches anhydritiques pures ne gonflent pratiquement pas. Par contre, les roches anhydritiques, dont le contenu en argile depasse 5 pourcents causent des pressions de gonflement excedant 1MPa. Les roches se composant de 10 à 15 pourcents d"argiles et de 70 à 75 pourcents d"anhydrite presentent un potentiel de gonflement maximum. Dans ces cas, les pressions de gonflement excèdent 4MPa. Lorsque le contenu en argile depasse 15 pourcents, le potentiel de gonflement diminue à nouveau.

ZUSAMMENFASSUNG:

Das Quellverhalten toniger Sulfatgesteine ist nicht nur vom Ueberlagerungsdruck abhangig, sondern auch von der mineralogischen Zusammensetzung und vom Gefuege des Materials. Das Gefuege bestimmt den zeit lichen Verlauf und die Ausnuetzung des vorhandenen mineralogischen Quellpotentials. Gesteine aus massigem Anhydrit sind nicht quellfahig. Ein Tongehalt von 5 Prozent verursacht schon Quelldrucke von 1MPa. Ein Maximum an Quellvermögen haben Gesteine mit einer Zusammensetzung von 10 bis 15 Prozent Ton und 70 bis 75 Prozent Anhydrit. Hier wurden Quelldruecke grösser als 4MPa gemessen. Bei Proben mit einem Tongehalt grösser als 15 Prozent, resp, einem Anhydritgehalt kleiner als 70 Prozent, nimmt das Quellpotential wieder ab.

1. INTRODUCTION

Since the construction of the first tunnels in Switzerland more than 100 years ago, problems with swelling rocks have been well known, Beck and Golta (1972). Swelling of rock is a time dependent volume increase involving physico-chemical reactions with water. Swelling is pronounced in rock whose mineralogy favours such reactions, especially those containing clay minerals or anhydrite. The rocks that cause the most difficulties in Switzerland are the smectite containing marlstones of Tertiary age ("Molasse"),. the Jurassic opalinum shale with mixed-layer minerals illite/smectite and the Triassic clay-sulfate rocks of the "Gipskeuper" and "Anhydritgruppe" formations; Einstein (1979), Grob (1976), Madsen and Nueesch (1989).

As shown by Madsen and Mueller-Vonmoos (1985 and 1989) the swelling of clay rocks is caused by water uptake by osmosis. From known mineralogical and chemical parameters the swelling stress may be calculated as an osmotic pressure. In the clay-sulfate rocks the swelling is also caused by the transformation of anhydrite into gypsum. Therefore the relationship between swelling behaviour and mineralogical composition of clay-sulfate rocks is more complicated than that of pure clay reeks. Also, it has been shown by Madsen and Nueesch (1990) that the texture of clay-sulfate.

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