The Swelling Behavior of Clay-Sulfate Rocks

Clay-sulfate rocks are often encountered in engineering projects such as underground construction and waste repositories. The unique combination of clay swelling and chemical transformation of anhydrite into gypsum results in a swelling behavior that cannot be explained using conventional rock mechanics. The goals of this study are: 1) to investigate the strength characteristics of reconstituted clay-sulfate rocks and 2) to assess the influence of shearing on the gypsification of anhydrite. The specimens used in this testing program consist of 85 % anhydrite and 15 % clay and were hydrostatically formed at 100 MPa. Undrained and drained triaxial compression tests were performed on these specimens. The results showed that these clay-sulfate rocks have a normalized undrained shear strength of 0.504-0.792 and a friction angle a of 17.20. The normalized undrained strength of this predominantly anhydritic rock is low compared to natural shales of the same OCR. Depending on the shearing rate, specimens exhibited either ductile or brittle failure. Specimens sheared at faster rates failed brittlely, while those sheared at slower rates bulged at failure