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. Water content analysis was used to determine gypsification levels and involved isolating gypsum water from pore and adsorbed water. Due to cracking of gypsum shells caused by shearing, sheared specimens showed higher gypsification levels than unsheared specimens. In specimens that bulged, gypsification levels were relatively uniform throughout the specimen, with slightly higher levels at the boundaries. Specimens that failed with shear planes showed generally higher gypsification levels than bulged specimens. The brittle specimens also showed enhanced gypsification in the shear plane. This increase in gypsification is due to dilation, which draws water into the shear zone and allows more gypsification to occur. Like the bulged specimens, specimens with shear planes also had slightly higher levels of gypsum water at the boundaries. Thesis Co-Supervisor: Professor Herbert H. Einstein Title: Professor of Civil and Environmental Engineering Thesis Co-Supervisor: Dr. John T. Germaine Title: Principal Research Associate in Civil and Environmental Engineering Special thanks to -Professor Herbert H. Einstein, for your unyielding guidance and support in my research Dr. John T. Germaine, for your ingenuity and patience with my research (and me) Dr. Rolf Niiesch, for your encouragement and expertise The Lab Rats -Lana Aref, Greg Da Re, Erin Force, Laurent Levy, Catalina Marulanda, Marika Santagata, Joe Sinfield, Kurt Sjoblom, Guoping Zhang -for smiles, laughs, and support My parents, for giving me the opportunity to do anything I can dream of