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Shear Behavior of Bedded Salt Interfaces under Direct Shear Loading

Sobolik, Steven R.; Reedlunn, Benjamin R.

Bedded salt contains interfaces between the host salt and other in situ materials such as clay seams, or different impurities such as anhydrite or polyhalite in contact with the salt. These inhomogeneities are thought to have first-order effects on the closure of nearby drifts and potential roof collapses. Despite their importance, characterizations of the peak shear strength and residual shear strength of interfaces in salt are extremely rare in the published literature. This paper presents results from laboratory experiments designed to measure the mechanical behavior of a bedding interface or clay seam as it is sheared. The series of laboratory direct shear tests reported in this paper were performed on several samples of materials from the Permian Basin in New Mexico. These tests were conducted at numerous normal and shear loads up to the expected in situ pre-mining stress conditions. Tests were performed on samples with a halite/clay contact, a halite/anhydrite contact, a halite/polyhalite contact, and on plain salt samples without an interface for comparison. Intact shear strength values were determined for all of the test samples along with residual values for the majority of the tests. The results indicated only a minor variation in shear strength, at a given normal stress, across all samples. This result was surprising because sliding along clay seams is regularly observed in the underground, suggesting the clay seam interfaces should be weaker than plain salt. Post-test inspections of these samples noted that salt crystals were intrinsic to the structure of the seam, which probably increased the shear strength as compared to a typical clay seam. ACKNOWLEDGEMENTS This research is funded by radioactive waste repository programs administered by the Office of Nuclear Energy of the U.S. Department of Energy. The authors would like to acknowledge and thank Stuart Buchholz, Evan Keffeler, and Scyller Borglum of RESPEC Inc. in Rapid City, South Dakota. They performed the laboratory tests documented in this SAND report under a contract with Sandia, and were co-authors on a U.S. Rock Mechanics Symposium paper reporting the results. We would also like to thank Courtney Herrick of Sandia National Laboratories; Frank Hansen of Sandia National Laboratories and RESPEC; Sean Dunagan from Sandia's WIPP organization; Andreas Hampel of Hampel Consulting; and the US-German collaboration on repositories in salt for their review and support of this work.