Sandia National Laboratories and RESPEC personnel designed an experiment to establish a plausible lower bound of the shear strengths of clay seams at the Waste Isolation Pilot Plant (WIPP). Artificial clay seams were prepared from brine mixed with bentonite and applied between salt specimens with machined asperities. The artificial seams were consolidated at 3000 psi hydrostatic stress, and the excess pore fluid was drained. During consolidation, the clay seams hardened and reduced to about one-fourth of their unconsolidated thickness. Direct shear tests were performed on the consolidated specimens at nominal normal stresses of 500, 1000, and 1500 psi. The tests included pre- and post-failure measurement of normal and shear stiffness. The resulting peak shear stress failure envelope had a friction angle of 9 degrees (coefficient of friction of 0.16) and cohesion of 125 psi when using nominal stresses, which provides a realistic lower bound to the shear strength of clays seams in bedded salt. These results will be incorporated into calculations of the long-term behavior of the WIPP.

1. Introduction

Sandia National Laboratories (SNL) is tasked with modeling the long-term future behavior of the Waste Isolation Pilot Plant (WIPP). A shortcoming that hinders the accuracy of the geotechnical analyses is a lack of publicly available shear strength and deformation properties for geologic contacts and clay seams in bedded salt formations. SNL personnel have been unable to recover clay seams from the WIPP for testing because of operational constraints at the repository. In 2018, SNL commissioned RESPEC to obtain representative core samples from a potash mine near WIPP and perform direct shear tests on the geologic contacts and clay seams in those samples. RESPEC collected 12-inch (30-cm) diameter core samples from the rib of a decline in the potash mine. The natural clay seams and geologic contacts that were recovered from the potash mine had intergrown halite crystals, and the shear strengths of the interfaces were similar to that of intact salt. Consequently, these strength results are likely an upper bound of the interface shear strength in bedded salt deposits. The results of this first phase of direct shear testing are summarized in Sobolik et al., 2019.

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