We investigate the uniaxial compressive strength (UCS) anisotropy and elastic constants of the Poorman formation foliated schist at the 4850’ level of Sanford Underground Research Facility (SURF) in Lead, South Dakota. As seen in rock cores retrieved from the experiment site, the foliation characteristics vary from distinct planar foliation bands to tight folding with thicknesses varying from the centimeter to meter scale. From each sample group, 1” diameter and 2” long samples were sub-cored with planar foliation oriented 0°, 30°, 45°, 60°, and 90° from the sub-core axis to the foliation plane normal (beta angle, ß). Two additional groups of seemingly “isotropic” samples were prepared from monitoring boreholes where the foliation is tightly folded and deformed at the centimeter scale. We found UCS is weakest when the normal to the foliation plane is oriented 45° to 60° from the sample axis and the resulting failure plane follows the foliation orientation. Dynamic Young's modulus was estimated using elastic modulus relationships for isotropic rocks. To compare the estimate with the true dynamic Young's modulus, we assumed the planar samples behave as transversely isotropic material and utilized the 5 samples in each group to solve for the independent stiffness constants (c11, c33, c13, c44, c66). Here we present our findings on the influence of foliation orientation on UCS and static and dynamic Young's modulus measured under triaxial stress conditions similar to the in-situ stress conditions at the EGS Collab Experiment 1 field site.

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