MECHANICAL PROPERTIES OF POORMAN SCHIST AND WESTERLY GRANITE

Abstract

We investigate the influence of foliation orientation and fine-scale folding on the static and dynamic elastic properties and unconfined strength of the Poorman schist. The Poorman schist is the dominant rock type at the Enhanced Geothermal Systems Collaboration (EGS Collab) in-situ hydraulic stimulation experiment on the 4850-foot depth level of the Sanford Underground Research Facility, South Dakota. Measurements from triaxial and uniaxial laboratory experiments reveal a significant amount of variability in the static and dynamic Young’s modulus depending on the sample orientation relative to the foliation plane. Dynamic P-wave modulus and S-wave modulus are stiffer in the direction parallel to the foliation plane as expected for transversely isotropic mediums with average Thomsen parameters values 0.133 and 0.119 for epsilon and gamma, respectively. Static Young’s modulus varies significantly between 21 and 117 GPa, and a peculiar trend is that some foliated sample groups show an anomalous decrease in the static Young’s modulus when the symmetry axis is oriented obliquely to the direction of loading. Utilizing stress and strain relationships for transversely isotropic medium, we derive the analytical expression for Young’s modulus as a function of the compliance tensor components (s11, s13, s33, and s44) and sample orientation to fit the static Young’s modulus measurements. The regression of the equation to the Young’s modulus data reveals that the decrease in static Young’s modulus at oblique symmetry axis orientations is directly influenced by a low shear modulus, G13, which we attribute to shear sliding along foliation planes. We argue that such difference between dynamic and static anisotropy is a characteristic of near-zero porosity anisotropic rocks. The uniaxial compressive strength also shows significant variability and ranges from 21.9 to 194.6 MPa across the five sample locations, and is lowest when the symmetry axis is oriented 45º or 60º from the direction of loading.