Anisotropic Strength of Poorman Formation Rocks, kISMET Project

As part of Lawrence Berkeley National Laboratory’s kISMET (Permeability (k) and Induced Seismicity Management for Energy Technologies) project, laboratory rock mechanics experiments were conducted on cores taken from the Sanford Underground Research Facility (SURF) (formerly of Homestake Mine) in Lead, South Dakota. Strength properties and their anisotropy were investigated in order to aid the interpretation of the hydraulic-fracturing stress measurements conducted in boreholes drilled down 70-meters from the 4850’ level (1478 meters depth) of the mine. Brazilian disc and triaxial lab tests were conducted on these cores as well as cores previously obtained as part of the Deep Underground Science and Engineering Laboratory (DUSEL) program. Emphasis on how the orientation of the foliation in the schist relative to the loading direction affects the respective strengths of the rocks was monitored. Brazilian disc tensile strengths ranged between 2.7-4.84 MPa when loaded parallel to the foliation, whereas strengths ranged between 5.82-12.01 MPa when loaded perpendicular to foliation, indicating tensile strength anisotropy. Triaxial tests also indicated anisotropy in static and dynamic elastic properties and strength anisotropies when loaded parallel and perpendicular to the foliation direction. UCS results yielded an average value of 107.4 MPa for triaxial samples loaded parallel to foliation, and 88.1 MPa for samples loaded foliation perpendicular to foliation. Young’s Modulus results indicated a 17.5% increase between samples loaded parallel to foliation compared to perpendicular.