Mechanical and Geophysical Characterization of Damage in Anisotropic Rock

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In rock, changes in applied stress often result in damage caused by the creation of new cracks in the intact material, initiation of new cracks from pre-existing fractures, or slip along pre-existing discontinuities. Ultimately coalescence of pre-existing and/or newly created cracks can induce failure in the rock mass. The Deep Underground Science and Engineering Laboratory (DUSEL) requires the excavation of very deep large underground cavities for the proposed physics experiments. Such excavation poses a significant engineering challenge because of the unprecedented combination of high stress and the large size of the caverns. The caverns will be excavated in the Yates formation, which opens additional questions because of the anisotropy in rock properties. The objectives of the research are to determine: fracturing mechanisms in anisotropic rock; the effect of shear cracks on fracture coalescence; and the geometrical and mechanical properties of fractures prior to and during formation using active geophysical monitoring. The results of the research will provide key understanding of fracturing for open and closed (frictional) fractures in anisotropic rock, and will support the design of experiments for DUSEL to field-test these theories. The research is multi-faceted, consisting of experimental observations and measurements at the micro- and meso-scales, and complemented with numerical simulations of the experiments. Laboratory experiments coupled with geophysical monitoring will be conducted to identify and classify the coalescence mechanisms in systems with multiple non-persistent open and closed discontinuities in anisotropic brittle rock, and are designed to determine the changes in seismic wave transmission with crack initiation, slip and coalescence. The material used for the experiments is rock from the Yates formation, i.e. the host rock where the large caverns at DUSEL will be excavated. In addition to the benefits for DUSEL, this represents much-needed research that will unify what is known of fracturing of open flaws in isotropic rocks with open and closed flaws in anisotropic rocks. Interaction of engineers and geophysicists is highly desirable and eventually must lead to the application of engineering techniques to geophysical methods and geophysical measurements to the validation of theories and methodologies. The work is particularly important for the field of rock mechanics where there is no direct access to the rock undergoing damage. Knowledge transfer from research to academia and practice is integrated into a series of modules that include all stakeholders, from K-12 to undergraduate and graduate students and to rock mechanics professionals. The educational outreach component of the project includes a site visit to DUSEL in Lead, SD. A diverse group of undergraduate students, graduate students, science teachers in local schools and the two PIs will participate.