Tests of Rock Cores, Duluth-Vermillion Study Area, Minnesota.

Laboratory tests were conducted on rock core samples received from six core holes in the Duluth-Vermillion study area of Koochiching, Lake, and St. Louis Counties, Minnesota. Results were used to determine the quality and uniformity of the rock to depths of 200 feet below ground surface. The rock core was petrographically identified as predominantly tonalite and gabbro, with relatively minor amounts of amphibolite, granite, and gneiss. Many specimens contained fractures ranging in orientation from vertical to horizontal. Several specimens contained bands andor contacts with other types of rock. Evaluation on a hole-to-hole basis indicates the tonalite represented by specimens from Hole DV-CR-19 to be quite uniform and very competent. This material should offer very good possibilities as a competent hard rock medium. The tonalite, medium-grained gabbro, and granite and granitic gneiss representing Holes DV-CR-17, -40, and -9, respectively, exhibited physical properties typical of relatively competent to very competent material, and all should offer reasonably good possibilities as competent media. The coarse-grained gabbro from Hole DV-CR-24, and the amphibolite and tonalite from Hole DV-CR-39, were generally marginal compressive strength 8,000-12,000 psi to relatively competent compressive strength 12,000 psi in quality, with only one specimen DV-CR-39, Specimen 7, an amphibolite yielding an ultimate uniaxial compressive strength characteristic of incompetent rock.