TESTS FOR STRENGTH CHARACTERISTICS OF A SCHISTOSE GNEISS. MECHANICAL ANISOTROPY.

The testing program was conducted to evaluate the mechanical anisotropy of a schistose gneiss and to develop and refine procedures for future studies of anisotropy. This Second Interim Report concerns tests on a schistose gneiss from a surface excavation, whereas the First Interim Report concerns the study of a schistose gneiss obtained from depths of 2,500 to 5,050 ft. Six different inclinations of schistosity were tested. Direct tension, unconfined compression, and triaxial compression tests were conducted. Results of the testing program on near-surface schistose gneiss show that it is much weaker than the deep rock. Its strength is not appreciably affected by specimen size in the range of 1-in. to 2 18-in. diameters. Specimens having schistosity inclined at 45 degrees have the least strength in triaxial compression tests. However, evidence is presented suggesting that anisotropy is controlled by a plane of weakness inclined at about 10 degrees to the plane of schistosity. The variation in strength with orientation is greater for higher confining pressure. There is a threefold variation in tensile strength due to orientation. Youngs Modulus decreases with increasing inclination of the weakest plane. Wet specimens are significantly weaker than airdry specimens. It is concluded that, when openings in anisotropic rock cannot be favorably oriented, it is important to conduct strength tests and a petrofabric analysis to determine whether planes of weakness exist which are not visually apparent. Author