Most granites have preferred directions ("rift", "grain", and "hardway") that affect the ease with which the rock can be split. In this study, we present a series of uniaxial and triaxial tests on the directional-dependent properties of Sierra White Granite, a rock that has been chosen as a laboratory analog for the FORGE site granitoid. We performed triaxial tests on 1.5" by 3.5" cylindrical cores involving a test program including isotropic compression, cyclic loading, and triaxial compression in three preferred directions under low to high confining pressure. In these tests, we measured stress, displacement, axial and circumferential strain, and P-wave velocity. We found no significant degree of direction dependency in the deformation (stiffness) and strength parameters. On the other hand, the P-wave velocity results show the largest velocity along (equation) (direction perpendicular to plane C) then (equation) and (equation). More tests are needed to accurately quantify the direction-dependent behavior of the given test material.
The direction dependence (anisotropy) of granite is often reported in the literature, both in relation to practical engineering and scientific aspects. With respect to practical engineering, the best known are the orthogonally oriented so-called "quarry planes" rift, grain, and hardway, indicating the ease of splitting or cleavage in this sequence (i.e., rift the easiest, and hardway the hardest). In quarrying, this direction dependence is often used to split the rock by a hammer blow in the rift direction.
From the geotechnical viewpoint, one wants to know what causes the direction dependence, if it affects other small- and larger-scale properties, and what these effects are. Moreover, understanding the material reaction to external influences such as stress, temperature, electrical currents, and chemical agents, is also important to better design geotechnical structures. Most researchers agree that the mechanical properties along the rift orientation depend on the orientation of microcracks and fluid intrusions – which in turn are affected by the direction dependence of quartz (see e.g. Douglass & Voight, 1969; Osborne, 1935). The reasons behind the other two directional dependencies are less clear, but most likely involve the effects of foliation, large-scale stress, and/or temperature changes (Osborne, 1935). It should be noted, however, that some granite samples used in laboratory experiments do not show preferred directions (Chen et al., 1999).
