Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
9716488 | International Journal of Rock Mechanics and Mining Sciences | 2005 | 13 Pages |
Abstract
We conducted laboratory rock strength experiments in two ultra-fine-grained brittle rocks, hornfels and metapelite, which together are the major constituent of the Long Valley Caldera (California, USA) basement in the 2025-2996Â m depth range. Both rocks are banded, and have very low porosity. Uniaxial compression tests at different orientations with respect to banding planes reveal that while the hornfels compressive strength is nearly isotropic, the metapelite possesses distinct anisotropy. Conventional triaxial tests in these rocks reveal that their respective strengths in a specific orientation increase approximately linearly with confining pressure. True triaxial compression experiments in specimens oriented at a consistent angle to banding, in which the magnitudes of the least (Ï3) and the intermediate (Ï2) principal stresses are different but kept constant during testing while the maximum principal stress is increased until failure, exhibit a behavior unlike that previously observed in other rocks under similar testing conditions. For a given magnitude of Ï3, compressive strength Ï1 does not vary significantly in both Long Valley rock types, regardless of the applied Ï2, suggesting little or no intermediate principal stress effect. Strains measured in all three principal directions during loading were used to obtain plots of Ï1 versus volumetric strain. These are consistently linear almost to the point of rock failure, suggesting no dilatancy. The phenomenon was corroborated by SEM inspection of failed specimens that showed no microcrack development prior to the emergence of one through-going shear failure plane steeply dipping in the Ï3 direction. The strong dependency of compressive strength on the intermediate principal stress in other crystalline rocks was found to be related to microcrack initiation upon dilatancy onset, which rises with increased Ï2 and retards the failure process. We infer that strength independence of Ï2 in the Long Valley rocks derives directly from their non-dilatant deformation.
Related Topics
Physical Sciences and Engineering
Earth and Planetary Sciences
Geotechnical Engineering and Engineering Geology
Authors
C. Chang, B. Haimson,