Eighty-nine tests were run on cores of laminated dolomitic limestone from the Manlius Formation (Devonian, eastern New York) at confining pressures of 1, 200, 300, 400, 500, 600 and 800 bars, and with the angle between laminae and maximum principal stress varying from 0° to 90°. Percentage of dolomite varies markedly both between specimens and within a single specimen. Results indicate that faults will form parallel to laminae at inclinations of laminae to maximum principal stress between about 13° and about 51°, and that the laminae exert some control over fault orientation well beyond this range. Within the range tested, confining pressure does not suppress or enhance the ability of the laminae to control fault orientation. At 800 bars, the behavior of the rock is changing, with development of ductile faults or significant ductile flow.
Compressive strength depends primarily on confining pressure and orientation of the laminae, but also varies significantly with Young's modulus. The data are consistent with the Coulomb-Navier criterion of brittle fracture, assuming lower values for internal friction and cohesion parallel to the laminae than in all other directions. Observed and calculated compressive strengths agree more closely if the Young's moduli of the specimens are taken into consideration.
The significant effect of the irregular and discontinuous laminae in the test material on rock strength and fault orientation suggests caution in assuming the usual 30° angle between maximum principal stress and faults when conducting fracture analysis in areas of anisotropic, inhomogeneous rocks (a description that fits most sedimentary rocks).