A three-dimensional, transversely isotropic yield condition is combined with a plane of weakness to describe the initial yield limit for Green River Shale. This theory is compared to experimental results for two different qualities of oil shale, including true three-dimensional tri-axial stress tests. It is interesting to note that a decrease in the anisotropy of the material with increasing mean stress is predicted by the yield condition and is borne out by the experimental results. For large confining stresses, the material ceases to fail preferentially along the plane of weakness.

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