A Strength Criterion for Anisotropic Rocks

Two intrinsically anisotropic phyllites obtained from Chamera hydro-electric project site in the Himalayan region were tested in high pressure triaxial apparatus at confining pressures up to 70 MPa and specimen orientations varying from 0 to 90 degrees with respect to the direction of major principal stress. On the basis of these experimental results and the available triaxial data on various anisotropic rocks, a simple strength criterion has been proposed. The strength predictions made on the basis of the proposed criterion show a good agreement with the experimental results. In contrast to the various existing theories, i. e. ; Walsh-Brace theory, single plane of weakness theory, variable coefficient of internal friction and cohesive strength theory and McLamore-Gray theory, the proposed criterion takes into account all the types of anisotropies resulting from 'cleavage', 'bedding-planes' and multiple sets of discontinuities, and has been found valid for jointed rocks as well. Another additional advantage is that the minimum input data requirement is only of two triaxial test results at a single orientation (i.e., beta = 90˚) and uniaxial compressive strength at different orientations. This test data is adequate for strength prediction at any confining pressure and specimen orientation.