There have been extensive efforts to develop empirical criteria by a number of rock mechanics researchers as the theoretical ones are not adequately capable to demonstrate the response of intact or jointed rock masses at a wide range of confining pressures. Amongst such expressions proposed Ramamurthy's criteria are easy to use and yields sufficiently accurate response for intact and jointed rocks. However this research indicates that the said criterion could be further refined to estimate even more precise compressive strength of anisotropic rocks. An intensive laboratory investigation was undertaken to test jointed specimen at various discontinuity angles. Fabricated artificial samples which included two perpendicular joints were prepared by Plaster of Paris. These specimens were used for simulation of mechanical anisotropy subjected to uniaxial and triaxial tests. A term called anisotropy effect (Ae) was used and strength reduction factor (cj) was defined in order to evaluate and modify Ramamurthy criterion based on the results of these tests. The investigations indicated that the criterion deviates from estimating the strength of the mechanically anisotropic specimen for some discontinuity angles. The paper shows that the modified expression yields more precise estimation.

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