The objective of this study is to determine the effects of intermediate principal stresses on the tensile strength of rocks. The laboratory testing involves four-point bending test, Brazilian tests with axial compression, circular plate bending test. Uniaxial, biaxial and triaxial compressive strengths of rocks are also determined to correlate their results with those of the tensile testing. The four-point bending and Brazilian tensile strengths under compression provide a linear transition with the triaxial extension test. Based on the Coulomb criterion, the circular plate bending tensile strength can well correlate with the conventional uniaxial and triaxial compressive strengths of the rocks. The results indicate that the intermediate principal stresses do affect the rock tensile strengths. The compressive and tensile strengths and cohesion obtained from the triaxial extension tests are greater than those obtained from the triaxial compression tests. Both stress conditions give similar internal friction angle. More important the results indicate that the Brazilian tensile strength can not be correlated with the two stress conditions. It is recommended that an extension of the Coulomb criterion into the tensile region should be correlated with the tensile strength obtained from the circular plate bending test rather than the Brazilian tension test.

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