Effects of intermediate principal stress on tensile strength of rocks

Abstract

The objective of this study is to experimentally determine the effects of intermediate principal stresses on the tensile strength of rocks. The results are used to assess the predictive capability of the Coulomb criterion when one or more principal stresses are in tension. Four rock types have been tested. The laboratory testing involves four-point bending test, Brazilian tension tests with axial compression, and biaxial flexural tensile strength tests. Uniaxial, biaxial and triaxial compressive strengths are also determined to correlate their results with those of the tensile testing. Results indicate that the four-point bending and Brazilian tensile strengths with axial compression provide a linear transition with the triaxial extension test results. This is because they are all under the condition where ${\sigma }_1 ={\sigma }_2 >{\sigma }_3$. Based on the Coulomb criterion, the biaxial flexural tensile strength correlates well with the conventional uniaxial and triaxial compressive strengths of the rocks. The compressive and tensile strengths and cohesion obtained from the triaxial extension tests $({\sigma }_1 ={\sigma }_2 )$ are greater than those from the triaxial compression tests $({\sigma }_2 ={\sigma }_3 )$. Both stress conditions give similar internal friction angle. More important the results indicate that the Brazilian tensile strength can not correlate 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 strengths obtained from the biaxial flexural tensile strength test rather than the Brazilian tension test.