Effects of intermediate principal stress on tensile strength of rocks

摘要

The objective of this study is to experimen tally 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 labo ratory testing involves four-point bending test, Brazil ian 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 test ing. Results indicate that the four-point bending and Brazilian tensile strengths with axial compression pro vide a linear transition with the triaxial extension test results. This is because they are all under the condi tion where σ_1 = σ_2 > σ_3- Based on the Coulomb criterion, the biaxial flexural tensile strength correlates well with the conventional uniaxial and triaxial com pressive strengths of the rocks. The compressive and tensile strengths and cohesion obtained from the tri axial extension tests (σ_1 =σ_2) are greater than those from the triaxial compression tests (σ_2 =σ_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 ofthe 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.