Universal confined tensile strength of intact rock

Strength criteria for intact rock are essential for the safe design of many engineering structures. These criteria have been derived mainly from tests in the compressive stress region. Very few results have been published for confined, direct tensile tests on intact rock. No appropriate criteria are available for addressing the issue on tensile strength of intact rock at current stage. We present the results of direct triaxial tensile tests on Longmaxi Shales under varying confining stresses. These and the results from previous tests in marble and sandstone prove that the phenomenon of "tension cut-off" at low confining stress and the positive correlations between confining stress and tensile strength above the confining stress threshold for brittle rocks occur also in more ductile rocks like shales. Such findings are consistent with the concept that tensile failure processes for intact rock are universal. Our results demonstrate that friction processes still have a significant role on intact rock strength in the tensile region which is leading to confined tensile failure and transitioning to a purely tensile mode. Further, strength criteria are presented which consider the frictional processes leading to failure under confined, direct tension tests and validated against published tensile strength data.