True triaxial tests in two porous sandstones: experimental failure characteristics and theoretical prediction

The failure of rocks subjected to a general state of stress (σ1 ≥ σ2 ≥ σ3) has been studied since the pioneering true triaxial experiments conducted by Mogi [1]. In true triaxial tests, cuboidal specimens are subjected to fixed, and typically unequal, σ3 and σ2, while σ1 is increased until failure occurs (expressed by σ1 at failure, or σ1,peak). For each constant σ3, σ2 is varied from test to test between σ2 = σ3 and σ2 approaching σ1. The important effect of the intermediate principal stress (σ2) on σ1,peak has been demonstrated in a number of rocks [1-6]. As σ2 is raised above the fixed σ3, σ1,peak first rises, but reaches a plateau at an intermediate σ2 level, beyond which it gradually declines. The variation of σ1,peak with σ2 contradicts the Mohr-Coulomb failure criterion, which neglects the role of σ2 in the failure process.