The effect of the intermediate principal stress on fault formation and fault angle in siltstone

We conducted true triaxial compression tests on specimens prepared from two siltstone core sections, one above and one below the Chelungpu Fault, Taiwan. For different constant σ₂ and σ₃ magnitudes, the maximum principal stress (σ₁) was raised until a post failure stage was reached, and a through-going fault had developed. Despite differences between the properties of the two cores, in all tests peak σ₁ increased as σ₂ was set at higher levels than σ₃, in contrast to Mohr-Coulomb condition predictions. The fault-normal vector was aligned with the σ₃ direction and made an angle (θ) with σ₁ direction. The angle θ, which corresponds to fault dip in case of normal faulting, increased monotonically with σ₂ for fixed σ₃, a variation that is also inconsistent with Mohr-Coulomb theory. The results of shear band localization theory are used with fault angles observed for axisymmetric compression and deviatoric pure shear to infer properties of the inelastic constitutive behavior. These properties are significantly different for the two cores. Using them to predict θ for other deviatoric stress states yields good agreement with the observations for core II and acceptable agreement for core I. The results are used to predict the angle variation for constant mean normal stress (θ decreases as the deviatoric stress state varies from axisymmetric extension to axisymmetric compression) and at fixed deviatoric stress state (θ decreases monotonically with increasing mean normal stress).