Effects of inherently structural anisotropy and stress anisotropy on strength and fracture characteristics of transversely isotropic rocks

To investigate the influences of structural anisotropy and stress anisotropy on the strength and fracture characteristics of transversely isotropic rocks, a series of true triaxial tests, covering a full range of intermediate principal stress (σ₂) that varies from the generalized triaxial compression to the generalized triaxial tensile stress states, were conducted on slate specimens with different foliation orientations (θ₁-θ₂-θ₃). The results show that the elastic modulus of slate specimen increases with σ₂, and is affected by its foliation orientation with the influence of θ₂ less significant than θ₁. θ₁ significantly influences rock failure characteristics: typically, ductile behavior occurs when θ₁ < 60°, while brittle behavior occurs when θ₁ ≥ 60°. The fracture patterns of slate specimens are closely related to σ₂ and foliation orientation. To clarify the mechanism by which the two factors influence fracture patterns, a numerical approach is proposed to quantitatively analyze the effect of σ₂ on normal and shear stresses on the foliation plane of specimen. Additionally, incorporating the structural anisotropy into the linear Mogi-Coulomb criterion, a true triaxial strength criterion is proposed and validated against the true triaxial data on slate. It is also found that the sensitivities of parameters of this criterion to θ₁ are significantly higher than θ₂. Finally, the acoustic emission (AE) results reveal that the shear fracture along foliation plane and the mixed shear fracture involve a sudden energy release in the late stage, while the shear-through and tensile fractures show a gradual AE activity.