Influences of Schistosity Structure and Differential Stress on Failure and Strength Behaviors of an Anisotropic Foliated Rock Under True Triaxial Compression

Foliated rocks are often encountered in underground engineering, and the spatial orientation relationship between its foliation and in situ stress controls the stability of the surrounding rock. This is closely related to the anisotropy of the mechanical properties of foliated rocks. The anisotropic mechanical properties of thin foliated rock under the influences of foliation and stress can be obtained by laboratory testing; however, there have been few experimental studies on foliated rocks under true triaxial compression at present. Foliated rocks are in a three-dimensional unequal stress state in deep excavation engineering; thus, to evaluate the stability of surrounding rock and scientifically guide the support design, a systematic true triaxial test considering the loading orientations (beta, omega) of schistosity for a foliated gneiss was conducted. The results show that the strength and failure of the gneiss are greatly affected by inherent structure and stress conditions. More specifically, the larger the omega and sigma(2), the greater is the strength, and the failure mode tends to be controlled by the differential stress. Finally, a new empirical true triaxial anisotropic failure criterion was proposed according to the variations of strength with loading angle and stress conditions. This criterion can reflect the tendency and sensitivity of the change in the strength to sigma(2) at different omega and beta, and can satisfy the need to model degradation from the true triaxial stress state to the conventional triaxial stress state. This criterion provides a new approach to characterize the strength of anisotropic rocks and improve the design of engineering works in practice.