Elliptical anisotropic strength criterion of similar weakly cemented soft rock under directional shear stress path

The surrounding rock stress will redistribution during the underground engineering excavation process. This will affect the strength of weakly cemented soft rock, leading to uneven and large deformation of the surrounding rock of the tunnel, and even causing safety hazards such as collapse and roof fall. In this study, similar weakly cemented soft rock samples were synthesized using a mixture of bentonite, aeolian sand, gypsum, and talcum powders. Then, the directional shear tests with different direction angle of major principal stress were conducted by using GDS SSHCA hollow cylinder torsional shear apparatus. The stress vs. strain curves, peak stress variations, and shear strength are investigated and explored the strength anisotropic characteristics of similar weak cemented soft rock induced by the direction of major principal stress. The results show that both axial compressive strain and shear strain simultaneously generated when 0°< α <45°(α is the angle between the major principal stress and the vertical direction). And the axial strain gradually decreases, while the shear strain gradually increases with the increases of α ;If α = 45°, shear strain predominated and accompanied with volume expansion phenomenon. The samples were subjected to tensile stress and torsional shear stress, which lead to the axial tension and shear strains when 45 °< α < 90 °. The peak shear stress ratio(τzθ / p')max increased with α =30°, after which it decreased until α = 90°. The peak shear stress ratio is 0.81, which highlights the rock's pronounced anisotropic behavior. Utilizing the above findings and assumed the envelope of(τzθ / p')max in the deviatoric plane follows elliptical shape. Then, the elliptical anisotropic strength criterion of weakly cemented soft rock under directional shear stress path was established to describe the anisotropic strength characteristic of weakly cemented soft rock. The ellipse major and minor axis parameters can be obtained by using triaxial and pure shear tests. The validity of this model is confirmed by the results of directed shear tests. This research provides a theoretical basis for controlling deformations and preventing disasters in the field of underground engineering. © 2024 Academia Sinica. All rights reserved.