# Experimental Study of Failure Differences in Hard Rock Under True Triaxial Compression

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## Abstract

In view of a previous study of the intermediate principal stress effect at a limited *σ*_{2} range, a series of true triaxial tests, covering a full range of intermediate principal stresses that vary from the generalized triaxial compression stress state (*σ*_{2} = *σ*_{3}) to the generalized triaxial tensile stress state (*σ*_{1} = *σ*_{2}), was carried out on sandstone and granite samples. The experimental results revealed that the deformation, failure strength and failure mode have a significant dependence on the stress state. As an effect of the intermediate principal stress on crack evolution, the deformation difference known as stress-induced deformation anisotropy occurred and should be considered when developing the mechanical model. Moreover, a post-peak deformation with a step-shaped stress drop is observed and illustrates that there will be a multi-stage bearing capacity after the rock failure. The peak strength is non-symmetrical with the increasing *σ*_{2} and is closely related to the Lode angle. Based on the final fracture surface and SEM analysis under true triaxial compression, three failure modes and failure zones, including tension failure, shear failure and mixed failure, are delineated and discussed. Combining the failure mode and the strength under true triaxial compression, it is found that the strength variation exhibited a close relationship to the failure mechanism.

## Keywords

True triaxial Strength Deformation anisotropy Failure mechanism Hard rock## List of Symbols

*σ*_{1},*σ*_{2}, and*σ*_{3}Maximum, intermediate, and minimum principal stresses

*τ*_{oct}and*σ*_{oct}Octahedral shear stress and octahedral normal stress

*σ*_{m,2}Mean effective normal stress

*ε*_{1},*ε*_{2}, and*ε*_{3}Maximum, intermediate, and minimum principal strains

*ρ*Unit weight

*K*_{12}and*K*_{13}Deformation moduli in the

*σ*_{2}and*σ*_{3}directions- A
Deformation anisotropy coefficient

*µ*Lode stress parameter

*σ*_{µ}Peak strength under different Lode stress parameters

*λ*Strength-increasing coefficient

*τ*_{eff}Effective shear stress

*σ*_{e}Tensile stress

## Notes

### Acknowledgements

The authors gratefully acknowledge the financial supports of the 111 Project under Grant No. B17009, the National Natural Science Foundation of China under Grant No. 11572083 and the State Key Research and Development Program of China under Grant No. 2016YFC0600707.

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