Mechanical properties and deformation damage characteristics of deep rock under unloading and compensation paths

To study the effect of stress compensation on the mechanical properties and deformation damage characteristics of unloading sandstone, a true triaxial perturbation unloading rock testing system was used to carry out unloading experiments on sandstone with different intermediate principal stresses and rock damage experiments under stress compensation paths, and the effects of the two stress paths on the strength parameters of unloading rock were investigated based on the Mg-C strength criterion, and the mechanical properties and deformation damage characteristics of unloading rock were analysed after the stress compensation. The results show that the increase of the intermediate principal stress σ2 can effectively improve the bearing capacity and stability of the rock and transform the rock from plasticity to brittleness in a certain range, even in the unconventional true triaxial state; There are differences in the damage patterns of unloading rocks under the two stress paths. The overall damage pattern of rocks develops from tension to tension-shear composite to shear damage with the gradient increase of σ2, and the crushed area near the unloading surface gradually shifts from deep to shallow, while the overall damage pattern changes from tension-shear composite to shear damage with the gradient increase of compensating stress \begin{document}$\sigma'_3 $\end{document} at the unloading surface, and the crushed area near the unloading surface gradually shifts from shallow to deep. During the unloading and stress-compensated support of the rock on one side, accompanied by the expansion and compression of ε3, there is no significant change in ε2, but ε1 and εv undergo two significant rebound deformations, and the deformations of ε1 and εv are synchronized with that of σ3; Stress compensation can effectively compensate the rock stress loss caused by unloading, improve the cohesion and damage strength of sandstone, and gradually increase the compensation coefficients of sandstone during loading and unloading, η(m1), η(m3), and η(mv), but have less effect on the internal friction angle and η(m2); The fracturing coefficients ξ(m2) and ξ(m3) are both negative under arbitrary compensating stress conditions, and the rock is in dilatation in the σ2 and σ3 directions, whereas ξ(m1) and ξ(mv) are just the opposite and are both positive under arbitrary compensating stress conditions, and the rock is in compression.