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During the exploitation of deep resources, rock masses are under three-dimensional anisotropic stress conditions owing to tectonic conditions and engineering disturbances. Owing to the advantages of high heating efficiency and selective volume heating, microwaves have broad application prospects in fields such as unconventional low-permeability reservoir fracturing. Therefore, it is of great significance to study the mechanical behavior of rocks under true triaxial stress after microwave irradiation. In this study, true triaxial tests of sandstone under different microwave irradiation conditions were conducted. The results showed that the strength of the irradiated sandstone first increased and then decreased. The strength of the irradiated sandstone was significantly lower than that of the unirradiated sandstone. The pre-peak brittleness of the sandstone gradually increased with an increase in σ₂. The brittleness of the irradiated samples was lower than that of the unirradiated samples. For the post-peak brittleness index of the sandstone, with the increase of σ₂, the microwave irradiated and unirradiated sandstone both gradually decreased first and then slowly increased. The residual strength of the unirradiated sandstone showed a trend of first increasing and then stabilizing, while the residual strength of the irradiated sample showed a greater dispersion and fluctuation. And, the residual strength of sandstone first increased and then decreased with the increase of microwave duration. In addition, a new double logarithmic linear strength model was proposed, which is in good agreement with the experimental results. The true triaxial stress induced anisotropic damage and deformation of sandstone are critical to rock fracture. The stress difference induced by water vapor expansion and mineral thermal expansion is the main factor for microwave breaking water-bearing sandstone. This work is of great significance to the research and application of microwave assisted breaking hard rock and fracturing deep low-permeability reservoir rocks. • Mechanical properties of natural sandstone irradiated by microwave under true triaxial stress were investigated. • Effect of intermediate principal stress on strength and anisotropic deformation of sandstone was analyzed. • The Mogi and SMP criteria were compared and a new linear double logarithm strength model was proposed. • The mechanism of microwave breaking of water-bearing sandstones was investigated. [ABSTRACT FROM AUTHOR]

The effective stress law and the permeability play a notable role in dealing with the gas-solid coupling problem of porous media. The key parameter to quantify the influence of pore pressure to effective stress is the effective stress coefficient α ij. However, current relevant researches are mainly focused on the pore elastic flow characteristics in the elastic stage or hydrostatic pressure under conventional triaxial stress state, which are difficult to reflect the actual formation stress condition. To this end, the present study was conducted to investigate the influence of induced damage on the effective stress coefficient and the permeability of sandstone under true triaxial stress. The results showed that the effective stress coefficient and the permeability were closely related to the damage evolution and fracture propagation. α ij exhibited obvious anisotropy, α 1 was larger than α 2 and α 3 , and α 3 > α 2 appeared in the horizontal direction. α ij increased with increasing the fracture density and opening, even greater than unity. Increasing the initial horizontal stress could reduce α ij under the damage state. The elastic modulus E ij also exerted anisotropy during loading, E 1 basically exhibited an increasing trend followed by a decrease as the axial strain increased, E 2 and E 3 were continue to degrade with increasing the axial strain. Loading the deviatoric stress could cause damage, which induced the initiation and propagation of microcracks, and in turn led to the nonlinear stress-strain relationship and volume expansion of the rock, so the permeability increased, and it had increased sharply before reaching the peak strength. Under the influence of σ 2 , α ij first increased and then decreased with increasing the intermediate principal stress coefficient b under the damage stage. The porosity effect also affected the relationship between α ij and the permeability related to the induced damage, and α ij revealed an increasing trend as the permeability increased. • Effective stress coefficient a ij increases with fracture growth, even beyond unity. • a ij and E ij exhibit anisotropy with loading. a 1 is the largest, and a 3 > a 2. • Increasing the horizontal stress can reduce a ij related to the induced damage. • a ij first decreases and then increases with increasing b under the damage state. • a ij increases with the increase in permeability attributed to the induced damage. [ABSTRACT FROM AUTHOR]

In deep underground coal mining, engineering activities are performed within anisotropic in-situ stress fields due to engineering disturbances and tectonic stress. Many such activities involve the development of excavations in soft rock and anisotropic coal. Accordingly, studying the mechanical properties of soft rocks is important for the stability of deep underground excavations. In this study, the deformation, strength, and failure characteristics of soft sandstone and raw coal under two different true triaxial loading paths were investigated using a self-developed true triaxial test apparatus. The results indicated that the inelastic strain in the pre-peak stage of sandstone and coal gradually increased with increasing intermediate principal stress. Also, the strength-drop in the post-peak stage increased. The crack initiation stress, crack damage stress, and peak strength of sandstone and coal first increased and then decreased with increasing intermediate principal stress for a given σ 3. Moreover, with increasing intermediate principal stress, the failure mode of sandstone and coal changed from shear to tensile shear, and from brittle to semi-brittle. The linear Mogi criteria as found to characterize the true triaxial strength of coal well, while the modified Lade criteria was more applicable to soft sandstone. Owning to the symmetry assumption, the linear Mogi criteria predicted low strength when the intermediate principal stress coefficient exceeded to 0.5. In addition, the peak strength curve of rock on the π plane and the influence of weak structures on the failure mode of anisotropic coal was discussed. Weak structures have an important influence on the failure mode of coal, which depends on the strength difference between the structural plane and the coal rock mass. The strength envelope on the π plane had a significant stress Lode angle effect, which gradually decreased as the mean stress increased. • Influence of the intermediate principal stress on deformation, strength, and failure mode of sandstone and coal. • Different Mogi-type strength criteria were discussed and compared. • The interval effect of intermediate principal stress on coal rock strength was discussed. • The envelope of true triaxial strength of rocks on the π plane was discussed. • The influence of weak structures on the failure mode of coal was discussed. [ABSTRACT FROM AUTHOR]