# Experimental Study on Anisotropic Mechanical Characteristics of Shale under Triaxial Loading
## 5 Discussion  
Based on the experimental results and analysis, the reason for the compressive strength \(\sigma_{\rm cs}\), elastic modulus \(E_{\Theta}\), anisotropic index of \(\sigma_{\rm cs}\) and \(E_{\Theta}\), and failure modes of shale samples having different sensitivities to the variation of confining pressure \(\sigma_{3}\) is systematically analyzed:  
(1) The reason for the \(\sigma_{\rm cs}\) and \(E_{\Theta}\) of shale samples having confining pressure effects is that the spherical stress \(\sigma_{\rm m}\) whose value is equal to \(\sigma_{3}\) has a compaction effect on the samples. Under the action of spherical stress \(\sigma_{\rm m}\), the microcracks inside the samples are closed, and the density, compressive strength \(\sigma_{\rm cs}\), and elastic modulus \(E_{\Theta}\) increase, which is the same for homogeneous rock and shale. (2) The first difference is that the basic failure mode of shale changes from shear failure to tensile failure as the bedding angle \(\theta\) increases. According to Table 10, it can be seen that the failure surface tendency of all samples is the same as the bedding planes' tendency except for the samples with a \(0^{\circ}\) bedding angle, because shale is more prone to deformation along the bedding planes under deviatoric stress. Therefore, the three-dimensional stress condition of the samples can be simplified to the plane stress condition (ignoring the influence of the bedding planes' direction), and it is easy to ascertain that the plane spherical stress \(\sigma_{\rm m}\) has a limited constraint effect on the shear stress generated by the deviatoric stress on the shear failure surface. The radial stress component of the spherical stress \(\sigma_{\rm m}\) can effectively suppress the radial strain of the samples, thereby reducing the possibility of the tensile failure of the sample. It is found that the spherical stress \(\sigma_{\rm m}\) has different restraining performance on the shear stress and radial strain, which are the main factors causing shear failure and tensile failure, and as the confining pressure increases from 0 to 40 MPa, the increase percentages of the compressive strength \(\sigma_{\rm cs}\) and elastic modulus \(E_{\rm\theta}\) of the shale samples with bedding angles are 0\({}^{\circ}\), 30\({}^{\circ}\), 45\({}^{\circ}\), 60\({}^{\circ}\), and 90\({}^{\circ}\) are 62.8%, 88.87%, 86.7%, 114.1%, and 155.2% and 24%, 22%, 20.6%, 13.9%, and 33.9%, respectively, so the sensitivity of the shale samples with a high bedding angle (90\({}^{\circ}\)) dominated by tensile failure to the change in confining pressure is significantly greater than that of the samples with medium and low bedding angles (0\({}^{\circ}\)\(\sim\)60\({}^{\circ}\)) dominated by shear failure. (3) The second difference is that the failure site of shale samples changes from the combination of the rock matrix and bedding planes to bedding planes. Combined with the failure images of shale samples shown in Table 10 and the values of cohesion \(c\) and internal friction angle \(\varphi\) in Table 4, it can be seen that the shear strength of the bedding planes is significantly weaker than that of the rock matrix. With the increase in the bedding angle, the shear failure characteristics of the samples are gradually dominated by the bedding planes, which leads to the phenomenon that the sensitivity of the shale samples to the change in confining pressure decreases when the basic failure mode is the shear failure. At the same time, it is observed that the failure modes of shale samples with 0\({}^{\circ}\)\(\sim\)30\({}^{\circ}\) low bedding angles are the same, so the sensitivity of compressive strength \(\sigma_{\rm cs}\) and elastic modulus \(E_{\rm\theta}\) to confining pressure changes little in this range. (4) The anisotropy index of the shale mechanical parameters is obviously affected by confining pressure, and the variation of anisotropy index of different mechanical parameters is different with the increase in confining pressure.