# Experimental Study on Anisotropic Mechanical Characteristics of Shale under Triaxial Loading
## 4 Analysis of Anisotropic Mechanical Characteristics of Shale
### Anisotropy Characteristics of Shale Compression Failure Modes  
The strength and deformation characteristics of rock are closely related to the failure mode. Unlike isotropic materials, the failure modes of shale samples in uniaxial and triaxial compression tests are affected by the confining pressure \(\sigma_{3}\) and the bedding angle \(\theta\), and there are many kinds of failure modes. Table 10 lists the failure modes of representative shale samples in uniaxial and triaxial compression tests, and the main fractures of the failure samples are marked by a red curve, and the bedding angle \(\theta\) of each sample is marked by a white straight line. Figure 13: Variation of shale anisotropic indexes with the confining pressure \(\sigma_{3}\): (**a**) Compressive strength \(\sigma_{\text{cs}}\); (**b**) Elastic modulus \(E_{\theta}\). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Bedding Angle** & \multicolumn{4}{c}{**Confining Pressure \(\sigma_{3}\) (MPa)**} \\ \cline{2-5}
**\(\theta\) (\({}^{\circ}\))** & **0** & **10** & **20** & **30** & **40** \\ \hline
0 & & & & & \\ \hline
30 & & & & & \\ \hline
45 & & & & & \\ \hline
60 & & & & & \\ \hline
90 & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Failure modes of representative shale samples under uniaxial and triaxial compression. The composition of shale can be simplified into the shale matrix and bedding planes, and the failure modes of shale can be decomposed into four basic failure modes--tensile failure and shear failure of shale matrix and tensile failure and shear failure of bedding planes [33]--while the different failure modes of shale samples presented in Table 10 are combinations of these four basic failure modes. According to Table 10, the following conclusions can be drawn:  
(1) Consistent with isotropic materials, confining pressure \(\sigma_{3}\) can significantly change the failure modes of shale samples. Compared with triaxial compression, there are more penetrating fractures in shale samples under uniaxial compression, and their distribution is more complex. The reason for this phenomenon is that the uniaxial compression experiment lacks the constraint effect of radial stress. (2) The uniaxial failure mode of the samples with a \(0^{\circ}\) bedding angle is a composite failure of tension and shear, and the friction cone at the end is the product of the hoop effect, and this failure mode is relatively complex, including four types of basic failure modes. With the application of confining pressure \(\sigma_{3}\), the failure mode of the samples with a \(0^{\circ}\) bedding angle changes to the through-bedding plane shear failure mode with a single fracture, and the fracture is not parallel to the bedding planes, which is the shear failure mode of rock matrix. In addition to the secondary fractures produced by uniaxial compression, the failure mode of samples with a \(30^{\circ}\) bedding angle is the same as that of samples with a \(0^{\circ}\) bedding angle, which is dominated by the shear failure mode of the shale matrix. (3) The failure mode of the samples with a \(45^{\circ}\) bedding angle is mainly a composite failure mode of shear failure through the bedding planes and shear failure along the bedding planes. The failure surface of the composite failure mode is approximately composed of two failure angles; one is the angle of the bedding planes, and the other is the angle higher than the bedding angles. At the same time, a small number of shale samples with a \(45^{\circ}\) bedding angle also show pure shear failure mode along the bedding planes. The failure mode of the samples with a \(60^{\circ}\) bedding angle is mainly shear failure along the bedding planes. In addition, a small amount of composite shear failure between rock matrix and bedding planes occurs in samples with a \(60^{\circ}\) bedding angle.