# Experimental Study on Anisotropic Mechanical Characteristics of Shale under Triaxial Loading
## 1 Introduction  
Shale, as a common sedimentary rock, has a layered structure formed by an ordered parallel arrangement of a high-strength rock matrix and low-strength bedding planes [1, 2]. Due to the different physical and mechanical properties of the rock matrix and bedding planes, there are significant differences in the mechanical and deformation properties of the shale between the parallel bedding planes direction and the vertical bedding planes direction, resulting in the macroscopic mechanical properties of shale being controlled by bedding planes, exhibiting significant anisotropic mechanical characteristics [3, 4, 5]. Excavating a tunnel in shale formations can cause disturbances to the surrounding rock mass, leading to the opening or sliding of the bedding planes within the shale, reducing its macroscopic mechanical parameters such as strength and modulus, resulting in damage and deterioration of surrounding rock mass [6]. This leads to engineering disasters such as tunnel collapse, asymmetric large deformation, and support cracking, which not only seriously affects the construction progress, but also endangers construction safety. Therefore, it is vital to systematically investigate the anisotropic mechanical properties of shale under triaxial loading, which is meant to provide a theoretical basis for the design of support and excavation parameters for tunnels in shale or other layered rock formations. Anisotropy of rock materials refers to the characteristics of mechanical parameters, structural properties, and constitutive relationships that change with direction, and numerous scholars have carried out studies on the anisotropy of rock materials [7; 8; 9]. Meanwhile, shale is often known for its remarkable intrinsic anisotropic properties, including strength, brittleness, acoustic velocity, permeability, fracture toughness, etc. [10; 11; 12; 13; 14; 15]. Various studies have first focused on the anisotropy of the basic mechanical properties of shale. For example, [PERSON] et al. [16] studied the elastically anisotropy of Chicopee shale and found that the elastic anisotropy of shale is due to the combined effects of pores or cracks and mineral grain orientation and decreases with increasing confining pressure. [PERSON] et al. [17] investigated the anisotropic mechanical behavior of the Tournemire shale and found that the plastic deformation and failure mode of the Tournemire shale is anisotropic and strongly depends on confining pressure. [PERSON] et al. [18] addressed that the brittleness anisotropy of Longmaxi shale samples was bedding angle-dependent, and the anisotropy of brittleness decreased with increasing confining pressure through the triaxial experiments under various confining pressure. [PERSON] et al. [19] conducted an experimental study on the mechanical behavior and brittleness characteristics of Longmaxi shale, and the anisotropic behavior of shale specimens under conventional triaxial compression, including the strength, brittleness, deformation, and failure behaviors, was analyzed. [PERSON] et al. [20] studied the damage evolution and acoustic emission (AE) characteristics during failure process of anisotropic shale and revealed the effect of different layer inclination angles between the loading direction and the bedding planes. [PERSON] et al. [21] analyzed the characteristics of the anisotropic tensile strength of layered rocks and three different anisotropic tensile failure criteria were reviewed and compared. [PERSON] et al. [22] investigated the tensile mechanical properties and damage patterns of shale with different various orientations of the bedding planes by conducting a Brazilian splitting test and an AE test. [PERSON] et al. [23] conducted a high-temperature triaxial permeability testing machine to study anisotropic permeability of oil shale at stresses that emulate in situ conditions. [PERSON] et al.