## 3 Results
### Failure Mode  
Figure 7 shows the morphology of the samples after failure from different angles. The dotted lines indicate the location of the main cracks. The relationship between the extension direction of the crack and the position of the interface can be considered as the failure mode of the sample. There are three main forms of deformation and failure: compression, tension, and shear. Shear deformation is the main deformation on the interface, and tensile fracture is the main fracture in the sample body. The different fracture development characteristics can be subdivided into three types according to the form of movement: compression fracture, slip fracture, and bending fracture. Compression cracking is distributed in all samples, mainly in the vertical direction, which is related to the sample material. The cracks generated by slippage and tension are evenly distributed near the interface and can be clearly observed in the 15\({}^{\circ}\), 30\({}^{\circ}\), and 45\({}^{\circ}\) samples (Figure 7c-e). The cracks are arranged in a pinnate pattern pointing to the direction of deformation. In the process of shear deformation, the sample tended to slide along the contact face. As the axial load continued to increase, the normal load on the interface also increased, the interface was tightly connected, and the friction resistance increased. The sliding force was not sufficient to resist friction but reached the shear strength of the sample material; therefore; many cracks were generated on both sides of the interface. The bending fracture was mainly caused by the influence of the tunnel, which changed the overall stress distribution, and the stress concentrated around the tunnel, resulting in bending cracking. It should be noted that crack development is more complex in samples at 45\({}^{\circ}\) and 60\({}^{\circ}\). When the angle of the structural plane is 45\({}^{\circ}\), it is easy to produce stress concentration and fracture at the tip of the sample under the action of load. This situation also occurs at the position where the tunnel contacts the structural plane. The shear slip deformation occurs along the structural  
plane. Under the influence of the principal stress and the friction force of the structural plane, a large number of tensile shear cracks are generated inside the rock, which extends outward from the structure plane. When the angle of structural plane is 60\({}^{\circ}\), the influence of principal stress is more significant. According to the preliminary test, the included angle between the principal stress of the sample and the horizontal direction is 63.1\({}^{\circ}\), which is very close to the dip angle of the structural plane. Therefore, in the process of compression deformation, the structural plane becomes the principal stress plane. A conjugate shear fracture is formed with the principal stress surface generated in the other direction. On the outside of this set of conjugate fracture planes, longitudinal tensile cracks through the whole rock mass appear, resulting in significant rock mass failure. The three-dimensional distribution of fractures can effectively reflect the failure characteristics (Figure 8). The samples without an interface mainly developed vertical fractures, which constituted the fracture surfaces in the yz and xy directions, and the samples were cut into multiple blocks. Cracks appeared at the top, bottom, and arch of the tunnel. The fracture development of the sample with an interface inclination of 0\({}^{\circ}\) was centered on the tunnel and distributed symmetrically, and fracture surfaces mainly developed in the yz and xy directions. Cracks developed at the top and bottom of the tunnel, and particularly near the interface. The rock surrounding the tunnel was severely fractured. Out of the two fracture groups in the sample with an inclination angle of 15\({}^{\circ}\), one group was parallel to the x to y direction and the other group was perpendicular to the x to y direction.