## 3 Test Results and Discussion
### Crack Behavior and Failure Pattern  
The failure patterns of pre-holed specimens with different filling modes and hole angles are shown in Figure 12. Generally, the types of macro cracks in rock are mainly identified by the crack orientation in experimental tests [40; 41]. Cracks with a propagation path parallel to the loading direction and a smooth and clean surface are considered as tensile cracks, whereas cracks propagating obliquely with respect to the loading and forming visible shear bands are regarded as shear cracks. It can be seen from Figure 12a,e that the failure mode of the specimens with a square hole is mainly the shear dominated mixed failure. There are three main macro cracks leading to the final rock failure, including two diagonal shear cracks intersecting at both sides of the square hole and one wing crack initiated from the corners of the hole. Especially for the square hole with \(\alpha=0^{\circ}\), splitting fractures first occur on the sidewalls of the hole as a result of the high compressive stress concentration, and then slabbing starts at the wall and develops inwards under the combined effect of shear stress, eventually forming a V-shaped notch (Figure 12b) [42; 43]. Figure 12: _Cont._  
From Figure 12c, it can be considered that the failure mode of the specimen with \(\alpha=0^{\circ}\) under the Filling Mode B is tensile dominated mixed failure, while the specimen with \(\alpha=45^{\circ}\) is still dominated by shear failure. There are also three main macro cracks, including one tensile crack initiated from the upper or lower part of the rectangular inclusion, and two shear or mixed cracks that initiate from the inclusion corners. Different from the unfilled hole, the tensile cracks initiated from the upper and lower parts of the filled hole plays a key role in the rock failure. It should be noted that due to the relatively poor mechanical properties of the Type I infilling material, the inclusions are easily damaged by shear cracks. As shown in Figure 12f,g, the inclusion in \(45^{\circ}\) hole is cut into several fragments by shear cracks, accompanied by many particles or debris from the fracture surface of the infilling material. For the specimen with \(\alpha=0^{\circ}\) under the Filling Mode C, as shown in Figure 12d, the failure mode is shear dominated mixed failure. The specimen is basically cut into four parts by four conjugate shear or mixed cracks, showing a X shape, and obvious spalling can be observed along the crack path. While the failure pattern of the specimen with \(\alpha=45^{\circ}\) under the Filling Mode C is composed of one tensile crack and two shear cracks sliding along the inclined interface of the inclusion, showing an inverted Y shape (Figure 12h). However, the inclusions are not penetrated through by the macro cracks due to the better mechanical properties of the Type II infilling material, but the interface debonding or slipping occurs. In conclusion, the crack initiation, propagation and final failure pattern of the specimens with a square hole are affected not only by the filling mode but also by the hole angle. However, under different filling modes and hole angles, the failure mode of the specimens can be generally considered as mixed tensile-shear failure, and the difference is dominated by tensile or shear. In addition, it is found that the filling effect can effectively restrain the spalling on sidewalls of the square hole. The underlying mechanism is closely related to the stress distribution around the hole or inclusion. Notably, the effect of end friction on the crack propagation should not be ignored. Taking the specimen Sq-45-B as an example, the macro tensile crack initiated from the lower corner of the hole propagates parallel to the loading direction at first, but gradually deflects under the end friction effect when it approaches the bearing end of the specimen, finally forming a curved crack.