## 3 Test Results and Discussion
### Strength and Deformation Properties  
The effect of filling mode and hole angle on the strength and deformation parameters of sandstone specimens under uniaxial compression are investigated, as shown in Figure 6, and the specific parameters are listed in Table 2. It is clear from Figure 6 that the peak strength, peak strain and elastic modulus of specimens containing a square hole are all lower than that of the intact specimen. In Figure 6, the average UCS of the intact specimen is 75.03 MPa, while that of the specimens Sq-0-A and Sq-45-A is 53.59 MPa and 46.22 MPa, with a decrease of 28.58% and 38.40%, respectively. Similarly, the decrease extent of peak strain is 14.00% and 21.85% (Figure 6), and the decrease in elastic modulus is 16.41% and 24.29% (Figure 6), respectively. It can be seen that the strength and deformation properties of rock are weakened more obviously by the square hole with \(\alpha=45^{\circ}\) than that with \(\alpha=0^{\circ}\)  
Figure 5: Typical stress–strain curves of specimens containing a square hole under different filling modes: (**a**) \(\alpha=0^{\circ}\); (**b**) \(\alpha=45^{\circ}\). After filling, the differences of mechanics properties between intact specimens and pre-holed specimens decrease as the filling mode changes from B to C. For the pre-holed specimens under the Filling Mode B, the UCS of Sq-0-B and Sq-45-B slightly increase compared to the unfilled specimens, and the increasing degrees are only 4.96% and 3.98%, respectively. However, for the specimens under the Filling Mode C, Sq-0-C and Sq-45-C, the UCS increase significantly by 12.65% and 14.58% respectively. The variation of peak strain and elastic modulus of the pre-holed specimens with different filling modes is similar to that of UCS. The strength and deformation properties of the specimen with Type II inclusion is the largest, followed by the Type I inclusion, and finally the unfilled holes among the three filling modes. Therefore, one can be concluded that the strengthening effect of relatively rigid inclusion on the mechanical properties of the pre-holed specimens is obviously stronger than that of the weak inclusion. This is mainly because the rigid infilling material can significantly improve the internal stress state of rocks [15; 22]. In addition, the mechanical properties of the specimens with 0\({}^{\circ}\) inclusions are always larger than those with 45\({}^{\circ}\) inclusions, and the enhancing effect of filling on the mechanical properties of the specimens with \(\alpha=0^{\circ}\) and 45\({}^{\circ}\) is almost the same at the same filling mode. In order to better understand the effect of filling and hole angle on hole deformation, two virtual extensometers with length D of 14.3 and 19.8 mm were arranged on the boundary of 0\({}^{\circ}\) and 45\({}^{\circ}\) holes respectively by DIC, to obtain the variation of hole aperture with time in horizontal and vertical directions (\(\Delta\mathrm{D_{x}}\) and \(\Delta\mathrm{D_{y}}\)) before the peak stress, as shown in Figure 7. It can be seen that before about 5 s, the deformation of the hole in the horizontal and vertical directions is basically 0. After that,  
Figure 6: Effect of filling mode and hole angle on the mechanical parameters of specimens: (**a**) peak strength \(\sigma_{c}\); (**b**) peak strain \(\varepsilon_{c}\); and (**c**) elastic modulus \(E_{\mathrm{t}}\). the value of \(\Delta\)D\({}_{\rm x}\) is always positive and increases with time, which indicates that the hole expands horizontally under the external force, resulting in interface debonding or slipping between the rock matrix and inclusion [26]. On the contrary, the value of \(\Delta\)D\({}_{\rm y}\) is always negative and decreases with time, indicating the hole shrinks in the vertical direction under the external force. Obviously, the deformation of the holes under the three filling modes is different. Generally, the deformation of the unfilled hole is the largest, followed by the hole filled with Type I inclusion, and finally with the Type II inclusion.