## 4 Results and Analysis of Impact Tests
### Failure Features of Samples under Impact Loading
When the loading time reaches 2506 \(\upmu\)s, the two holes are penetrated by the two horizontal tensile cracks 1\({}^{\text{a}}\) and 1\({}^{\text{b}}\) between them, and the shear cracks 2\({}^{\text{b}}\) and 2\({}^{\text{c}}\) intersect with the right hole and the left hole, respectively. Meanwhile, a tensile crack 1c at the bottom of the sample propagates horizontally from left to right. Finally, the tensile crack 1c reaches to the right end, and the shear crack 2\({}^{\text{a}}\) coalesces with the shear crack 2\({}^{\text{b}}\). Obviously, we can classify the failure pattern of the specimen G3-C as tensile-shear failure. In contrast, the failure state of the specimen containing two circular holes arranged obliquely is similar to that of the specimens G2-C and G3-C at the start of dynamic loading. As the loading time rises, one shear crack 2\({}^{\text{a}}\) appears between the two holes and intersects them. After that, the other four shear cracks 2\({}^{\text{b}}\)-2\({}^{\text{e}}\) occur on the diagonals of the specimen at the moment of 2436 \(\upmu\)s. Finally, the shear cracks 2\({}^{\text{d}}\) and 2\({}^{\text{c}}\) penetrate the hole in the upper-right corner of the sample, while the shear crack 2\({}^{\text{b}}\) penetrates the hole in the lower-left corner of the sample and intersects with the shear crack 2\({}^{\text{e}}\). Thus, the failure mode of the specimen is shear-dominated failure. For the specimen with two holes arranged in a vertical direction, the initial fracture process is similar to that of the above samples with openings. In other words, the spalling cracks occur first, and then the shear cracks along the diagonals of the specimen develop towards the holes. The difference is that the rock bridge between the two cavities is not penetrated by a tensile or shear crack directly. Instead, it is seen that the upper hole in specimen G5-B is coalesced by the shear cracks 2\({}^{\text{b}}\), 2\({}^{\text{c}}\) and 2\({}^{\text{e}}\), whilst the lower hole is intersected by the shear cracks 2\({}^{\text{a}}\) and 2\({}^{\text{d}}\). The coalescence of the two holes is through the connection of the two shear cracks 2\({}^{\text{c}}\) and 2\({}^{\text{d}}\). In addition, a horizontal tensile crack 1a is found propagating from the left end to the right end of the specimen. In a word, the failure mode of the specimen can be treated as tensile-shear failure. With regard to the sample containing three holes, it is observed that spalling cracks appear first at the roofs and floors of the holes, and then the hole in the upper-left corner of the sample and the hole in the lower-left corner of the sample are, respectively, connected with the right hole in the sample through two shear cracks 2\({}^{\text{a}}\) and 2\({}^{\text{b}}\). Subsequently, the shear cracks 2\({}^{\text{d}}\) and 2\({}^{\text{c}}\) generated on the diagonals of the sample intersect with the left two holes in the sample, respectively. In the meantime, a tensile crack 1\({}^{\text{a}}\) extends from the right sidewall of the hole to the right end of the specimen. The specimen fails once the tensile crack 1a reaches the right end. Therefore, the failure type of the specimen could be regarded as tensile-shear failure. In terms of the specimen with four holes, the failure behavior is complex. Spalling cracks emerge first on the top and bottom locations of each hole. At the moment of 1964 \(\upmu\)s, tensile cracks 1a and 1b appear between the horizontal holes, and three shear cracks 2a-2c appear on the diagonals. Further on, the upper two horizontal holes are fully connected by two tensile cracks 1a and 1c, and the lower two horizontal holes are fully connected by two tensile cracks 1b and 1d. At this moment, two shear cracks 2a and 2d are connected to the tensile crack 1a, while the other two shear cracks 2b and 2c are found to be connected to the tensile crack 1b. Finally, there is also a tensile crack 1c generated in the middle of the sample, which propagates from the left end to the right end of the sample.