## 3 Experimental Results Analysis
### Crack Propagation and Coalescence Analysis  
The influence of preexisting fissure length, angle and distance on crack coalescence is discussed below.  
A greater preexisting fissure length more significantly affected the crack propagation and coalescence of rock-like materials. The experimental results of rock-like materials with a fissure angle of 15\({}^{\circ}\) and a distance of 20 mm are taken as examples. As illustrated in Figure 7, long preexisting fissures played a key role in the propagation and coalescence of rock-like materials. With increasing the preexisting fissure length, an increasing number of newly generated cracks were connected with the preexisting fissure, especially for failure specimen 15-20-50. Most newly generated cracks were connected with long preexisting fissures, while only one newly generated crack propagated across a short preexisting fissure. In contrast, when the long preexisting fissure was not long enough, more newly generated cracks appeared around the short preexisting fissure, as demonstrated by failure specimen 15-20-30. Therefore, the length of preexisting fissure greatly affects the propagation and coalescence of rock-like materials containing two unequal parallel preexisting fissures.  
Figure 5: UCS of rock-like materials with variations in the preexisting fissure angle. (**a**) Preexisting fissure length is 10 mm; (**b**) 20 mm; (**c**) 30 mm; (**d**) 40 mm; and (**e**) 50 mm.  
Figure 6: UCS of rock-like materials with variations in fissure distance. (**a**) 10 mm; (**b**) 20 mm; (**c**) 30 mm; (**d**) 40 mm; and (**e**) 50 mm.  
As illustrated in Figure 8, with an increase in the preexisting fissure angle, the short preexisting fissures played an increasingly significant role in rock propagation and crack coalescence. When the preexisting fissure angle was small, long preexisting fissures dominated the crack propagation and coalescence. With increasing the preexisting fissure angle, an increasing number of newly generated cracks appeared in the short preexisting fissures. Taking the failure specimens with a fissure distance of 20 mm and a fissure length of 30 mm as examples, few or even no newly generated cracks appeared in the fissures when the fissure angle was small (i.e., 0\({}^{\circ}\), 15\({}^{\circ}\), 30\({}^{\circ}\), or 45\({}^{\circ}\)). Short preexisting fissures began to develop increasing numbers of cracks as the preexisting fissure angle increased, especially in specimens with fissure angles of 90\({}^{\circ}\). It was clear that short preexisting fissures play a significant role in crack propagation and coalescence and specimen failure.  
The fissure distances also influenced the crack propagation and coalescence of rock-like materials. Short preexisting fissures played an increasingly significant role in crack propagation and coalescence by increasing the fissure distances. Examples were specimens containing fissures at an angle of 30 mm and a length of 30 mm (Figure 9) when the distance between two fissures was 5 mm, 10 mm, 15 mm, or 20 mm. Long preexisting fissures were more crucial to crack expansion and coalescence. No newly generated cracks propagated  
Figure 7: Influence of preexisting fissure length on crack propagation and coalescence. **(a)** Failure specimen 15-20-10; **(b)** 15-20-20; **(c)** 15-20-30; **(d)** 15-20-40; **(e)** 15-20-50.  
through short fissures. However, when the distances between the two preexisting crack were 25 mm, 30 mm, 35 mm and 40 mm, the short fissures had an impact on the growth of cracks and the coalescence of rock-like materials, with newly generated cracks connecting with short preexisting fissures.  
Figure 8: Influence of the preexisting fissure angle on crack propagation and coalescence. (**a**) Failure specimen 0-20-30; (**b**) 15-20-30; (**c**) 30-20-30; (**d**) 45-20-30; (**e**) 60-20-30; (**f**) 75-20-30; and (**g**) 90-20-30.