## 4 Results and Discussion
### Damage Effects of Rocks under Different Water Interval Charge Configurations
#### 4.2.1 Length of Water Interval at the Bottom of the Blast Hole  
In tunnel blasting, the explosive charge at the bottom of the blast hole is relatively concentrated and far from the free surface, which is easily caused by excessive damage to the rock near the bottom of the blast hole. To avoid over-excavation of the rock at the bottom of the borehole, using the control variable method, 10 cm, 20 cm, and 30 cm water bags are set here to protect the rock while moving the explosive center of gravity upwards to improve the distribution of explosive energy. The charging structure is \(CS\#1\), \(CS\#2\), and \(CS\#3\) (\(CS\) is the abbreviation of Charge Structure, and \(CS\#1\) means the first charging structure), as shown in Figure 7. Figure 8 shows the cloud diagram of surrounding rock damage at 0.6 ms.  
Figure 8 shows that as the center of gravity of the explosives rises, the damage range shifts upwards, and the damage width at the bottom decreases. With the distance between the upper and lower explosives reducing, the damage in the middle of the hole is aggravating, which is not conducive to the damage of other parts of the rock body and the reasonable distribution of explosive energy.  
Extract the peak tensile stress \(\sigma\)tmax for 8 units in the PT partition, and extract the peak value of the damage variable \(D_{max}\) for all the monitoring units. Figure 9 shows the variation of \(\sigma_{tmax}\) and \(D_{max}\) with the depth of the borehole.  
Figure 7: Different water media length at the bottom of the blast hole.  
Figure 9 shows that in \(CS\#1\), the tensile stress fluctuates obviously in the range of 0.6 m\(\sim\)1.0 m, and \(\sigma_{tmax}\) is equal to \(\sigma_{td}\) at the hole depth of 0.9 m. Because the middle water medium is long, the stress wave generated by the upper and lower parts of the explosive cannot be effectively superimposed, and underexcavation may occur. The trend of \(\sigma_{tmax}\) of \(CS\#2\) and \(CS\#3\) is similar, which is greater than the dynamic tensile strength; a comprehensive evaluation of the damage to the rock by \(D_{\max}\) is needed. \(D_{\max}\) increases rapidly to 0.75 at the hole depth of 1 m \(\pm\) 5 cm, indicating that the rock has been completely damaged at this location. According to Figure 9b, the \(C_{D}\) values of \(CS\#1\), \(CS\#2\), and \(CS\#3\) are 50%, 70%, and 50%, and the variance \(D_{(CD)}\) values are 0.14, 014, and 0.18, respectively. In addition, the mean value of the maximum value of the damage variable \(D_{max}\) values in \(CS\#2\) is about 13.3% and 21.4% higher than that of \(CS\#1\) and \(CS\#3\), while \(CS\#3\) from 1.75 m to the bottom of the hole \(D_{max}\) values are less than 0.75. Large clumps at the bottom of the hole can easily lead to underbreak.  
At 8 cm below the hole bottom, \(D_{\max}\) is equal to 0.75 and tends to decrease rapidly under the \(CS\#2\) condition, which reduces the damage to the retained rock mass while breaking the rock. Considering the changes of \(\sigma_{tmax}\) and \(D_{max}\) values together, \(CS\#2\) can obtain a better blasting effect.