## 4 Discussions
Naturally, the blast-induced vibration velocity of underwater blasting is smaller than that of land blasting due to the attenuation of water fluctuation [20]. Considering that region V and region VI were composed of PC and RCC, which were located in shallow water, the charge weight per delay should meet the following formulas:  
\[PPV_{\text{limit}}\geq\max\{PPV_{x},PPV_{y},PPV_{z}\} \tag{14}\]  
that is:  
\[PPV_{\text{limit}(PC)}\geq\max\{35.23[SD]^{-1.31},198.09[SD]^{-1.84},74.12[SD ]^{-1.52}\} \tag{15}\]  
\[PPV_{\text{limit}(RCC)}\geq\max\{16.58[SD]^{-0.95},135.03[SD]^{-1.65},27.60[ SD]^{-1.04}\} \tag{16}\]  
Variation of maximum charge weight per delay with distance predicted by fitting formula are shown in Figure 10.  
Aiming to meet the requirements of each structure for blast-induced vibration, explosives charge weight per delay were calculated by [PERSON]'s empirical formula as shown in Table 3. The maximum explosives charge weight per delay in region V and VI was 61 kg and 27 kg, respectively. Figure 10: Prediction value of explosives charge weight per delay in different regions. (**a**) Region V and (**b**) region VI. Figure 9: The explosive unit consumption in land blasting and underwater blasting. The underwater blasting parameters were designed accurately, hole spacing was 1.4 m, row spacing was 1.2 m, hole depth was 12 m, and the explosives charge weight per hole was 18 kg and 23.4 kg in regions V and VI. The width of the cofferdam was 12 m with 10 rows of drill hole named K1-K10. Among them, K1-K6 rows were arranged in region VI and K7-K70 rows were arranged in region V. The blast network was shown in Figure 11. The total explosives charge weight in region V and VI was 4953 kg and 6934.7 kg, respectively. Consequently, three holes per delay were charged with 54 kg to decrease blast-induced vibration effectively in region V, while one hole per delay was charged with 23.4 kg except row K4. Row K4 was divided into two parts by the gallery, the upper explosives charge weight was 6.71 kg, and the lower explosives charge weight was 6.69 kg in region VI (Figure 12). Surface connectors of 17 ms were chosen in the whole surface detonation network. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Location**} & \multirow{2}{*}{\(PPV_{\text{limit}}\) **(cm/s)**} & \multirow{2}{*}{**Radial Distance (m)**} & \multicolumn{2}{c}{**Explosive Charge Weight Per Delay (kg)**} \\ \cline{5-5}  & & & **Region V** & **Region VI** \\ \hline Concrete structure & \textless{}10 & 250 & 120,052 & 137,560 \\ Grouting of dam foundation, & \textless{}2.5 & 95 & 687 & 606 \\ stilling basin and factory building & \textless{}10 & 20 & 61 & 70 \\ Rock mass-infrastructure surface & \textless{}10 & 80 & 3933 & 4507 \\ Stilling basin foundation & \textless{}0.5 & 142 & 148 & 27 \\ Precision control instrument & \textless{}0.5 & 142 & 148 & 27 \\ (underground power house) & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Permissible blast vibration values of hydropower station. Figure 11: Recommended blast network of region V and region VI. Underwater blasting was carried out with recommended parameters in region V and VI, and blast-induced vibration was monitored at point 1-9. Among them, point 7-9 were too far from the source explosion to measure any data. The maximum blast-induced vibration velocity in region V and VI are shown in Figure 13 and \(PPV_{\text{max}}\) was 7.65 cm/s and 4.21 cm/s, respectively. All the measured values were less than the maximum allowable values. Mean percent error (MAPE) was 13.92% and 17.26% after analyzing the prediction values and measured values of blast vibration velocity by Equation (17). The results showed that the underwater blasting parameters obtained by land blasting test can meet the blast requirements.