# Numerical Study of Pressure Attenuation Effect on Tunnel Structures Subjected to Blast Loads
## 4 Results
### Pressure Reduction Module Effect Analysis
#### 4.1.3 Linear Tunnel with Double-Pressure Relief Orifice Plate Explosion  
The variation of blast attenuation inside the tunnel of this experiment is shown in Figure 9. According to the experimental results, the transfer of the blast inside the tunnel will decrease as the distance increases. In order to know the blast attenuation characteristic of the double-pressure relief orifice plate, as designed by reducing the section, we analyzed and discussed the blast transfer rate and variation rate of the double-pressure relief orifice plate. In terms of the blast transfer rate, the linear tunnel with a double-pressure relief orifice plate was tested, and when the blast was transferred from the larger tunnel section (pressure transducer position P3) to the double-pressure relief orifice plate (pressure transducer position P4), the blast transfer attenuation in P3 and P4 was analyzed. In terms of the pressure attenuation rate, the linear tunnel with a single-pressure relief orifice plate and the pure linear tunnel were tested, and the pressure attenuation in position P4 was analyzed and discussed. The pressure transducer positions P3 and P4 were taken as examples to describe the effect of a double-pressure relief orifice plate on the blast transfer rate. When the quantity of C-4 explosive was 100 g, before the blast was transferred to the double-pressure relief orifice plate, the measured blast value in pressure transducer position P3 was 272.79 kPa. When the blast was transferred through the double-pressure relief orifice plate (pressure transducer position P4), the measured blast value was 38.18 kPa. Therefore, the blast transfer rate in pressure transducer position P4 was 0.14. In other words, when the blast was transferred from position P3 of the larger tunnel section (side length 30 cm) to position P4 of the smaller tunnel section after the path was changed, the double-pressure relief \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Weight of C-4 (g)**} & **100** & **150** & **200** & **250** & **350** \\ \hline \multirow{5}{*}{Position} & P1 & Experiment & \multirow{2}{*}{Explosion pressure (kPa)} & 784.67 & 1164.00 & 1777.00 & 1763.02 & 2443.99 \\ \cline{2-7}  & Simulation & & 792.96 & 1006.73 & 1623.96 & 1721.41 & 2175.51 \\ \cline{2-7}  & P2 & Experiment & \multirow{2}{*}{Explosion pressure (kPa)} & 457.43 & 642.93 & 760.66 & 908.14 & 1502.18 \\ \cline{2-7}  & Simulation & & 584.15 & _739.17_ & 863.49 & 960.17 & 1126.15 \\ \cline{2-7}  & P3 & Experiment & \multirow{2}{*}{Explosion pressure (kPa)} & 300.49 & 367.77 & 454.65 & 482.50 & 747.77 \\ \cline{2-7}  & Simulation & & 568.95 & 617.27 & 655.25 & 702.23 & 837.73 \\ \cline{2-7}  & & Explosion pressure (kPa) & 123.89 & 170.62 & 242.00 & 272.76 & 348.09 \\ \cline{2-7}  & Experiment & \multirow{2}{*}{Blast transfer rate (\(R_{trans/a}\))} & 0.41 & 0.46 & 0.53 & 0.57 & 0.47 \\ \cline{2-7}  & & \multicolumn{5}{c}{Average: 0.49} \\ \cline{2-7}  & & Explosion pressure (kPa) & 117.91 & 144.36 & 166.38 & 182.05 & 211.96 \\ \cline{2-7}  & Simulation & & 0.21 & 0.23 & 0.25 & 0.26 & 0.25 \\ \cline{2-7}  & & \multicolumn{5}{c}{Average: 0.24} \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of experiment and numerical results of steel tunnel with single-pressure relief orifice plate. - orifice plate reflected the blast, reducing and detouring the throughput, and the blast attenuation amplitude was 86%. The blast transfer rate (\(R_{transfer}\)) is expressed as follows:  
\[R_{transfer\_double\_orifice}=\frac{P4_{double\_orifice}}{P3_{double\_orifice}} \tag{7}\]  
where \(P3_{double\_orifice}\) and \(P4_{double\_orifice}\) are the measured blast at the pressure transducer position, \(P3\) and \(P4\), in the linear tunnel with single orifice plate attenuator, respectively. Figure 8: Blast transfer of numerical simulation of linear tunnel with single-pressure relief orifice plate explosion (100 g C-4).