# Numerical Study of Pressure Attenuation Effect on Tunnel Structures Subjected to Blast Loads
## 4 Results
### Pressure Reduction Module Effect Analysis
#### 4.1.1 Linear Tunnel with Expansion Chamber Explosion
When the quantity of the explosive was \(\leq\)350 g, the one-pressure relief orifice plate design mode can reduce the blast transfer rate to 0.49, meaning the blast was transferred from P3 to P4, and the blast could be attenuated by 51% by reducing the tunnel section. The effect of the one-pressure relief orifice plate on the pressure attenuation rate was tested using the pure linear tunnel and the linear tunnel with one-pressure relief orifice plate, and the pressure attenuation in position P4 was analyzed and discussed. When the quantity of C-4 explosive was 100 g, the measured blast value of the one-pressure relief orifice plate (P4) was 123.89 kPa, and the measured blast value of the pure linear tunnel (P4) was 271.77 kPa. Therefore, in pressure transducer position P4, the pressure attenuation rate of the one-pressure relief orifice plate was 0.46, as compared with the pure linear tunnel. The blast attenuation rate (\(R_{attenuate}\)) is expressed as follows:  
\[R_{attenuate\_single\_orifice}=\frac{P4_{sigle\_orifice}}{P4_{linear}} \tag{6}\]  
According to Table 2, when the quantity of explosive was \(\leq\)350 g, the pressure attenuation rate in P4 was 0.56, meaning with the one-pressure relief orifice plate, the blast in P4 was attenuated by 44%, as compared with the pure linear tunnel (without pressure relief orifice plate). In addition, according to a comprehensive comparison of pressure transducer positions P1 to P3, before the blast was transferred to the single-pressure relief orifice plate, as the cross-section was consistent, the blast transfer of the pure linear tunnel was approximate to that of the linear tunnel with one-pressure relief orifice plate (pressure attenuation rate was 0.98 to 1.08), which matched the estimated result. Figure 7: Comparison diagram of explosion experiment results of linear tunnel with single-pressure relief orifice plate. In terms of numerical simulation, the numerical simulation result of the linear tunnel with a single-pressure relief orifice plate and the experimental blast are compared in Table 2. According to this numerical simulation, when the quantity of explosive was \(\leq\)350 g, the blast was transferred from position P3 to position P4, and the blast transfer rate of the one-pressure relief orifice plate was 0.24. Therefore, as predicted by numerical simulation using tunnel section reduction, the blast attenuation amplitude can be 76%. In terms of the blast transfer rates of the experiment and numerical simulation, the blast transfer rate of the one-pressure relief orifice plate (position P4) obtained by experimental analysis was 0.49 (blast was attenuated by 51%). Thus, the blast transfer rate predicted by numerical simulation was 0.24 (blast is attenuated by 76%). In contrast, the blast attenuation predicted by numerical simulation was larger. Both the experimental and numerical simulation results showed that the one-pressure relief orifice plate, as designed by reducing the tunnel section, was surely effective on blast attenuation. The transfer of the blast inside the tunnel is shown in Figure 8.