## 3 Test Results and Analysis
### The Structural Dynamic Response Analysis
Simultaneously, it can reduce the pressure, isolate the vibration, and improve the stress distribution around the supporting structure, thus indirectly enhance the structural resistance of the composite structural test section. By analyzing the stress response of the structural inner layer of the three different types of structures, it is found that among these three structures, the composite structure possesses the most effective wave absorption and energy dissipation effects. After the foam concrete energy dissipation layer is added to the composite structure test section, the mean attenuation rate of the peak stress of the structure achieves 55.34%. Based on the research results of existing scholars on the anti-explosion performance of underground structures, the attenuation rate of the foam concrete backfill layer to the peak stress is summarized in Table 8.  
The time-history curves of the measured vertical relative displacement between the vault and the floor and the measured transverse displacement between the left and right straight walls for the structures, after enduring the explosion load, are shown in Figure 13. It can be seen from Figure 13 that the vertical relative displacements of the test sections of the C30 reinforced concrete, the high-performance reinforced concrete, and the composite structure are individually 52.9 mm, 31.9 mm and 19.9 mm, and the transverse ones are 26.8 mm, 46.48 mm and 3.27 mm, respectively. Under the same loading conditions, the measured relative displacement and residual displacement of the three types of structural test sections are different. Among them, for the high-performance reinforced concrete test section and the C30 reinforced concrete one, after suffering the peak values of vertical and transverse relative displacements, the plastic deformations of the structure produce irrecoverable residual displacements, while the residual displacement of the composite structural test section is small, and the total displacement mainly appears as an elastic deformation. \begin{table}
\begin{tabular}{c c c c} \hline
**No.** & **Author** & **Location of Measuring Points** & **Attenuation Rate** \\ \hline
1 & [PERSON] [26] & Mean value of measurement points & 68\% \\
2 & [PERSON] [27] & Mean value of measurement points & 95\% \\
3 & [PERSON] [28] & vault & 98\% \\
4 & [PERSON] [29] & Mean value of measurement points & 89.24\(-\)93.42\% \\
5 & [PERSON] [30] & vault & 90\% \\
6 & [PERSON] (This article) & vault & 61.8\% \\  & & Mean value of measurement points & 55.34\% \\ \hline \end{tabular}
\end{table} 
Table 8: The attenuation rate of the foam concrete backfill layer to the peak stress. In Figure 13a, there is an abnormal phenomenon in the data after the peak vertical relative displacement value in the C30 reinforced concrete test section, because it is known from the macro failure phenomenon of the structure that downward displacement occurs at the vault, while the time-history curve of the vertical relative displacement shows no residual displacement. Considering the serious damage of the structure, it is very likely that the slungshot produced by the peeling affected the test system; therefore, only the first peak displacement is used as a reference. As shown in Figure 13a, the vertical relative displacement of the high-performance reinforced concrete test section is 31.6 mm, which is 40.3% less than that of the C30 reinforced concrete test section. Based on the analysis of the macroscopic damage phenomenon of the C30 reinforced concrete test section, the compression shear damage zone appears at the hance and arch spring of the C30 reinforced concrete test section and the structural damage is serious, while the internal surface of the high-performance reinforced concrete test section is complete, with a vertical crack with a maximum width of 5 mm at the vault, and an intermittent crack at the arch spring.