## 4 Conclusions  
In order to study the damage characteristics, dynamic responses and damage mechanisms of different types of underground structures in rock medium under the action of the plane charge explosion, a prototype test for three types of straight-wall-arch structures under the plane charge loading was carried out. The conclusions of this research are as follows:  
(1) From the perspective of the structural macroscopic damage phenomenon, under the same plane charge explosion loading, the damage characteristics of the three types of structural test sections are significantly different. From the analysis of structural damage grade and damage degree, the damage of the C30 reinforced concrete structure belongs to a medium damage mode, while that of the high-performance reinforced concrete structure and the composite structure is classified as a slight damage mode, and the overall damage degree of the composite structure is weak. Comparing with the C30 reinforced concrete structure, the high-performance reinforced concrete structure and the composite one have excellent levels of anti-explosion performance, both of which can effectively reduce the damage grade.  
(2) From the stress response analysis of the structural inner layer, the peak stress at the arch spring of the high-performance reinforced concrete structure is greater than that of the C30 reinforced concrete structure, while the peak stress values at the inner layer of the vault, the hance, the straight wall midpoint, and the floor midpoint are less than that of the C30 reinforced concrete structure. The high-performance concrete improves the resistance of the structure by enhancing the structural strength and increasing the energy dissipation. The foam concrete soft backfill layer reduces the maximum peak stress of the structural inner layer, and can reduce pressure, isolate vibration, and improve the stress distribution around the supporting structure, thus indirectly improving the structural resistance of the composite structure.  
(3) From the analysis of the relative displacement response of the internal surface of the structure, the vertical relative displacement of the high-performance reinforced concrete test section is smaller than that of the C30 reinforced concrete test section, while the transverse relative displacement is larger than that of the C30 reinforced concrete test section. However, brittle damage does not occur in the structure. The vertical and transverse relative displacement of the composite structure are smaller than that of the high-performance reinforced concrete structure and the C30 reinforced concrete structure, and the composite structure has a better anti-explosion performance and less effect of injury.  
Conceptualization, X.N.; methodology, N.J. and R.Z.; software, N.J.; validation, N.J. and X.W.; formal analysis, N.J.; investigation, C.Z. and Y.Z.; resources, X.W. and Y.Z.; data curation, N.J.; writing--original draft preparation, N.J.; writing--review and editing, N.J.; visualization, N.J.; supervision, X.W. and R.Z.; project administration, X.W.; funding acquisition, X.W. All authors have read and agreed to the published version of the manuscript.  
This research was supported by the Postgraduate Scientific Research Innovation Project of Hunan Province (CX20200648).  
Not applicable.  
The data used to support the findings of this study are available from the corresponding author upon request.  
The authors declare no conflict of interest.