## 6 Conclusions  
Presenting good compressibility, the use of the EPP foam concrete as a structural material for energy absorption and as a pressure-relief buffer layer can be justified in large cross-section tunnels such as high-stress soft rock highways and railways. Therefore, based on the obtained result, the following conclusions are drawn:  
1. The increase in the EPP particle volume content decreases the strength parameters of the EPP concrete. This is while the elastic modulus and Poisson's ratio of the concrete decrease with an increase in the EPP particle volume content;
2. Good compression and energy absorption performance are exhibited by the EPP concrete under limited compression conditions;
3. The compressive performance of the EPP concrete can be reflected by dense volume strain index \(\theta_{d}\);
4. An increase in the EPP particle content results in an increase in the strain energy density. Moreover, the group with the highest EPP particle content (64%) presents the highest strain energy density (2396.5 kJ/m\({}^{3}\)). Also, the ratio of the concentration to the total strain energy in the compression platform section also increases, indicating the energy absorption characteristics of the EPP concrete to be determined by the internal pores;
5. Similar to the axial compressive strength, the compressive strength of the composite specimen depends on the strength of the EPP concrete layer. Furthermore, the strength of the composite specimens is not only influenced by the content of the EPP particles but is greatly related to the aspect ratio of the EPP concrete layer;
6. The higher content of EPP particles results in a higher rate of energy dissipation. Moreover, the energy dissipation rate of the group with the highest EPP particle content (64%) is 60.6%, which approximately varies linearly with the EPP particle content.  
Figure 27: Relationship between energy dissipation rate and EPP particle content of laminated specimens.  
Figure 26: Contrast diagram of strain energy and density dissipation of laminated specimens.  
However, it worth mentioning that due to the lack of time, complexity of constructing engineering demonstrations of relationships and lack of practical supporting data, the findings of the present study are limited and have not been practically verified in engineering applications.  
Finally, due to its good physical, mechanical and, especially, compressibility properties, the EPP foam concrete was shown to absorb the stress energy of the surrounding rock, suggesting its broad application prospects to be researched in future studies. So, future studies are recommended to focus on the practical applications, improving the relevant practical parameters and providing better references for the formation of similar projects in the future.  
Conceptualization, C.J. and J.L.; Methodology, C.J.; Formal analysis, C.J.; Writing--original draft, C.J.; Writing--review & editing, J.L., H.L. and Z.M.; Visualization, H.L. and Z.M.; Supervision, J.L.; Funding acquisition, J.L. All authors have read and agreed to the published version of the manuscript.  
The authors acknowledge the National Natural Science Foundation of China (Grant: 51978088) and the SKLGP State Key Laboratory's independent project of China (Grant: 2021Z007).  
Not applicable.  
The data presented in this study are available in the article.  
The authors declare no conflicts of interests.