# Seismic Response Analysis of Concrete Box-Type Subgrade in High-Speed Railways
## 5 Conclusions  
The box-type subgrade is a novel subgrade structure proposed in China in recent years and is currently in the early stages of research. This paper takes the lead in establishing a nonlinear integrated spatial dynamic model of ballastless track and box-type subgrade and systematically analyzes the seismic response characteristics of the box-type subgrade structure under multi-dimensional seismic actions. A comparative analysis with the seismic response of commonly used bridges is also conducted. The study outcomes are mentioned below:  
1. Due to the contact connection between the box-type subgrade and the underlying foundation, the vibration response of the box-type subgrade under seismic action is primarily the rigid body motion. The deformation of various components, such as the top slab, side walls, and bottom slab, of the box-type subgrade under seismic action is small. Overall, it can meet the seismic defense requirements of \"no damage in minor earthquakes, repairable in moderate earthquakes, and no collapse in major earthquakes\".
2. Under seismic action, there is tensile stress concentration at the contact area between the bottom of the top slab and the side walls of the box-type subgrade, but it is far below the designed strength value, with the bottom slab being in compression. In terms of vibration amplification factors, the acceleration amplification factor of the box-type subgrade is just 1.44 at resonance. Compared with traditional simply supported beams, the seismic amplification factor of the box-type subgrade is relatively small, indicating better adaptability in strong earthquakes.
3. The analysis shows that the frequency characteristics of the acceleration response time-history curve of the box-type subgrade under seismic action are basically consistent with the input seismic wave; therefore, it is suggested that the seismic alarm threshold setting for the box-type subgrade system uses the original acceleration ground motion parameters.
4. With an increase in seismic intensity, the vibration and deformation of the box-type subgrade change linearly. The deformation occurs mainly in the vertical displacement, with the horizontal displacement being relatively small and the growth rate relatively low. However, the structural force increases nonlinearly, exhibiting a slower growth at low seismic intensities. Once the design basic acceleration peak crosses 0.2 g, the force on the box-type subgrade structure increases significantly, and the rate of damage accelerates.  
Y.-Y.C.: conceptualization, software, supervision, methodology, visualization, writing--review and editing. H.X.: formal analysis, funding acquisition, methodology, investigation, writing--original draft. X.-G.S.: data curation, methodology. M.M.N.: writing--review and editing. S.-J.G.: data curation. B.-E.L.: methodology. M.M.N.: writing--review and editing. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the Open Project Fund of the National Engineering Research Center for Digital Construction and Evaluation Technology of Urban Rail Transit (Grant No. 2023 JS01) and the Basic Scientific Research Operating Expenses of Central Universities (Science and technology leading talent team project) (Grant No. 2022 JBXT010), and the National Key Research and Development Program of China (Grant No. 2023 YFB2604302).  
The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.  
Authors [PERSON] and [PERSON] were employed by the company China Railway Design Corporation Limited. The remaining authors declare that the re-search was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.