## 5 Conclusions  
This study proposed a damage evaluation technique based on the displacement behavior of a subway box structure. We conducted in situ measurements (leveling and crack investigation) and numerical analysis. The conclusions are as follows.  
1. The cross-sectional analysis condition, which is a current evaluation technique in structure diagnosis in Korea, cannot reflect the displacement behavior characteristics of a particular location of the subway box structure, which demonstrates a continuous behavior in the longitudinal direction. Our analysis results show that the evaluation technique, where the maximum displacement (point settlement) in a specific cross-section and an empirical value for the subgrade modulus were applied, could not consider the relative displacement difference based on the structure location. Therefore, it was concluded that it was difficult to obtain a realistic damage evaluation for the structures.
2. The validity of the evaluation technique proposed in this study was verified through a comparison between the onsite damage and computed result of the subgrade modulus (boundary condition), where the structure's displacement behavior characteristics were reflected by deriving a displacement function using a displacement measurement of the subway box structure, and applying this to the continuous elastic foundation beam theory.
3. The numerical analysis results after replacing the leveling results with the location of the box structure with the displacement function and the damage location and scale that occurred in the real subway box structure were consistent. Therefore, we confirmed that even the damage at an invisible location, which may occur in the real subway box structure, can be predicted based on 3D numerical analysis, where the displacement function is calculated using the leveling result, and the realized subgrade modulus is applied depending on the location.
4. Based on the leveling results, a displacement function was derived, and the boundary conditions of the modulus of the subgrade reaction were applied to calculate the modulus of subgrade reaction and the stress for each location through continuous elastic beam analysis. Continuous damage evaluation is possible by considering the relative displacement difference based on the location of the underground structure.
5. Using the proposed technique to analyze the damaging impact, the external boundary conditions that may induce major deformations in a subway box structure can be computed, and the members and locations where the damage may occur can be predicted and evaluated. Therefore, we can achieve reasonable damage evaluation and maintenance of the subway box structure, where the displacement behavior characteristics are reflected by repairing and reinforcing the damaged part at an accurate location through damage impact analysis techniques.  
Conceptualization, J.-Y.C. and G.-N.Y.; methodology, J.-Y.C.; software, G.-N.Y.; formal analysis, J.-Y.C. and J.-S.C.; data curation, J.-S.C. and S.-H.K.; investigation, H.-H.L.; funding acquisition H.-H.L.; writing--original draft preparation, J.-Y.C. and S.-H.K.; writing--review and editing, J.-Y.C. and S.-H.K. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.