## 5 Conclusions  
This study systematically defined the methodology of implementing an underground utility tunnel, which is vulnerable to disasters and accidents, through a digital twin, and proposed the process for generating such a model. Since a digital twin has been increasingly applied for maintenance and asset management in the construction industry. This study aimed to sequentially implement the digital twin for underground facilities. Therefore, the major layers needed for configuring an underground utility tunnel digital twin are data acquisition, modeling, and service layers, which have been analyzed according to each process as follows. * Data acquisition: The key role of the data acquisition layer is collecting facility data, sensor data, and general information. Facility data can be acquired based on existing 2D or 3D drawings or through a scanning process using LiDAR technology. * Modeling: Modeling is proceeded based on data building. The key role of the modeling layer is to model the underground utility tunnel by using BIM (infrastructure \begin{table}
\begin{tabular}{p{142.3 pt} p{227.6 pt}} \hline
**Category** & **Description** \\ \hline \multirow{6}{*}{Facility management} & Status inquiry of management and accommodation facilities \\ \cline{2-3}  & Inspection of management and accommodation facilities \\ \cline{2-3}  & State information display of management and accommodation facilities \\ \cline{2-3}  & Search firefighting facility control data \\ \cline{2-3}  & CCTV control \\ \cline{2-3}  & Register field situations \\ \cline{2-3}  & Inquiry of workers’ locations \\ \hline \multirow{2}{*}{Safety management} & Check for abnormalities \\ \cline{2-3}  & Request on-site support \\ \hline \multirow{2}{*}{Abnormal signal management} & Check for abnormal situations \\ \cline{2-3}  & Propagate situations \\ \hline \multirow{6}{*}{Simulation management} & Guide evacuation routes \\ \cline{2-3}  & Suppress disaster spread \\ \cline{2-3}  & Predict damage spread \\ \cline{2-3}  & Visualization of analysis results \\ \hline \end{tabular}
\end{table} 
Table 6: _Cont._and sensor data) and GIS (spatial information). Additionally, modeling was efficiently performed by configuring a DB architecture for storing and linking relevant data and for providing services. * Service: Lastly, underground utility tunnel management technology using a digital twin consists of an underground utility tunnel management service and a response service for abnormal situations. An underground utility tunnel management service provides manned and unmanned surveillance, diagnoses, and risk inference services through a digital twin. In particular, abnormal situations in underground facilities include fire detection, spatial object displacement, earthquake disaster, and flooding. This study is significant in that the findings and proposed methodology can be utilized in the future when implementing a digital twin of underground facilities and underground utility tunnels. Disasters or abnormal situations in an underground utility tunnel can lead to serious casualties and property damage since key elements of a city such as power, communications, water supply, and heating facilities are collectively accommodated in a certain underground space. As a further step, the digital twin model proposed in this study needs to be validated in terms of its ability to accurately represent the actual site and reproduce real-world situations. In this process, it is necessary to validate and compare the amount of data actually built into the database, the accuracy of the 3D model, and its ability to accurately depict abnormal situations. In particular, studies must be additionally performed for abnormal situations in which research must focus on distance and object recognition and the displacement detection of facilities and structures based on a digital twin. Conceptualization, J.L. and C.H. ; methodology, J.L. and C.H. ; investigation, J.L., C.H. and Y.L. ; data curation, J.L. ; writing--original draft preparation, J.L. and Y.L.