## 5 Conclusions  
In this study, the mutual interaction of tunnels, piles, and soil following the installation of a pile in the soil above an existing tunnel by applying allowable and ultimate loads was analyzed through a laboratory model test and a finite-element numerical analysis. The conclusions of the study are as follows:  
1. A laboratory model test was conducted to measure the effect on an existing tunnel of installing a pile nearby. When the axial force was measured using the strain gauges attached to the tunnel, the axial force acting on the side wall was larger than the axial force acting on the crown of the tunnel, and it sharply decreased as the horizontal offset increased. Under the allowable load, the decrease was up to 68.9% at the crown and 80.3% at the side wall; under the ultimate load, it was up to 55.5% at the crown and 71.9% at the side wall. Therefore, it is important to have a sufficient horizontal offset when a structure is installed near an existing tunnel.
2. The results of the vertical and horizontal displacement contours using numerical analysis show that the tunnel belongs to the influence area by a pile installation. The closer the separation distance, the closer the tunnel will be within the direct influence range. However, where the separation distance is sufficient (case 3-3), it can be seen that the effect of the pile installation is small. So, a sufficient distance between the tunnel and piles is important for the stability of the tunnel.
3. When finite element numerical analysis was conducted under the same conditions as the laboratory model test, the axial force decreased by up to 59.5% (allowable load) and 55.6% (ultimate load) at a horizontal distance of 2.0 D, and by up to 69.3% (allowable load) and 70.0% (ultimate load) at a horizontal distance of 3.0 D. This indicates that the regulations of Seoul, which designate the area within 2.0 D of the tunnel diameter as a zone that allows partial construction, and the area beyond 2.0 D as one that allows construction, are reasonable.
4. In the case of constructing piles around an existing tunnel, the suitability of existing regulations on the offset between the existing tunnel and the position of pile installation were determined using a model test and numerical analysis. Through this study, construction with a distance of 2.0 D was considered safe. However, these regulations are conservative, and construction can be performed outside the area that cannot be penetrated by structures at the discretion of the engineer at the site, depending on the site situation or economic factors. Additionally, the case where construction was performed within the offset of 1.0 D was considered dangerous because the reduction rate of the axial force generated at the top and side walls of the tunnel was less than 10%. Moreover, the standard for areas that cannot be penetrated by structures in each country (UK, Japan, and Korea) was less than 1.0 D; hence, additional research on the subject is required.  
[PERSON] wrote the manuscript and performed the numerical analyses; [PERSON] investigated and reviewed the material parameters required for the numerical and other analyses; [PERSON] and [PERSON] researched and analyzed the existing literature and relevant data; [PERSON] was responsible for the research concept, design, supervision, and final review. All authors have read and agreed to the published version of the manuscript.  
This research was supported by the Korea Agency for Infrastructure Technology Advancement (KAIA) (Project: Research of Advanced Technology for Construction and Operation of Underground Transportation 339 Infrastructure, No. 21 UUTI-B157786-02).  
Not applicable.  
The authors declare no conflict of interest.