# Stability Evaluation of TBM Pilot Tunnels to Rear Blasting Using the Protection Shield
## 6 Conclusions  
This study aimed to minimize the effect of NATM enlargement blasting on a pilot tunnel by using a protection shield during the simultaneous excavation of a TBM pilot and NATM enlargement. Accordingly, the effects of the presence of the protection shield and the separation distance between the blasting point and the protection shield were analyzed through the vibration velocity produced by blasting, the plastic zone, and the crown settlement. The following conclusions were derived from this study:  
1. The vibration velocity at point A (i.e., crown part of the pilot tunnel in front of the protection shield) decreased by 36.02%, 49.29%, 58.86%, and 65.85% as the separation distance between the blasting point and the protection shield increased to 2 m, 4 m, 8 m, and 20 m, respectively, and there was almost no vibration on the ground surface directly above point A. In addition, minimal vibration was found to occur at point B (face) regardless of the use of the protection shield. Therefore, the use of a protection shield was considered beneficial in reducing the effect of the blasting on the pilot tunnel.
2. Comparisons of the plastic zone with different blasting distances showed that the plastic zone included the TBM location when the separation distance was 2 m. Further, the plastic zone did not reach the pilot tunnel and TBM behind the protection shield for a separation distance of 8 m or higher. Therefore, a distance of at least 8 m between the blasting point and the protection shield was predicted to be safe.
3. Although the underground settlement at point A was within the settlement range regardless of the use of the protection shield, it decreased by 34.02% when the protection shield was used (the result of comparing Case 1-1 and Case 2-1 with the closest separation distance). In addition, the difference in settlement amount depending on the use of a protection shield was not significant as the separation distance increased to 8 m or higher, and the difference was predicted to be insignificant at point B. Therefore, although the use of a protection shield is advantageous for stability, the pilot tunnel and TBM are considered safer if the separation distance is at least 8 m.  
Figure 12: Fabrication of protection shield.  
* As a follow-up study, a simulation test will be conducted after fabricating the protection shield. A field test will be conducted using a TBM with the same size (i.e., 3 m diameter). In the field test, the field applicability and economic feasibility will be analyzed, and an optimal separation distance will be obtained. These results should enhance the reliability of the present study.  
[PERSON] wrote the manuscript and performed the numerical analyses; [PERSON] and [PERSON] researched and analyzed existing literature and relevant data; [PERSON] and [PERSON] investigated and reviewed the material parameters required for the numerical and other analyses; [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 conducted by the research team for the \"Development of high-speed excavation technology (20 TBIIP-C155615-01) for long tunnels using Protection Shield (simultaneous excavation of TBM pilot and NATM enlargement)\" of the Ministry of Land, Infrastructure and Transport (Korea Agency for Infrastructure Technology Advancement) Technology Business Innovation Program. We would like to express our appreciation for their support on this research.  
Not applicable.  
The authors declare no conflict of interest.