## 6 Conclusions  
The purpose of this study was to minimise the effect of NATM widening blasting on the pilot tunnel and TBM surroundings by using a protective shield during TBM and NATM widening parallel excavation. Accordingly, after comparing the numerical analysis results under the same conditions with the actual field measurement data, half-section blasting was modelled and compared with the results using the TBM and NATM parallel construction method. In addition, the effect according to the tunnel excavation depth and NATM and TBM excavation cross-sectional size was compared and analysed. The main results are as follows. 1. Vibrations from the actual excavation were measured on the ground surface of the GTX-A construction site where tunnels were excavated only by blasting, and it was confirmed that similar results were obtained from modelling and analysis under the same conditions (the maximum vibration was measured at 0.11 cm/s). Based on this, the difference in vibration between the TBM and NATM parallel construction method using the protective shield and when the tunnel half section was excavated only by blasting excavation were compared, and the results were organised according to the tunnel excavation depth. As a result of measuring the vibration at the same point as the field measurement, when the tunnel excavation depth was 20.0 m, the vibration velocity was reduced by 75.23% by the parallel excavation method, compared to blasting excavation. Further, it was observed that the greater the depth, the greater the reduction (30.0 m depth = 79.16%, 40.0 m depth = 81.81%, 50.0 m depth = 83.33%). Therefore, it is judged that large-depth tunnel excavation has a greater vibration reduction effect than low and medium-depth tunnel excavation. 2. In the case of an expansion blasting cross-section of 10.0 m, as the pilot tunnel size increased from 3.0 m to 5.0 m, the vibration velocity measured at the rear of the TBM and the excavation surface of the TBM decreased by 22.17% and 15.55%, respectively. When the expansion blasting cross section was extended to 15.0 m and 20.0 m, the vibration occurred more extensively, and as the TBM pilot tunnel size increased from 3.0 m to 5.0 m, the vibration reduction width also increased along with the expansion blasting cross-section. Due to the condition of the TBM and NATM construction method, which is blasted with an outer shell hole, it seems that the vibration velocity increases as the expansion blasting cross-section and the number of blasting holes increase. The tendency for the vibration velocity to decrease as the size of the TBM pilot tunnel increases to 3.0 m and 5.0 m is considered to be because securing a free surface has a great effect on reducing the vibration velocity. 3. These results show that the vibration velocity reduction width increases as the expansion blasting cross-section increases, but when the expansion blasting cross-section is 20.0 m or more, it exceeds the maximum domestic standard of 4.0 cm/s, as well as foreign standards. Thus, actual construction is expected to be difficult. Therefore, when using the TBM and NATM parallel construction method using a protective shield, the area of the widening blasting surface should be preferably 10.0 m or less, which is measured at a vibration velocity of 1.0 cm/s. Further, considering the cost aspect, it is effective to construct the free surface as large as possible. 4. This study was conducted based on the measurement data of a specific site, and further research on various ground and blasting conditions is needed. Moreover, when excavating an underground tunnel, we plan to study a method to protect the TBM backup device from blasting rockslide during the TBM and NATM parallel construction method. In addition, studies on the force (the torque, thrust force, chamber pressure, and upward force) acting on the TBM and tunnel lining, which were not considered in this study, will be added. **Funding:** This research was supported by the Korea Agency for Infrastructure Technology Advancement (KAIA).