## 1 Introduction  
With the development of urban areas, the above-ground space has reached a saturation point, and as an alternative to this, the development of underground space is receiving significant attention. In recent years, underground spaces have been used not only for transportation infrastructure, but also for underground facilities such as power stations, communications, water supply and sewage, and complex residential and cultural areas.  
Excavation methods using the new Austrian tunnelling method (NATM) and tunnel boring machine (TBM) are representative construction methods for excavating such underground spaces. NATM is a method based on equilibrium theory, which leads to new equilibrium elastic or elasto-plastic states, contrary to conventional tunnels designed and constructed based on the relaxation load theory [1]. NATM is advantageous for cross-sectional construction, and realises excavation relatively cost-effectively in various ground conditions. However, it has the disadvantages of high overbreaks, low excavation velocity, and noise and vibration due to blasting. Compared to the blasting construction method, mechanised excavation using TBM has the advantage of low noise and vibration, higher safety, and cost- and time-effectiveness in long tunnels. However, it is difficult to change the excavation section, and the initial investment cost is high. Thus, TBM is not economical for tunnels with short distances.  
To compensate for the disadvantages of the blasting construction method and mechanical excavation using TBM, several construction methods have been developed, and a representative construction method is the TBM and NATM parallel excavation method. TBM and NATM parallel excavation is superior in that it exhibits low noise and vibration as TBM mechanical excavation, and economic feasibility and flexible excavation cross-section of blasting can be realised. TBM and NATM parallel excavation is employed in performing preliminary excavation using a small-diameter TBM, and simultaneously enlarging the excavation cross-section through back explosive blasting according to the desired tunnel size and shape. This method can obtain a free surface and exclude centre-cut blasting that causes vibration, which significantly reduces noise and vibration. Additionally, it has the advantage of enabling follow-up measurements according to the condition of the bedrock, because it is possible to investigate the ground condition before wide excavation through the TBM preceding tunnel.  
However, there is a lack of research on TBM and NATM parallel excavation according to various ground conditions, tunnel depth, size and shape, and the effect of back blasting using explosive charges on the front TBM mechanical device and preceding tunnel. Moreover, the required minimum offset from the blasting point needs further clarification. Such results are expected to enhance cost-effectiveness by securing the stability of the construction method and shortening the existing construction period.  
In this study, 3D numerical analysis was used based on the vibration data measured on the upper ground surface of the site using the NATM blasting construction method to compare the existing blasting method and the TBM and NATM parallel construction method. For the application of the TBM and NATM parallel construction method, the vibrations generated during the blasting of the upper halves were compared, and the reduction rate of vibrations measured on the ground surface according to the excavation depth of the tunnel was analysed. In addition, based on the experiment conducted in [2], the blasting vibration reduction effect due to pilot tunnel sizes for a total of six cases was predicted by modelling pilot tunnels of 3.0 m and 5.0 m excavated using TBM, and 10.0 m, 15.0 m, and 20.0 m using NATM tunnels. This data was compared to the standard about vibration. This study suggests the appropriate depth and size of the tunnel excavation. It is expected to increase reliability and safety.