## 1 Introduction  
Underground tunnels can be excavated in a variety of ways, and their construction has continuously evolved. Tunneling methods can be classified into two categories: conventional and mechanized. Numerous tunneling projects have been completed in South Korea using the New Austrian tunneling method (NATM), which is a conventional tunneling method based on blasting. Blasting is an effective excavation method, but the vibrations and noise generated during the process affect the surrounding ground and structures and are a major source of complaints. To overcome these shortcomings, various approaches have been proposed, including the typical application of a tunnel boring machine (TBM) in conjunction with the NATM. The TBM and NATM parallel excavation method has been recognized for its ability to combine the advantages of mechanical and blasting excavation methods. Additionally, a free surface can be achieved using a small-diameter TBM to excavate a pilot hole, which reduces vibration and noise because center-cut blasting, which is the main cause of vibration, can be eliminated. Furthermore, it is possible to investigate ground conditions before widening excavation using a TBM to create a pilot tunnel, which provides an opportunity to determine the appropriate steps to be taken based on rock conditions. Based on this analysis, different shapes and sizes can be applied during NATM widening excavation. Currently, there is a lack of research on parallel excavation methods using TBM pilots and NATM widening. Studies on the effects of rear blasting on TBMs and pilot tunnels must also be conducted to determine the stability of this method and evaluate the extent to which construction periods can be shortened. Additionally, although there is a standard for permissible vibration aboveground, there is no standard for the vibration induced in underground spaces during excavation, which necessitates additional research. Accordingly, this study employed numerical analysis to analyze the effects of vibration caused by blasting on a TBM, backup equipment, tunnel face, ground surface, and other components in the case of parallel excavation following pilot excavation using a TBM. Furthermore, the effects of various ground conditions with different rock types and explosive charges were compared. Underground blasting excavation sites are primarily located in urban areas, and because urban construction involves many aboveground and belowground structures, vibration and noise complaints are increasing. However, there are no regulations regarding vibrations that affect the ground and equipment--such as TBMs--during blasting, and there has been little research on this subject. Therefore, this study investigated the permissible standards for blasting in South Korea and abroad to compare them with numerical analysis results and assess the effectiveness of using a TBM and the NATM in parallel to reduce vibration. Table 2 summarizes the permissible blasting vibration standards set forth in tunnel standard specifications. Table 3 provides a summary of acceptable blasting standards used during the Seoul and Busan subway construction projects in South Korea [13,14]. Table 4 lists the standards for acceptable blasting vibration values in various countries [15]. The aforementioned South Korean domestic standards are generally stricter than foreign standards. Domestic standards specify permissible vibration values according to the materials and usage of a structure, whereas foreign standards specify values according to population density, frequency, and other factors. \begin{table}
\begin{tabular}{c c c c} \hline
**Project Name** & **Period** & **TBM Size (m)** & **Construction Length (m)** \\ \hline New town bypass in Haeundae & December 1992–1995 & 4.5 & 2200 \\ Northern urban highway (Section 2) & December 1991–December 1996 & 6.5/7.0 & 1783 \(\times\) 2 \\ Line 5 of Seoul Subway (Sections 5–21) & December 1990–December 1993 & 3.5 & 628 \(\times\) 2 \\ \hline \end{tabular}
\end{table} 
Table 1: _Cont._