## 2 Literature Review  
Due to the increasing urban development worldwide, the demand for underground space facilities is also increasing, and excavation methods, equipment, and related regulations for safer and more economical construction are continuously being studied. As of 2020, a total of 3526 road and railway tunnels were built in South Korea, with a total length of 3062.3 km [3]. In 2018, South Korea enacted the Special Act on Underground Safety, and mandated underground safety impact assessment and design. To this end, construction companies independently have been managing subsidence, vibration, and noise more strictly than domestic standards [4]. In Korea, the NATM construction method has been continuously used, starting with some subway constructions in Seoul and Busan in 1983. However, the NATM method has faced civil complaints due to blasting vibration and noise, and a poor working environment due to dust and gas. Unexpected geologic characteristics, planning and specification problems, calculation and numerical errors, construction problems, and management and control problems can cause collapse accidents caused by explosive blasting in NATM [5; 6]. Ref. [7] developed a drilling path and ground exploration complex system for identifying the geological characteristics of the excavation surface. In this case, the MSP method was used for drilling a large-diameter armed hole to reduce tunnel blasting vibration. Ref. [8] reviewed major disaster-influencing factors when excavating NATM tunnels. The measurement indicators were specified through literature and expert surveys, and those were prioritized in order of disaster impact factors with the greatest impact through surveys and analysis. In addition, many studies have been conducted to overcome the disadvantages of blasting vibration and NATM [9; 10]. TBM, which was used to excavate underground in the River Thames in England in 1825, rotates a cutter head equipped with several disc cutters to crush and excavate bedrock, and has advantages such as higher velocity, improved safety, and reduced vibration and noise compared to the NATM construction method. However, it has disadvantages such as expensive equipment, limited excavation cross-section and route, and difficulty in preparing for stratum change. Thus, extensive research on tunnel excavation using TBM is required [11; 12; 13; 14]. Ref. [15] studied the effects on the torque, thrust force, chamber pressure, and upward force acting on the TBM due to excavating complex ground. The coupled method of the DEM (discrete element method) and FDM (finite difference method) was used for the TBM excavation model. Ref. [16] studied that the face pressure of an EPB shield TBM was predicted using the geological and operational data acquired from a domestic TBM tunnel site. Consequently, the TBM and NATM parallel construction method, which was first developed by the Italian Railways in 1982 as a method to solve these disadvantages, can check unexpected geological characteristics through TBM pre-excavation, and has the advantages of securing a free surface due to the pilot tunnel, reducing vibration and noise due to the exclusion of centre cut blasting, and diversifying the size and shape of the tunnel by rear NATM excavation. It was shown in ref. [17] that it is possible to reduce construction costs by 50% per unit volume using TBM and NATM parallel construction compared to NATM blasting in good quality rock conditions. Moreover, it was shown in ref. [18] that when a small diameter of 3.5-5.0 m is employed in the TBM pilot tunnel, the constructability becomes excellent. Specifically, a 20% construction cost reduction can be realised due to the preliminary ground investigation of the pilot tunnel. In ref. [2], 3D numerical analysis was used to probe the impact of blasting vibration on TBM and backup devices depending on the type of bedrock and the power factor using the TBM and NATM parallel construction method. It was confirmed that the vibration velocity increased in high-density bedrock more than that in mud and soil.