# Stability Evaluation of TBM Pilot Tunnels to Rear Blasting Using the Protection Shield
## 2 Literature Review  
NATM is based on the equilibrium theory in which an equilibrium state is induced in an elastic or elastoplastic state; by contrast, a conventional tunnel is designed and constructed based on the tunnel load theory [1]. Because the surrounding ground primarily supports the tunnel in NATM, the strength of the rock mass should be preserved as much as possible and support materials, such as rock bolts and shotcrete, should be installed to minimize the ground deformation during tunnel excavation. In addition, NATM affords good constructability and economic efficiency because it can be easily applied to a wide range of cross sections and it is widely used owing to its adaptability to changes in ground conditions and its versatility with auxiliary construction methods. Therefore, as NATM construction methods are finding increased use, studies have focused more on the development of NATM-based excavation methods and their support materials [2; 3; 4; 5; 6]. TBM is widely used for excavation because it affords advantages such as high-speed and stable excavation, minimization of ground and environmental damage, noise, and vibration. Therefore, TBM has been increasingly used for tunnel construction worldwide [7; 8]. Although TBM is an effective tunnel excavation method, it requires equipment to be setup according to the ground and site conditions, and accidents have often occurred due to unexpected ground conditions and equipment defects during construction. Therefore, many studies have investigated TBM tunnel excavation designs [9; 10; 11]. The TBM and NATM simultaneous excavation method is safe because the ground conditions can be investigated through the TBM pilot tunnel, and the support material can be installed in advance when a fracture zone is observed. Further, vibration and noise reduction effects can be obtained because the free face can be secured from the pilot tunnel, and center-cut blasting can be avoided. In addition, it provides the advantages of both NATM blasting and TBM excavation, such as shortening the construction period through simultaneous excavation. However, if a certain excavation length is not secured, the relatively high initial cost of using the TBM results in poor economic feasibility. The TBM and NATM simultaneous excavation method was first developed by the Italian Railways Authority in 1982. This method can reduce construction costs by 50% per unit volume  
Figure 1: Simultaneous excavation using tunnel boring machines (TBM) and New Austrian tunneling method (NATM) enlargement with protection shield. compared to those of NATM blasting for high-grade rock (e.g., rock mass with a high RMR or Q classification score) [12]. Further, the constructability can be improved when the TBM pilot tunnel has a small diameter of 3.5-5 m; in particular, the construction cost can be reduced by 20% due to the pre-ground investigation of the pilot tunnel [13]. The TBM and NATM simultaneous excavation method has already been applied worldwide. For example, a railroad tunnel in Canada was constructed by simultaneously using NATM in the lower half of the excavation and TBM in the upper half. In Korea, the TBM and NATM simultaneous excavation method has been used in the construction of the Seoul subway, an urban expressway in northern Seoul, and a bypass in Busan Haeundae. However, the overall number of construction cases and studies remain insufficient. Further, the current TBM and NATM simultaneous excavation method does not prevent NATM enlargement blasting from causing some faults in the TBM and backup installations. In this light, this study fabricated and installed a protection shield between a NATM blasting point and a TBM. In keeping with previous studies, we used a TBM with a small diameter of 3 m. Further, the stability of the TBM and NATM simultaneous excavation method with the protection shield was evaluated by analyzing the required minimum distance and its effect on the ground behavior.