Several previous research works have discussed the advantages of stable

Several previous research works have discussed the advantages of stable tunnel excavation and the mechanisms of auxiliary methods. Since tunnels are often driven through soft ground containing groundwater and in locations close to various utilities and structures, Kimura, Ito, Iwata, and Fujimoto (2005) applied two methods, namely, special jet grouting for foot piles and long steel pipe fore-piling for preventing displacement, and a boring method for groundwater drainage. Oke, Valchopoulos, and Marinos (2014) analyzed literature and construction reports and discussed the effect of the umbrella arch (UA) by classifying three types of support elements: spiles, forepoles and grouted. Yoo (2002) investigated the behavior of a tunnel face reinforced by longitudinal pipes using a 3D finite quinacrine analysis. Based on the numerical results, he concluded that the face-reinforcement technique using longitudinal pipes could significantly reduce the deformation of the face and thus improve its stability. Kamata and Mashimo (2003) researched the effects of several auxiliary methods, such as face bolting, vertical pre-reinforcement bolting and forepoling, through centrifugal modeling tests on sandy ground and numerical simulation with DEM. They identified several favorable effects in terms of face stability. Taguchi et al. (2000) conducted model and full-size tests on a thin flexible pre-lining. They concluded that the pre-lining was effective for both the stability of the face and the prevention of ground surface settlement. They also proposed a quantitative estimation method for face stability. Kitagawa et al. (2009, 2010) performed trapdoor experiments and a numerical simulation to determine the effect of a reduction in settlement and the corresponding mechanism using a tunnel foot reinforcement side pile. Cui, Kishida, and Kimura (2008) performed numerical simulation of a tunnel excavation and a side pile with the aim to prevent surface settlement of the shallow overburden and the soft ground. Based on the numerical simulation, they proposed that the prevention of ground surface settlement and tunnel settlement, by the installation of a foot reinforcement side pile, affects the shear reinforcement, the load redistribution and the internal quinacrine pressure. They also advised that the foot reinforcement side pile should be installed across the shear zone during tunnel excavation.
Several tunnels constructed for the Tohoku bullet train in Japan, the so-called Tohoku Shinkansen Railway, between Hachinohe and Shichinohe-Towada, were constructed under the condition of shallow overburden and soft ground. In cases without any obstacles on the ground surface, the objective ground was improved using the shallow or deep mixing stabilization method. Then, the tunnel was excavated by NATM. This approach constitutes the ground improvement method of the excavation of a shallow overburden tunnel. Fig. 1 shows the construction process associated with this method. First, the ground is excavated to the upper part of the tunnel crown. Then, cement is mixed with the natural ground around the sidewall of the tunnel using the shallow or the deep mixing stabilization method. The premixed soil is spread and compacted by rolling it over the tunnel crown area. Finally, the excavated soil is backfilled and compacted by rolling it to the ground surface. The tunnel can then be excavated using NATM. Various combinations of improved areas and levels of strength of the improved ground were implemented in the field, and the tunnels were excavated successfully. The ground improvement method was employed after considering the conditions of the overburden, the geology, the ground surface, the allowed settlement, and data from several previously reported construction projects (Kitagawa, Isogai, Okutsu, & Kawaguchi, 2004; Nonomura, Iura, Okajima, & Kishida, 2011; Saito, Ishiyama, Tano, & Haga, 2011; Tadenuma, Isogai, Konishi, Nishiyama, & Okutsu, 2003). Without disturbing any buildings and houses on the surface, this method has the advantage of pre-knowledge of the geological structure. Consequently, this method is more advantageous in terms of construction costs than other auxiliary methods, as shown in Fig. 2.