A stability study of deep and double-deck tunnels considering shape and reinforcing method of an enlarged section by using numerical analyses

Recently, the necessity of deep and double-deck tunnels has been grown day by day due to the increase of traffic volume at metropolitans and thus the study on the divergence of those tunnels becomes required. Therefore sensitivity analyses were conducted with FLAC 2D program by selecting ground condition, coefficient of lateral pressure, support pattern, and depth of rock cover as parameters. Ultimately, this study is to find the optimal shape and support method of a diverged section. As the results of this study, it turned out that the box type gave higher stability of the section than arch type unlike the general thought. It can be explained that the arch type has about 30% bigger excavation area than the box type. When the ground conditions are poor, steel pipe grouting reinforcement gives higher stability than rockbolt reinforcement, but its thickness and range do not give a great influence on the stability of the enlarged section.