Numerical evaluation for dynamic behavior of shore connection of submerged floating tunnels

Abstract

The submerged floating tunnel is a novel design of tunnel that is operated while floating in the intended depth of water. The basic research related to the submerged floating tunnel has been conducted to materialize the design, but no actual structure has been constructed yet. As the submerged floating tunnel allow a higher level of displacement than other existing tunnels, it is essential to analyze its dynamic behavior and to propose a design to ensure dynamic stability. On the other hand, in order to provide an underwater path between continents through the submerged floating tunnel, a shore connection needs to be constructed between the submerged floating tunnel and the underground bored tunnel. There is a risk of stress concentration in the shore connection where the two tunnels behave under different constraint conditions. Accordingly, the need for research to analyze the stability of submerged floating tunnel at the shore connection is emerging. Because the shore connection is the connection point between the underground tunnel, which is mainly affected by the geotechnical characteristics, and the submerged floating tunnel, which is mainly influenced by the marine environment, it is necessary to analyze the shore connection behavior considering two environmental factors at the same time. In addition, the dynamic behavior of submerged floating tunnel needs to be evaluated considering shore connection characteristics. In the previous studies, the dynamic behavior of submerged floating tunnels has been analyzed through finite element analysis considering the fluid¬–structure interaction and the hydrodynamic load applied on the submerged floating tunnel according to ocean conditions. Structural factors regarding to the submerged floating tunnels have been evaluated, and the methods for designing mooring structure and cross-section of the tunnels have been proposed. The previous studies have been attempted to improve dynamic stability of submerged floating tunnels by considering various design loads. However, the boundary condition of submerged floating tunnels was assumed to be fixed boundaries without considering the shore connection. Thus, the previous study proposing a stable shore connection design or analyzing the dynamic behavior of the shore connection is lacking. In addition, the prediction of the hydrodynamic behavior of the submerged floating tunnel is uncertain because the previous studies did not consider the boundary conditions that change according to the characteristics of shore connection and ground adjacent to the shore connection. Therefore, in this thesis, the shore connection of submerged floating tunnel was numerically simulated to evaluate the dynamic behavior of submerged floating tunnel according to the characteristics of the shore connection. Also, the ground behavior at the shore connection was evaluated according to the dynamic load transmitted from the submerged floating tunnel. A preliminary study was conducted to find out the characteristics related to the shore connection and the stress concentration occurring in the shore connection. As the result, the use of elastic joint to relive stress concentration was suggested. Based on the results of the preliminary study, the influences of design parameters were reviewed by figuring out the behavior of the ground and tunnel around the shore connection according to the stiffness of the elastic joint installed at the shore connection. The dynamic characteristics of the submerged floating tunnel according to the elastic joint and ground stiffness were analyzed, then a method for deriving the natural frequency of the tunnel according to the shore connection design was proposed. When examining the behavior of the shore connection considering the ground nonlinearity, the behavior characteristics of the shore connection and the boundary condition, which means the constraint condition, of the submerged floating tunnel changed according to the strain level of the ground adjacent to the shore connection. Due to the change in the boundary conditions at the shore connection, the dynamic behavior of the submerged floating tunnel and the magnitude of the stress applied to the shore connection changed. These results demonstrated that the behaviors of both sides affect each other simultaneously. Therefore, a precise evaluation method for the dynamic behavior of the submerged floating tunnel considering the shore connection was newly proposed by combining the methodologies for simulating the hydrodynamic behavior of the submerged floating tunnel and for simulating the nonlinear behavior of shore connection including the ground and bored tunnel.