Submerged Floating Tunnel: Dynamic response due to earthquakes

Abstract

The submerged floating tunnel (SFT) is a conceptual idea that originates from the 19th century. The idea consists of a tunnel tube floating underwater where it is kept at a fixed depth by its buoyancy and by tethers that are anchored to the seabed and or by floating pontoons at the water surface. The tunnel is placed deep enough to avoid extreme weather conditions and to not hinder marine traffic but also not too deep to avoid high hydrostatic water pressures. The dynamic behaviour of this structure differs largely from classic immersed tunnels as it is not embedded in the bed and also from that of (suspension) bridges due to the hydrodynamic environment. This makes the dynamic behaviour of the SFT largely complicated and to this day still raises a lot of questions. This limited understanding contributed to the fact that worldwide not a single SFT has been constructed yet.
For this study, the dynamic response of a tether-supported SFT due to an earthquake will be analyzed. The main goal is to identify the main behaviour of an SFT due to these seismic events, to get an idea of how severe the damage could be and to know which measures in the design could be applied to reduce the impact.
To do so, a case study is introduced where the main dimensions of the tunnel are described. Next, a cross-sectional analysis is performed. This is done both analytically and by the use of the finite element model (FEM). The purpose of these analyses is to verify the input of the FEM. For the analytical model, a singular tether is described as an Euler-Bernoulli beam. By applying the Fourier transform method of analysis, the dynamic behaviour of this tether due to a simplified input signal is obtained. Subsequently, the same tether is modeled as a FEM by the use of DIANA FEA. By performing a time-history analysis for the same simplified input signal, the same results as for the analytical analysis are obtained which verifies the input. After that, the simplified model is used to model the entire tunnel which is used to study the dynamic behaviour of the SFT.
To perform the seismic analyses for the total tunnel, three different accelerograms of three different earthquakes are used. Before applying these signals to the model each of them is scaled to the spectra described by Eurocode to gain a more general input. The model is then used to perform two types of analysis. First, an eigenvalue analysis to gain the eigenperiods of the structure. Next, a time-history analysis to gain the seismic response of the structure. For the time-history analyses, the input is applied in the transverse and in the longitudinal direction, each as a separate analysis. From these analyses, the displacements and tension stresses in both the tunnel elements as in the tethers are obtained.
The last part of the study is used to analyze the effect of different design aspects of the tunnel. By using the case study as a starting point, multiple configurations are tested in where the effect of the number of mooring lines, number of tethers, tunnel alignment, tether inclination and the appliance of base isolation systems are examined.