Earthquake Induced Behavior of Submerged Floating Tunnels with Tension Leg Anchorage

Abstract

SUMMARY:The purpose of this study is to examine the behavior of a submerged floating tunnel (SFT) anchored by tension legs and subjected to seismic excitation. A proposal of a tunnel crossing the Høgsfjord in Norway is used as a case for the finite element model. The tunnel was proposed as 1345m long with a cross section diameter of 11.3 m. Furthermore, the tunnel should lie 25m under the sea surface to enable regular sea traffic. A seismic analysis of the SFT is performed using a pseudo-excitation method (PEM), which is based within the framework of random vibration methods. The coherency between different supports is studied, in particular the incoherence and wave passage effects. A simple constant incoherence model is introduced, for coherency between inter-components of motion. Three cases of the incoherence are then examined: (i) fully coherent, (ii) incoherent and, (iii) non-coherent. The seismic analysis is carried out for different velocities and angles of the propagating earthquake waves. The structure is modeled in Abaqus, by using beam elements for both the tunnel and the tension legs. The fluid/structure interaction is modeled and its effects on the structure are discussed. The SFT is modeled with both vertical and inclined tension legs and the behavior of the models are investigated, with regard to seismic events. The seismic analysis is performed in Matlab, by developing m-scripts based on the system matrices generated in Abaqus. The results from the analysis confirm the importance of accounting for the stochastic nature of seismic waves. Furthermore, the results suggest that submerged floating tunnels with tension leg anchoring are reliable when subjected to earthquake excitation. It is unlikely that earthquake action will be taken as the design load case for Norwegian conditions.