Several problems in refined local modeling for seismic response analysis of immersed tunnel

Based on the case study of a large-scale immersed tunnel and the previous research results, several problems in establishing the refined local model for seismic response analysis of the immersed tunnel are discussed emphatically as follows: (1) problem of model range size, (2) simplified method for hydrodynamic action of overlying water body, (3) the proper model of damping matrix in time domain and its effect, (4) selection of seismic input mode and its effect. The reasonable way for three-dimensional refined local modeling of the immersed tunnel is verified by engineering examples. The numerical results show that when the seismic responses of an immersed tunnel pipe is investigated carefully, at least the influence of other three pipes at two sides of the pipe must be considered and the effect of the limited soil layer whose horizontal length is at least 5 times the thickness of the soil layer at two sides of the pipe should be included. The method of equivalent additional mass can be used to simulate the overlying water body. When the fundamental frequency of the immersed tunnel-soil layer system is lower than the predominate frequencies of the input seismic wave, the ratio coefficients of the Rayleigh proportional damping matrix are determined by the optimization method based on the fundamental frequency of the system. It should be fully noted that the excitation of multi-support seismic input along tunnel axis has a great influence on the seismic response of the immersed tunnel.