The Influences of the Offshore Ground Motion and Site Factors on the Seismic Response of Immersed Tunnels

Because the seabed is covered with deep slit soil and seawater layer, the characteristics between offshore and onshore ground motions are significantly different. Besides, compared with sea-crossing bridges and other marine structures, the seismic response of immersed tunnels buried under seabed can be influenced by the site condition more significantly. Therefore, it is necessary to study the effects of seawater depth, site condition and offshore ground motion on the seismic response of immersed tunnels. Firstly, offshore strong motion records from the K-NET are grouped with different epicentral distances, magnitudes, and stations, and the response spectra of offshore ground motions in different groups are compared. Then, a numerical simulation model with the water layer, offshore soil layer and immersed tube tunnel coupling is established. The seismic responses of the tunnel and the seabed site are compared by inputting offshore and onshore ground motions respectively. Finally, the effects of seawater depth, incidence angle, slit soft soil layer and buried depth on the seismic response of tunnel and local seabed site are systematically studied by inputting P wave and SV wave pulses respectively. The results show that the silt soft soil layer will make the horizontal seismic response of the immersed tunnel more serious under SV wave incidence. The vertical seismic responses of the immersed tunnel and seabed are suppressed by the seawater layer under P wave incidence, and the response will decrease with seawater depth increasing. Besides, the effect of the buried depth and incidence angle on the seismic response of the immersed tunnel is limited.