Dynamic response and liquefaction mitigation of immersed tunnels under seismic loads: A coupled seawater-seabed-tunnel analysis

In recent years, immersed tunnels have been widely used in cross-river and cross-sea transportation projects due to their unique advantages. Unlike land-based tunnels, immersed tunnels are embedded in the shallow layers of nearshore seabeds, where the combined influence of seawater and seabed soil makes their seismic response more complex. Traditional seismic analysis methods for land tunnels are therefore less applicable. In this study, the DM04 model is adopted to simulate the mechanical behavior of marine sand, and the Coupled Acoustic-Structure (CAS) method is employed to model the dynamic interaction between seawater and the seabed, establishing a coupled system of seawater, seabed, and immersed tunnel. Using real seismic records from a marine region as input, this study investigates the effects of horizontal and vertical seismic intensities, overlying seawater, and sand compaction piles (SCPs) on the seismic stability of tunnel. The results indicate that under seismic loading, soil surrounding the tunnel liquefies earlier than soil in the far field, and significant seabed deformation may lead to buoyancy-induced instability. Stronger horizontal and vertical seismic motions increase the uplift and tilt of the tunnel. Seawater amplifies uplift displacement but reduces rotational motion during the uplift. SCPs effectively enhance seismic stability by suppressing soil liquefaction beneath the tunnel and limiting lateral soil flow, thereby mitigating uplift and rotation.