Marine structures are commonly situated near the mildly sloping sandy seabed characterized by the slope angles (alpha) not exceeding 10 degrees. The seabed liquefaction can be triggered due to the generation of the excess pore water pressure (EPWP), posing a threat to the stability of marine structures. This study focuses on the analysis of waveinduced liquefaction in the mildly sloping (MS) sandy seabed. A dynamic poro-elasto-plastic seabed model is developed to simulate the behavior of the MS sandy seabed under wave loading. The results indicates that the loading cycle required to trigger the initial liquefaction decreased as the position moved from the toe towards the crest of the MS sandy seabed. The amplitude of shear stress increases with the loading cycle and tends to increase with growing alpha before liquefaction, resulting in a slower accumulation of EPWP with larger alpha. Both the horizontal and vertical displacements induced by wave action reach the maximum at the crest of the sloping seabed. Notably, the horizontal displacement is much greater than the vertical displacement in the seabed under wave action. The displacement of the MS sandy seabed depends on not only the shear stress amplitude developed in the soils but also the accumulation of EPWP required to trigger the liquefaction in the seabed.

Pipelines are frequently embedded in layered sloping liquefiable seabed. This study proposed a dynamic effective stress numerical model for buried pipelines in the clay-over-sand sloping seabed under wave loading. The u-p formulation of Biot ' s theory was used to describe the porous saturated seabed. The elastoplasticity of clay was determined by Mohr -Coulomb yield criterion. The liquefaction characteristics of sands was modeled by a bounding surface plasticity constitutive model. The proposed model was validated through a wave flume model test and can capture the influence of pipelines on the excess pore water pressure (EPWP) in the liquefiable seabed. The pipeline-soil interaction can significantly increase the EPWP ratio at the bottom of the pipeline under wave action. The rate of increase in the EPWP ratio at the bottom of pipelines increased as the thickness ( h c ) of upper clay layer increased. Increasing h c caused a significant increase in the settlement of pipelines in the flat seabed and the horizontal displacement of pipelines in the sloping seabed. The study also highlights the significance of considering the slope angle, the relative density of sands, and the specific gravity of pipelines.

Liquefaction poses a potential threat to the safety of subsea tunnel during the earthquake. In the ocean environment, wave pressure is considered as a normal loading applied at the seabed surface. This study proposed a fully coupled dynamic effective stress finite element model for subsea tunnel in liquefiable layered seabed under combined earthquake and wave action. Biot u-p formulation integrated with a modified generalized plasticity model was adopted for liquefaction analyses of the seabed. The proposed model was validated through a wave flume model test in terms of excess pore water pressure (EPWP). The results showed that the tunnel - soil dynamic interaction significantly increased EPWP near the tunnel. Increasing the wave height significantly increased the liquefaction depth of the seabed; however, the wave loading can either suppress the displacement of the tunnel during the earthquake or increase the uplift of the tunnel during the subsequent wave loading. When investigating seismic response of subsea tunnels in liquefiable soils under combined earthquake and wave action, the ground motion was suggested to be input simultaneously with the wave pressure. This study also highlights the effect of the thickness of liquefiable layer and frequency content of earthquakes on dynamic responses of subsea tunnels.

The nonlinear variation of wave is commonly seen in nearshore area, and the resulting seabed response and liquefaction are of high concern to coastal engineers. In this study, an analytical formula considering the nonlinear wave skewness and asymmetry is adopted to provide wave pressure on the seabed surface. The liquefaction depth attenuation coefficient and width growth coefficient are defined to quantitatively characterize the nonlinear effect of wave on seabed liquefaction. Based on the 2D full dynamic model of wave-induced seabed response, a detailed parametric study is carried out in order to evaluate the influence of the nonlinear variation of wave loadings on seabed liquefaction. Further, new empirical prediction formulas are proposed to fast predict the maximum liquefaction under nonlinear wave. Results indicate that (1) Due to the influence of wave nonlinearity, the vertical transmission of negative pore water pressure in the seabed is hindered, and therefore, the amplitude decreases significantly. (2) In general, with the increase of wave nonlinearity, the liquefaction depth of seabed decreases gradually. Especially under asymmetric and skewed wave loading, the attenuation of maximum seabed liquefaction depth is the most significant among all the nonlinear wave conditions. However, highly skewed wave can cause the liquefaction depth of seabed greater than that under linear wave. (3) The asymmetry of wave pressure leads to the increase of liquefaction width, whereas the influence of skewedness is not significant. (4) Compared with the nonlinear waveform, seabed liquefaction is more sensitive to the variation of nonlinear degree of wave loading.