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The excess pore water pressure generated under the combined vertical and horizontal earthquake excitations is generally greater than that under the unidirectional excitation, which may further affect the dynamic response of soil-pile-superstructure-quay wall (SPSQ) system. This study conducted a coupled effective-stress analysis for the response of SPSQ system to liquefaction-induced lateral displacement under horizontal and vertical earthquake excitations. To accurately describe the lateral displacement behind the quay wall, the liquefaction behavior of soils was described by a modified generalized plasticity model with parameters properly determined from laboratory tests. The numerical model of the SPSQ system was validated against the previously reported centrifuge model tests. The results show that the earthquake excitation with abundant low-frequency component causes greater residual lateral displacements of pile cap (ux,RC) and quay wall (ux,RQ) than that with abundant high-frequency component. The ux,RC and ux,RQ tend to increase as the peak vertical acceleration increases for both vertical and inclined piles. Increasing the fixity of the quay wall significantly reduces the influence of vertical ground motion on the ux,RC and ux,RQ. The polarity reversal of the vertical ground motion also has a significant effect on the ux,RC and ux,RQ and should be considered in the analyses. In addition, the lateral displacement of liquefied ground causes an amount of rotation of the cap of the inclined pile, probably leading to the potential shear failure at the head of piles. Special consideration should be given to the earthquake-induced shear force at deeper location because the shear force could be significantly greater than that at the head of both inclined and vertical piles in certain cases. [ABSTRACT FROM AUTHOR]

In the marine environment, the seabed contains a certain amount of clay. Experimental studies show that the liquefaction susceptibility of the sandy seabed increases as clay content (CC) rises to a certain threshold, beyond which further increases in CC reduce liquefaction susceptibility. However, numerical models that describe the effect of CC on seabed liquefaction are very limited. This study proposed a dynamic poro-elasto-plastic finite element method model for analyzing liquefaction in the sandy seabed with CC below the threshold. Based on a series of undrained triaxial compression tests on sand-clay mixtures from existing literature, a unified constitutive framework was demonstrated to be effective for describing the liquefaction behavior of sand with low CC using one set of model parameters. Existing wave flume model tests validated the effectiveness of the proposed seabed model in describing the effect of low CC on excess pore water pressure (EPWP). Numerical results confirmed that adding a small amount of clay to the seabed increased the soil contraction and thus its liquefaction susceptibility. Wave-induced liquefaction was limited to a certain depth of the seabed, and the liquefaction depth was significantly affected by the CC. Adding a low content of clay the sandy seabed significantly increased both horizontal and vertical displacements under wave action, potentially leading to the instability of the seabed. This study provides a new method for accurately assessing the wave-induced stability of marine structures built on the sandy seabed containing certain amounts of clay. • Poro-elasto-plastic model evaluates effect of CC on wave-induced seabed liquefaction. • The methodology is validated at soil element and model test scales. • Liquefaction depth of seabed is greatly influenced by CC. • Higher CC increases horizontal and vertical displacements under wave action. [ABSTRACT FROM AUTHOR]

Marine structures are commonly situated near the mildly sloping sandy seabed characterized by the slope angles (α) not exceeding 10°. 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 wave-induced 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 α before liquefaction, resulting in a slower accumulation of EPWP with larger α. 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. • A dynamic poro-elasto-plastic seabed model was proposed for analyses of wave-induced liquefaction in mildly sloping sandy seabed. • Wave-induced liquefaction characteristics of the mildly sloping sandy seabed were investigated. • Effect of D r and α on the EPWP and the associated displacement of the seabed under wave action was analyzed. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. • A fully coupled dynamic model was developed for subsea tunnel in liquefiable layered seabed under combined earthquake and wave action. • Wave pressure can either suppress the displacement of tunnels during the earthquake or increase the uplift of tunnels during the subsequent wave loading. • The ground motion was suggested to be input simultaneously with the wave for seismic analyses of subsea tunnels in the liquefiable soils. • The effect of the thickness of liquefiable layer and the frequency content of earthquakes on the dynamic response of subsea tunnels was investigated. [ABSTRACT FROM AUTHOR]

This study proposes an integrated three-dimensional time-domain finite element model for offshore wind turbine (OWT) monopile in porous seabed under multi-directional seismic excitations. Biot's u - p formulation is used to describe the dynamic interaction between the soil skeleton and pore water. Potential-based fluid elements are used for seawater to accurately model fluid–structure interaction (FSI). Moreover, FSI interface elements are applied to the interface of seawater and pile and to the interface of seawater and mud line. To analyze the response of the OWT-monopile-seabed-seawater system to multi-directional seismic excitations, a viscous-spring artificial boundary is implemented in the finite element model. The integrated model was sufficiently verified in terms of the vibration characteristics of OWTs, the displacement and the excess pore water pressure using implemented viscoelastic artificial boundary, and the FSI interaction under both horizontal and vertical excitation. The results indicated that: (1) The hydrodynamic pressure and the excess pore water pressure varied with different positions along the OWT and the pile; (2) Adding the excitation of the EW component ground motion caused the change of the distribution of transient liquefaction with respect to that under the excitation of only NS component ground motion; (3) The vertical ground motion caused a substantial fluctuation of the vertical displacement of the OWT and larger area of transient liquefaction near the surface of the seabed and near the tip of the pile; (4) resonance occurred when the frequency of input motion was approaching to the first natural frequency of OWT, leading to extremely large acceleration and displacement of OWT. • An integrated model was proposed for the seawater-pile-seabed system under multi-directional seismic excitations. • A viscous-spring artificial boundary was implemented in the integrated model. • The vertical ground motion caused larger area of transient liquefaction near the surface of the seabed and near the tip of monopile. • Resonance occurred when the frequency of input motion was approaching to the first natural frequency of OWT. [ABSTRACT FROM AUTHOR]

The seabed could be liquefied due to the wave-induced accumulation of residual excess pore water pressure (EPWP), resulting in the instability of offshore and marine structures. Thus, it is of importance to consider both the buildup of EPWP and the associated deformation behavior of the seabed. In this study, a dynamic poro-elasto-plastic seabed model, integrated with a state-dependent plasticity model, is developed in COMSOL Multiphysics. The numerical model of seabed is validated against the analytical solutions and model tests. The elliptical trajectories of soil particle at different depths of the poro-elastic seabed under the progressive wave are illustrated. The ratio of the amplitude of vertical displacement to that of horizontal displacement generally decreases as the depth increases for the poro-elastic seabed. In the poro-elasto-plastic seabed, however, the soil particles tend to move following the direction of the wave propagation. Moreover, the soil particles at the shallow part of the seabed go downwards before the initial liquefaction, while the soil particles at the lower position initially go upwards and then turn to go downwards when the initial liquefaction occurs. Particularly, the surface seabed with lower relative density is more prone to lose the stability, resulting in a dramatic upward vertical displacement. • A dynamic poro-elasto-plastic seabed model, integrated with a state-dependent plasticity model, is developed for wave-induced liquefaction analysis. • The elliptical trajectories of soil particle at different depths of the poro-elastic seabed under the progressive wave are illustrated. • The effects of soil plasticity and relative density on the soil particle trajectories are investigated. • The evolution of the soil particle trajectory before and after the liquefaction is discussed. [ABSTRACT FROM AUTHOR]