Soils on seabed can be liquefied under the passage of extreme waves. In this study, the liquefaction parameter is proposed that can be used to characterize the initiation of soil liquefaction. The expression of liquefaction parameter under linear waves is derived analytically based on the previous studies. In general, the maximum value of liquefaction parameter is the function of wave parameters (e.g., wave height, wave number k, and water depth h) and soil properties (soil density, permeability, and elasticity). For saturated soils, however, the liquefaction criterion depends on wave parameters and soil density only. A numerical model that solves the averaged Navier-Stokes equations is first calibrated by the analytical expression derived above for the saturated soil. The agreements between the numerical results and analytical solutions are very good. The numerical model is then utilized to study soil response to wave-current interaction, where the analytical solution is not available. Special attentions are paid to the sub-liquefaction cases under small amplitude waves, where soil is not liquefied, a more commonly encountered situation in the field. The maximum value of liquefaction parameter and vertical velocity across the soil surface, which are believed to contribute to the enhanced rate of sediment re-suspension, are plotted as the function of current speeds by using data from a series of numerical experiments.
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