Wave & current-induced progressive liquefaction in loosely deposited seabed

Quaternary newly deposited loose seabed soil widely distributes in offshore area in the world. Wave-induced residual liquefaction in loose seabed floor brings great risk to the stability of offshore structures in extreme climate. Understanding of the characteristics of wave-induced residual liquefaction in loose seabed is meaningful for engineers involved in design of offshore structures. In this study, wave & current-induced residual liquefaction in loose seabed floor is investigated deeply and comprehensively adopting a validated integrated numerical model. The time history of wave & current-induced pore pressure, effective stress, shear stress, lateral pressure coefficient K-0, stress angle, displacement of seabed soil are all quantitatively demonstrated. The variation process of progressive liquefaction, stress path, as well as stress-strain relation also are illustrated in detail. The classic effective stress principle has been modified to describe the nonlinear phenomenon that the reduction rate of vertical effective stress sigma'(Z) is faster than that of horizontal effective stress sigma'(x) and sigma'(y) accompanying residual pore pressure build up. It is shown that the integrated numerical model FSSI-CAS 2D incorporating PZIII soil model can effectively and precisely capture a series of nonlinear dynamic response characteristics of loose seabed floor under wave & current loading. The computational results further confirm the wave & current-induced liquefaction in loose seabed soil is progressively downward, initiating at seabed surface. Besides, it is found that three physical processes, including vertical distribution of oscillatory pore pressure, time history of stress angle as well as lateral pressure coefficient K-0 could be taken as indirect indicator to judge the occurrence of wave-induced residual liquefaction, and predict the residual liquefaction depth in loose seabed. It is also found that the progressive liquefaction process is significantly affected by wave height, permeability and saturation of seabed soil.