title: Seabed Instability around a Submerged Breakwater due to Dynamic Loadings
Creators: name: Li, Zhengxu
Publisher: name: Griffith University
Year: 2019
Subjects: subject: Seabed instability
subject: Breakwater
subject: Dynamic loadings
subject: Seabed liquefaction

description: A breakwater is one of common offshore structures for protecting ports and coastlines. Dynamic response of a seabed around a breakwater caused by the interactions between waves and currents is a critical aspect in evaluating the stability of the breakwater foundation. The existence of breakwater does not only affect the propagation pattern of nearby waves but also has a particular influence on the stability of the surrounding seabed. Under the interaction of waves and currents, liquefaction of the seabed foundation is one of main causes of breakwater damage, which must be fully considered in the design and construction of breakwaters. The periodic motion of waves exerts a cyclical pressure on the interface between seawater and seabed. Due to the effect of the cyclic wave pressures, the wave-induced residual pore pressure will increase, and the effective stress will decrease in the seabed, which could cause soil displacements and seabed deformation. Thus, under certain conditions, the shear failure and liquefaction of the seabed will occur. Furthermore, under the action of cyclic wave pressure, the normal stress and shear stress of the soil element in the seabed are cyclically changed which will cause the principal stress axis continuously to rotate. As a consequence, the plastic deformation of the soil is more significant, and the seabed is more prone to liquefaction. In this study, a one-way coupled two-dimensional numerical model is established integrating the fluid model and the seabed model. The soil liquefaction caused by the excess pore water pressure in the seabed is calculated by using the elasto-plastic porous medium soil model. The feasibility of the model was verified by comparison with the laboratory experiments, the centrifuge tests, and the previous numerical model data. It is shown that the numerical model can simulate the dynamic response of the seabed under wave-current interaction with high accuracy. By adopting the integrated numerical model, the dynamic seabed response generated by the rotation of the principal stress (PSR) axis is analysed under the cyclic wave loading. It is found that the PSR will affect the seabed dynamic response significantly. The liquefaction depth of the case considered PSR is much deeper than the results which did not consider the PSR effects, since the plastic strain of the soil caused by the PSR is involved in. Secondly, the dynamic response of seabed under different uniform current velocity and different wave conditions are solved. It is found that the following current accelerates the accumulation of pore water pressure, increases the displacement of the soil, and makes the seabed easier to liquefy, while the opposing current has an opposite effect. Also, the dynamic response of the seabed under wave loading is calculated, and detailed parameter analysis of the liquefaction potential of the seabed are carried out, including wave parameters (wave height, wave period), and seabed parameters (soil permeability, degree of saturation). In order to figure out the influence of the breakwater, a new-developed coupled model is established to simulate wave-seabed-breakwater interactions under cyclic wave loading. Firstly, the consolidation of the seabed under the effect of the self-weight of the breakwater is calculated. The dynamic response of the seabed around the breakwater and the seabed liquefaction depth are computed after consolidation process. Secondly, the interaction between wave and submerged breakwater is studied by the coupled numerical model. The influence of the height and crest width of the breakwater on the wave propagation is analysed. The variations of wave height in front of and behind the breakwater are compared. At the same time, the influence of breakwater height and crest width on liquefaction depth and liquefaction potential of seabed under wave action is analysed.