Dynamic Response Of The Sandy Seabed To Combined Wave And Current Loading

In the study of marine geotechnical engineering, the seabed-structures -wave interaction has been the focus of the current research. The main factor that affects the seabed structure's stability is the stability of the seabed foundation, so the research of the stability of the seabed is essential in this field. The seabed soil, which is mainly saturated soil and the stress environment is complex, its stability performance mainly include the surface displacement, the shear modulus of elasticity and changes in responses of excess pore water pressure and the seabed's liquefaction. The liquefaction of the seabed will lead to a sharp decrease of the capacity of the seabed foundation. So, the seabed liquefaction is the focus of this paper. Based on the u-p dynamic theory, the dynamic FEM equations are deduced using the finite element method for the solid phase and the finite difference method for the fluid phase. Then a fully coupled dynamic FEM program can be developed, in which the stress induced anisotropic elastoplasticity constitutive model is implemented. The poro-elastoplastic model for a sandy seabed can be established to conduct the numerical simulation. Subsequently, the excess pore pressure can be obtained to compute the liquefaction depth of the seabed according to the theory of effective stress. Firstly, a confirmatory case is conducted and the program is proved to be valid and reliable by analyzing the numerical results'convergence. The mechanics of oscillatory excess pore water pressure in seabed and residual excess pore pressure due to wave/current are discussed and the liquefaction depth which changes correspondingly is obtained. A comparison between the elastic and elatoplastic sandy seabed's dynamic responds to wave/current loading is also presented. The influence of parameters of the wave/current on the liquefaction of the seabed is also studied in this paper, including current velocity, wave period and relative water depth. This paper studies the impacts of the seabed conditions on the seabed's liquefaction depth including seabed thickness, permeability coefficient of the seabed and the relative density of the seabed. In this paper, the silt and clay seabed response to wave/current loadings are studied to conclude that this constitutive model can simulate the silt and clay seabed dynamic responses. To study the influence of the elastic-plastic parameters on the seabed's liquefaction depth, the Japan's Kobe soil, which is a representative soil which experienced dramatic liquefaction during earthquake, is used. Through the above researches, we can conclude that: this constitutive model, which is based on the super/sub-loading surface and anisotropy, can effectively simulate the behavior of the seabed under wave/current loadings and the liquefaction depth is larger than the elastic seabed. The seabed's permeability coefficient determines the response of pore water pressure, and the seabed thickness and relative density have less impact on the seabed liquefaction. The bigger the current velocity is, the longer the wave period is and the smaller the relative seawater depth is, the liquefaction depth of the seabed is smaller. The conclusions of this thesis can contribute to the design and construction of the offshore structures and the safety of the costal line as theoretical guidance.