Numerical Analysis For Dynamic Response And Liquefaction Potential Of Seabed Under Wave Loading

The analysis for dynamic stability of seabed or offshore foundation under wave loading is of practical significance in the design and construction of offshore and coastal structures. When waves propagate over the ocean surface, a sequence of wave pressure is induced on the mudline or seafloor, which causes the coupling interaction between the deformation of soil skeleton and seepage movements of porous fluid. Different from cyclic loading acted on onshore foundation, the wave pressure plays double roles as: the surface loading imposed on the mudline and the boundary condition of excess pore pressure. During long-time storm, the excess pore pressures will be built up, then dissipated and re-distributed simultaneously due to partial drainage. Then it is possible the developments of excess pore pressures cause the liquefiable sandy seabed be liquefied. In this dissertation, the methods of linear and nonlinear analysis for dynamic responses of sandy seabed under waves loading are developed.During the past three decades, attentions have been paid to two different issues: analysis for free-field dynamic response of elastic seabed and numerical evaluation of residual pore pressure in the seabed. The former is based on Biot's consolidation theory, and the seabed soil is taken as linear-elastic material. According to this method, the distribution of amplitudes of transient pore pressures and transient effective stresses are assessed, while the residual pore pressure and accumulated deformation cannot be taken account. The latter is developed on the basis of the effective stresses analysis of dynamic response of soil structure and foundation under seismic loading in the field of earthquake engineering. The complex effective stress state of seabed soils under waves is simplified to pure shear stress. Accordingly, the parameters of equivalent visco-elastic model are determined through cyclic shear testing. The governing equation of the boundary-value issue for the response of seabed is separated into dynamic equation and consolidation equation, and the incremental excess pore pressure under undrained condition has to be included into calculation at the beginning of every time step. The mudline is treated as drained boundary. In fact, the constitutive models employed in both methods could not re-produce the dynamic behaviour of soil to non-proportional cyclic loading, such as waves loading. To simulate dynamic responses of elastic, visco-elastic and elasto-plastic sea beds, linear and/or non-linear numerical models based on generalized Biot's theory are developed together with stable and effective algorithm.The difficulty in numerical computation made the generalized Biot's theory be difficult to be put into practice in the field of geotechnical engineering. For the boundary-value problem,such as dynamic response of seabed, the finite element formulations of u~U and u~p forms of generalized Biot's theory are established in this dissertation. Usually the direct solution of u~p form is viewed as rather difficult. A procedure is developed by the author. In addition, the stagger solution procedure is enlarged to u~p form I and II. The direct solution for u~U and u~p form, the stagger solution for u~p form are compared in the convergency, stability and efficiency, from which it is concluded that the first solution used not widely before is characterized with weakly ill-conditioned stiffness and comparatively high efficiency. The finite element methods based on these three solving procedures are numerically implemented.The seabed soil under wave pressure with small amplitude can be viewed as elastic material. Depending on the numerical model proposed, the effect of accelerations of soil skeleton and porous fluid on the dynamic response of elastic seabed is examined. It is possible that the amplitudes of effective stresses in large-thickness seabed to high-frequency waves would be underestimated in some regions if the accelerations were overlooked. The responses of layered seabed, seabed with gentle slope and the sea...