Abstract

This dissertation analyzes the ocean wave-induced response of seafloor deposits. The linear wave theory is introduced for determination of the wave-induced pressure at the mud line. The seafloor deposit is assumed to be a saturated porous medium modelled by a two-phase system. The theory of mixture is used to derive the coupled governing equations. A simple material constitutive model is proposed based on the generalized plasticity-bounding surface formulation. Some comparisons of the model tests with experimental results are made. The simple model is capable of simulating both loose and dense sand behavior under monotonic and cyclic loading, drained and undrained conditions. A nonlinear finite element analysis procedure is presented. A fictitious freedom technique is used to treat the different nodal degrees of freedom due to the composition of coupled fields. A non-symmetric frontal solver subroutine is developed to accommodate the asymmetry due to the non-associated plastic flow rule. A simple artificial boundary is suggested to deal with the unbounded domain, which is easy to implement in the finite element program and universal to apply to nonlinear analysis. One important conclusion obtained from the numerical analyses is the influence of the coefficient of permeability on the susceptibility of the sand mass to liquefaction; for high values of permeability even a loose sand mass need not liquefy. This is in direct contrast to the commonly held view that the liquefaction potential is a function of density only. Two example cases are studied in detail. The first example examines the seabed response to the wave train at a pipeline site in Lake Ontario (Christian et al., 1974; Nataraja and Gill, 1983), where the deposit was measured to be loose and medium dense. The second example investigates the seafloor behavior under a storm wave as the Ekofisk tank site in the North Sea (Lee and Focht, 1975; Nataraja and Gill, 1983). Here the deposit was reported to be dense. The numerical analyses predict the susceptibility of the two sites to liquefaction which reflects the real situations. It helps build confidence in the numerical procedure adopted in this study and lends it credibility.