Numerical Evaluation For Dynamic Response And Liquefaction Behavior Of Seabed Under Random And Non-linear Wave Loading

The analysis of dynamic response of seabed due to wave loading is of practical significance in design and construction of marine structures and offshore installations. Reccently Considerable efforts for this problem have been made with growing interest by many researchers and marine engineers, and many representative results have been achieved. It is obvious that wave loading plays a significant role in the evaluation of construction safety and seabed instability. But there are few results of research and engineering design that can consider the feature of wave loading and soil parameters together. The purpose of this thesis is to establish a reasonable numerical model to simulate the characteristics of randomness and non-linearity of wave loading. The dynamic correaltion between wave and seabed can also be described through this model. Comparative parametric studies are principally made between the proposed analysis considering actual feature of ocean situation and conventional analysis based on linear theory of regular wave. The effect of randomness and non-linearity of wave loading on the dynamic response of seabed is investigated. The necessity is discussed about considering the influence of eliminating energy on propagating wave by porous seabed. The main investigations consist of the following parts:1. The analytical solutions based on quasic-static model and dynamic model are derived and verified. The necessary condition of adoping dynamic model is discussed. Modifed expression provides the acdemic basis for the following research.2. Based on quasi-static model and dynamic model, the analytical method with complex value is adopted to derive and establish wave dispertion eqaution, which can be used to consider the influence of porous seabed on wave propagating. The interactions between seabed and wave are described through numerous compared numerical resluts of parametric studies. The results show that the variation of soil parameters and wave diagnostic parameters have great effects on wave length ratio L/L₀ and the coefficient of energy attenuation e_α.The results of aforementioned two non-dimensional parameters based on dynamic solution are less than the results based on quasic-static solution. The distributions of L/L_o and e_αverus the parameters of wave and seabed are same with each other between analitcal results based on two kinds of model. When wave propagates over shallow water region, the wave energy loses much rapidly than on deep water region. It is expected that porous seabed almost has no effect on wave propagating in the region of very deep water. Among the parameters of soil, permeabilty and shear modulus have distinct effects on the coefficient of energy attenuation e_α. Compared with the sea wave of high frequence, the sea wave with low frequence is more readily to be influenced by porous seabed. The difference between proposed wave dispertion equation and traditional wave dispertion equation is checked, which is uesful for pratical marine engineering.3. In conventional analyses of seabed dynamics, wave loading is basically treated as a deterministic process and is usually taken into consideration by using linear theory of regular wave. In fact, ocean wave is of intrinsic randomness in both time sequences and spatial distribution. The random nature of both wave and wave-induced loading will subsequently affect dynamic behavior of seabed. In this thesis, the analyses which can consider characteristics of randomness of wave loading and dynamic interaction between seabed and wave together, are formulated in a stochastic framework. Integrated numerical analysis model is established by employing wave spectrum. The comparative studies are conducted among the methods of traditional random analysis, modified random analysis, and linear regular wave theory. The results show that the amplitudes of dynamic response of seabed subjected to random wave loading are larger than that of regular linear wave loading. Therefore the stochastic feature of wave loading has to be duly taken into account in the analysis for dynamic response of seabed.4. The numerical analyses adopting finite element method are performed for dynamic response of sandy seabed subjected to non-linear wave loading in the shallow water region. The non-linear wave loading is represented by second-order Stokes wave theory and first-order cnoidal-wave theory. Numerical results are calculated for comparative studiesbased on two non-dimensional parameters, L/d and T(1/2)g/d . Compared with the numericalresults from the linear wave theory, the amplitudes of dynamic responses computed by non-linear wave theories increase. It is shown that the linear theory of wave usually underestimate the dynamic response of seabed in the shallow water region. Therefore, the effect of non-linear characteristics of wave loading should be taken into consideration in planning and design of various marine or offshore constructions and facilities.