Study On Wave-induced Response Of Progressive Pore Pressure And Liquefaction In Seabed

The main objective of this research is to investigate the problem of wave-induced response of progressive pore pressure and liquefaction caused by the build-up of that type of pore pressure in the seabed. Both analytical and numerical approaches are used to investigate the problem.By using the method of mathematical physics, Terzaghi's theory is firstly adopted to study the consolidation behavior of the layered seabed. From the results of parametric study, it is concluded that the permeability, the compressibility and the thickness of each layer have great influence on the consolidation of layered soil. For layered seabed with sandwich, the effect of the permeability and compressibility of the sandwich layer cannot be reflected by traditional average consolidation coefficient method. To evaluate the spatial diffusion of pore pressure and the variation of consolidation degree of soil, an analytical solution with high precise is proposed for the layered seabed.On the basis of the solution for consolidation, combining appropriate pore pressure build-up pattern, the governing equation for the wave-induced progressive pore pressure response in layered seabed is obtained. Adopting Green function to solve the equation, the distribution and time history of residual pore pressure can be obtained. Case analysis is conducted to compare the time history of pore pressure in homogeneous seabed and layered seabed. From the calculated results, it can be concluded that the extent of pore pressure build-up in the seabed relies on the permeability and thickness of the cover layer, and that soil replacement is an efficient method to improve anti-liquefaction ability of seabed.The interface wave model for two-layered viscid fluid is proposed to simulate the dynamic properties of the "water-liquefied soil-subliquefied soil" system, which is formed when certain depth of seabed is liquefied. The disadvantage of the inviscid interface wave model is discussed. Taking into account the continuity of tangential velocity at the interface, there will be confusion for the solution of variables. Furthermore, considerable viscosity of liquefied sand was observed in shaking table tests. Rather than the inviscid fluid assumption, both seawater and liquefied soil are treated as viscid fluids and the laminar Navier-Stokes equations are used to describe the motion of the two-layered wave system. Considering the equation for mass conservation and boundary conditions at the free surface and the bottom, applying the continuity conditions at the fliud interface, the governing equation for the viscid interface wave model is solved and the dynamic characteristics are obtained, i.e. wave number, wave height, fluid depth, fluid pressure and the amplitudes of surface wave and interface wave.Applying the proposed viscid interface wave model, combined with Terzaghi's consolidation equation, considering the liquefaction front as dynamic boundary, the progressive liquefaction is simulated. The shear stress ratio in the seabed is calculated by using analytical solution. The generation and dissipation of residual pore pressure in the sub-liquefied soil layer are simulated by adopting the poro-elastoplastic model and pore pressure build-up pattern. Based on finite difference method, a program is coded. Case analysis and parametric study are conducted to investigate the effects of soil properties and wave conditions on time history and final liquefaction depth. It is found that the final liquefaction depth will be overestimated if the inviscid two-layered wave model is used. Moreover, the assumption of a constant P₀ will lead to an increase of wave height, which conflicts with the energy conservation law. A revised procedure is proposed, in which the amplitude of the surface wave is set as constant. It is found that the final liquefaction depth decreases with the increase of viscosity of the liquefied layer. The parameters in the pore pressure build-up model are very sensitive. Therefore, special attention should be paid when those parameters are determined. The influnces of analytical solutions to calculate the shear stress are compared and it is found that the liquefaction depth will underestimated if the infinite solution is adopted for a shallow seabed.The liquefaction criterion is discussed in this dissertation as well. It is pointed out that the criterion which is commonly used seems to be not right. The changes of normal stress and pore pressure from the initial state to the liquefaction state are analysed. Based on the analysis, the correct form is suggested. And the problems such as pore pressure build-up pattern and wave loading selection are discussed as well.