Seminar - Zhange Liu - Application of two layered wave system on analysis of progressive liquefaction
Wednesday 9 May 2007, 1.10 - 1.50 pm
Civil Engineering Lecture Theatre 3
The evaluation for wave-induced liquefaction behavior of seabed is of practical significance in the design and construction of marine structures and offshore installations. There are two different mechanisms of wave-induced liquefaction. One is caused by the oscillating excess pore pressure, named oscillating liquefaction or transient liquefaction. The other is caused by the residual pore pressure due to the plastic characteristic of seabed, which is called progressive liquefaction or residual liquefaction. In this seminor, I will give a presentation on the latter mechanism. As a widely used method, the pore water pressure build-up pattern of soil under undrained shearing is incorporated with consolidation equation to establish dynamic consolidation equation and then analytical or numerical method is applied to solve the equation. The critical limitation for such method is only the occurrence of liquefaction can be determined and the equations will be invalid after the occurrence of liquefaction.
In recent years, a series of centrifuge tests were conducted by Sassa and Skeguchi. Based on those tests, the importance of the cyclic plasticity of soil was emphasized and a new analysis method was proposed for progressive liquefaction. In their method, the liquefied soil is treated as a special inviscid fluid and Lamb’s theory of two-layered wave system is used to describe the problem after the occurrence of liquefaction. However, the viscosity of the fluid used to describe the liquefied soil is not considered in their research. It was proved that viscosity did exist for liquefied sand by experiments. Thus, the consideration of viscosity is necessary in the analysis. To take the viscosity effect into analysis, a two layered wave model based on Navier-Stokes equations is developed to describe the progressive nature of wave-induced liquefaction. In the proposed model, both water and liquefied soil are treated as viscous fluids, and finite difference method is employed to solve the problem. It is found that Sassa’s procedure will overestimate the liquefaction depth and the influence of viscous effects is significant.