Research On Mechanics Of Wave-induced Liquefaction In Saturated Sandy Seabed

Wave-induced liquefaction in saturated sandy seabed is one of the dominant factors which threat the stability of seabed as well as marine installations.However,the existing research on wave-induced dynamic behaviors of saturated sandy seabed is concentrated on pre-liquefaction behaviors and the excess pore pressure is decomposed into an oscillatory pore pressure and a residual pore pressure merely in view of field or laboratory phenomenon,resulting in unrevealed physical mechanism of wave-induced liquefaction.In this dissertation the basic laws and physical mechanism of wave-induced liquefaction in saturated sandy seabed as well as the interaction of seabed and pipeline induced by waves were investigated through numerical analysis of combining the Biot's consolidation theory and an elasto-plastic constitutive model for sand liquefaction proposed by Zhang's research group.The main achievements are concluded as follows:1.A new elasto-plastic method of dynamic analysis was put forward to investigate the wave-induced dynamic responses of saturated sandy seabed.This method is capable of reproducing the coupling mechanical behavior of seepage and plastic deformation of soil from pre-liquefaction stage to post-liquefaction stage.The spatial and temporal distribution regularities of liquefaction process in saturated sandy seabed induced by waves,such as cosine waves,random waves and etc,were found.The excess pore pressure accumulates or dissipates averagely and monotonously,meanwhile it fluctuates transiently;liquefaction occurs intermittently in the time domain and the region of liquefied soil propagates continuously in the space domain.Parametric analysis was conducted through numerical simulations under standing waves,thereby showing the quantitative influences of soil permeability,thickness of seabed,wave height and wave period on the occurrence and development of soil liquefaction in saturated sandy seabed.2.New physical conceptions including three different developing stages of liquefaction and three different components of excess pore pressure were proposed,which can reasonably reveal the underlying mechanism of wave-induced liquefaction in saturated sandy seabed.An entire process of wave-induced liquefaction can be divided into three developing stages according to the “threshold shear strain” and the “initial liquefaction”.The combination of excess pore pressure accumulation and seepage forces is the reason for occurrence of the liquefaction state of zero effective stress.Based on the fundamental causes of pore pressure variations,the excess pore pressure was decomposed into three kinds of pore pressures,defined as wave pore pressure,seepage pore pressure and structural pore pressure respectively.3.Numerical simulation was performed on wave-induced interaction of pipeline and seabed with emphasis on the accumulation of excess pore pressure and the developing process of soil liquefaction around the pipeline induced by linear waves in the liquefiable seabed of finite thickness.Several basic conclusions are drawn as follows: The distribution of liquefaction in the seabed is significantly influenced by the pipeline.Around the pipeline,liquefaction first occurs under it,and then transfers upward along the perimeter;meanwhile,liquefaction occurs at the seabed surface and then transfers downward.Eventually,the entire sandy seabed around the pipeline liquefies,which is the leading cause of pipeline flotation.The degree of liquefaction in the soil around the pipeline is intensified owing to the effects of pipeline.The depth of pipeline influences the relative rates of soil liquefaction at the seabed surface and beneath the pipeline.