Experimental Study For Seabed Response Under Wave Loading

The nearshore region is a special geological,physical and biological area,which is important in environment,economy and culture.With the increase exploitation of the ocean,the nearshore region plays a significant role in resources,transportation,energy,trade,fishing and tourism industries.Therefore,a growing number of marine structures(such as breakwaters,pile foundations,oil platforms,pipelines,etc)have been constructed in this region.Different from terrestrial environment,marine structures are always under complex wave loadings,then the seabed foundation of marine structures prone to liquefaction,which in turn reduce the effectiveness of marine structures significantly in a few years.Thus,the study on seabed response and stability under wave loadings is still one of the top subject in marine soil mechanics.This thesis firstly introduces the poro-elastic theory for the wave-seabed interaction theory.The study found that pore pressures change dramatically near the seabed surface in fine sand,i.e.,pore pressure discontinuity phenomenon.To date,there is no appropriate theory and reasonable explanation of this phenomenon.Therefore,this thesis applies one-dimensional cylinder experiment to study the homogenous seabed response under wave loadings,not only verify the related theory but also trying to measure and analysis the pore pressure discontinuity phenomenon.On this basis,the thesis also investigates the wave-induced seabed response in mixtures seabed and layers seabed.The major findings of this study are summarized here:(1)Through the comparison between experimental results and the previous analytical solutions for a homogenous seabed,the accuracy of experimental results has been verified.The pore pressure discontinuity phenomenon has been measured in some experiments,which confirmed the pore pressure discontinuity phenomenon calculated by related theory and present numerical analysis does exist.Seabed liquefaction is the main reason for this phenomenon.Furthermore,wave-induced pore pressure damping and phase lag are very obvious in the experiment,which is the main reason for oscillatory liquefaction.In addition,wave parameters(wave period,wave height,wave cycles)and soil parameters(soil permeability,saturation)have significant influence on seabed liquefaction,i.e.,with the increases of wave period,wave height and wave cycles,the maximum seabed liquefaction depth will increases;while with the increases of soil permeability and saturation,the maximum seabed liquefaction depth will decreases;(2)Clay content(CC)has significant influence on wave-induced seabed response and seabed liquefaction.The maximum amplitude of wave-induced pore pressures will decreases with the increases of clay content;while with the clay content continue growing to the threshold of clay content,the maximum amplitude of wave-induced pore pressures will increases.In addition,the maximum seabed liquefaction depth will increases with the increases of clay content;while with the clay content continue growing to the threshold of clay content,the maximum seabed liquefaction depth will decreases sharply.Finally,with the increases of plasticity index,the threshold of clay content will decreases.This conclusion links the plasticity index and clay content,i.e.,in any wave conditions,these two parameters will eventually determined the liquefaction in mixtures seabed;(3)Through parameter analysis of layers seabed can we known,with the increases of wave period and wave height,the maximum amplitude of pore pressures in layers seabed will increases.Furthermore,through liquefaction analysis of layers seabed can we known: Firstly,when the coarse sand is upper in three layers sandy seabed,the seabed liquefaction resistance is stronger;Secondly,the maximum seabed liquefaction depth will increases with the decreases of soil permeability,i.e.,dense seabed prone to be liquefied;Finally,when replace a certain height of liquefied protection layer in seabed,pore pressures near the seabed surface will increases and leading to decreases of excess pore pressures,which will greatly reduce seabed liquefaction.Taken this study as an example,when the replaced liquefied protection layer is 0.1 meters high,the seabed liquefaction will reduce 56%;when the replaced liquefied protection layer is 0.2 meters high,the seabed liquefaction does not occur absolutely;(4)No matter in homogenous seabed experiment,mixtures seabed experiment or layers seabed experiment,seabed settlement has been clearly observed.The combined effect of seabed liquefaction and changing of seabed relative density led to this phenomenon.The seabed liquefaction and seabed settlement are linked to each other,i.e.,seabed liquefaction leads to seabed settlement while seabed settlement in turn expands seabed liquefaction.In addition,the increases of wave period,wave height and decreases of saturation will expand seabed settlement,this is because liquefaction is the main reason for seabed settlement and changing of the above parameters lead to the maximum liquefaction depth expanded.