Mechanism Of Wave-induced Instability Of The Silty Seabed In The Yellow River Delta

With human activities turning towards the ocean and a great quantity of oceanstructures was built and being built, the response and the instability of seabed due towave actions has been become one of the more and more important research projects.The work of this dissertation was set in the Yellow River Delta, based on the existingunderstanding and research achievement, explorations were developed from aspectsof simulation tests in the laboratory and numerical analysis. Attention wasconcentrated on the physical mechanism and the effective mathematic model of theproblem, emphasis was put on the different wave conditions of extreme case and thecase with higher frequency and lower height, discussion was made on the mechanismof wave-induced seabed instability and corresponding evaluation method was given.The main work and conclusions can be drawn as follows:1. A new rectangle flume testing system was developed and applied to study thewave-soil interaction problem. Series of tests were carried out to prove that regularstanding waves could be obtained with the system and the system owns its advantagessuch as fewer premises, lower costs and simplicity of operation. The stress states ofone point in the seabed under progressive wave can be broke up into the stress stateunder the antinodes and the stress state under nodes of standing wave. With thecharacteristic of standing wave that it causes only additional cyclic normal stress inthe seabed under wave nodes and only additional cyclic shear stress under waveantinodes, a special role was played by the testing system in the investigation anddifferentiation of the contribution to the oscillation and the residual pore pressure inthe seabed due to the two kinds of wave-induced additional stress.2. In series of flume tests, seabed fluidization and remarkable pore pressureaccumulation was observed under wave nodes, yet no obvious phenomenon but onlyoscillation pore pressure was found under antinodes. The analysis indicated thatwave-induced additional cyclic normal stress in the seabed contributed to therecoverable elastic volume deformation, corresponding to the oscillation pore pressure;the cyclic shear strain caused by the wave-induced additional cyclic shear stresscontributed to the oscillation pore pressure, and meanwhile the unrecoverable plasticvolume deformation caused by the additional cyclic shear stress was the decisive factor of residual pore pressure. Under the wave condition of high frequency and lowheight, liquefaction could not occur in a silty seabed, the essence of fluidizationobserved in the tests was shear failure, and it should be estimated with the criticalstate parameter.3. Based on the mechanism of the contribution to the pore pressure by thewave-induced additional stress in the seabed, the dynamic consolidation analysis codeof saturated soil and the general plastic constitutive model of soil was introduced,finite element model of experiments with both progressive and standing wave influme was established. The finite element model was shown to be able to reproducethe observed phenomena and the behavior of pore pressure variations. The porepressure was coupled with the displacement and the pore pressure showed oscillationand accumulation simultaneously. The amplitude of oscillation component decreasedwith depth, the residual component increased with depth but the rate of pore pressureaccumulation decreased with depth.4. Finite element model of seabed response under wave action was set upaccording to the actual wave conditions and soil properties of the Chengdao sea areain the Yellow River Delta. Extreme wave conditions and the surface hard layer wereconsidered especially and the liquefaction potential was estimated using a total excesspore pressure criterion. Results showed that both oscillation and residual porepressure component should be considered, a greater liquefaction possibility and abigger liquefaction depth tend to occur while the three dimensional effect wasconsidered and the seabed was covered with a surface hard layer. The biggestliquefaction depth was about three meters under the fifty years once wave conditions.Attention should be paid to the liquefaction potential under extreme wave conditionswhile ocean engineering structures were to be constructed.