Investigation On Soil Dynamic Response And Liquefaction Mechanism Around A Pile Foundation Due To Wave And Current Loads

Marine pile foundations are normally subjected to waves,currents and other dynamic loads.The wave/current loads can not only cause the damage to strength and stiffness of the pile foundation,but also induce the changes of stress within the seabed around the structure.Under the cyclic wave loads,the pore-water pressure within the seabed may gradually increase and the effective stress will decrease correspondingly.Once the effective stress disappears,part of the seabed may get liquefied and lose the constraint capacity on the foundation.In addition,the coupled mechanism between seabed response and sediment erosion is a research hotspot in the field of offshore geotechnical engineering.In this thesis,both the numerical simulation and theoretical analysis are applied to investigate the oscillatory soil response,the residual soil response and the mechanism of soil liquefaction in the vicinity of the pile foundation.The main contents are as follows:1.A new three-dimensional oscillatory seabed model has been established within the framework of OpenFOAM,in which the Biot's poro-elastic equation(u-p approximation)is used to govern the transient soil dynamic response.The validity of the oscillatory seabed model has been verified by comparing the new model with the previous laboratory experiments.2.The distribution of wave/current-induced pore water pressure and effective stress have been studied using the new oscillatory seabed model.The effects of current,wave parameters,soil properties and mono-pile geometry on wave-induced oscillatory soil response have also been examined respectively.The mechanism of seabed liquefaction around the mono-pile foundation due to wave and current loads has been investigated.3.The definition of the shear stress ratio has been modified and the governing equation for the three-dimensional residual soil response under cyclic load has been derived.A 3D residual seabed model has been established using OpenFOAM and the accuracy as well as the efficiency of the new model have been validated with previous laboratory experiments.4.Based on the new 3D residual seabed model,the distribution of residual pore water pressure around a mono-pile under wave and current loads has been investigated and parametric studies are carried out to examine the effects of wave and soil properties on residual soil response.The existing liquefaction criterion has been improved to make it consider the influence of oscillatory pore pressure and residual pore pressure on seabed liquefaction when determining soil liquefaction potential.The mechanism of residual soil liquefaction in the vicinity of the mono-pile has also been revealed.5.Both the oscillatory soil response and the residual soil response around the group of pile foundations due to wave and current loads have been studied using the new numerical model developed in this thesis.The effects of the wave/current obliquity and the structure geometry on the soil dynamic response as well as the seabed liquefaction development in the vicinity of group of pile foundations have been investigated.