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Full Text, Thesis (PhD Doctorate), Doctor of Philosophy (PhD), School of Eng & Built Env, Science, Environment, Engineering and Technology, Seabed instability induced by flow fluctuations is of particular significance for coastal engineers involved in design the soil foundation of offshore infrastructures, such as pipelines, breakwaters, and anchors and piles. Soil liquefaction generally occurs under cyclic loading when the excess pore water pressure overcomes the effective stress. The liquefied foundation will be incapable of supporting marine constructions and consequently structure damage and destruction will be induced because the soil will behave like heavy fluid and the shear resistance will vanish. The present study provides a new numerical model to simulate the soil behaviour for the water-structure-soil interactions. The conventional approaches for the wave-induced soil response, such as finite element method, finite volume method and finite difference method, have been reported in the literature. These techniques have been evolved and matured over the past decades and reliable results can be accessed. However, meshing is a time-consuming procedure during establishing a model. Moreover, mesh singularity is a challenging issue although it appears easily for the computational domain with a large deformation. In contrast to that, meshless methods attracted growing attentions from researchers because of its strong features, such as handling cases with a complex boundary or large deformation precisely. Furthermore, the continuity problem of interpolation can be improved. The adoption of meshless methods will reduce data storage and computational time. In the present study, a meshless approach is applied to establish fluid-seabed-structure models for marine pipelines or breakwaters. The present numerical modelling framework involves two sub-models associated with the fluid and soil field. The first is developed within OpenFOAM, in which the VARANS equations are solved. Finite difference two-step projection method and the forward time difference method are used for the space and temporal discretizatio

Full Text, Thesis (PhD Doctorate), Doctor of Philosophy (PhD), School of Eng & Built Env, Science, Environment, Engineering and Technology, Seabed instability surrounding an immersed tunnel is a vital engineering issue regarding the design and maintenance for submarine tunnel projects. It has been recognised that the pore water pressures and stresses in seabeds are affected by the water pressures generated by the natural dynamic loading. If the pore water pressure reaches the initial mean stress, the liquefaction could occur with the effective stress in seabed vanishing. To avoid seabed instability around the immersed tunnel, the study of seabed dynamic behaviour is necessary under the real hydrodynamic loading. Two mechanisms of wave-induced liquefaction has been reported in the literature, based on a mass of laboratory tests and field exploration, which are transient liquefaction and residual liquefaction. The transient liquefaction is motivated by the oscillatory excess pore water pressures under wave pressure vibration which usually happens with amplitude reduction and phase lag of pore pressure in seabed soil. While the residual liquefaction is on the consequence of the excess pore water pressure build-up under cyclic wave loading. The liquefied seabed soil will behave like a heavy fluid without any shear resistance to supported structures on it, thus leading to catastrophic failure of the immersed tunnel. In the present study, the main objective is to investigate the mechanism of soil response and liquefaction caused by waves and currents in the seabed foundation around the immersed tunnel. An integrated numerical model is established to analysis the seabed behaviour under natural dynamic loading, including ocean waves and currents. In the integrated model, the fluid sub-model is responsible for simulating the two-phase incompressible flow motion inside and outside the porous media, which is governed by the VARANS (Volume-Averages Reynolds Averaged Navier-Stokes) equation, while the seabed model is established adopting the LRBFCM with Biot’s "u− p" approximation which considered the inertial term