Three-dimensional Numerical Model for Seabed Foundation Stability around Breakwaters

Abstract

With the increasing demand for coastal zones from human activities, a growing number of breakwaters have been constructed around the main beach and major estuaries to defend against wave erosion and damage. The vulnerability of the breakwater foundation can be associated with dynamic soil responses in the vicinity of structure when subjects to the consecutive ocean wave loading. For the severe situations, soil liquefaction may occur around the breakwater foundation, which is considered as a significant cause of catastrophic failures of many marine structures. Therefore, understanding and predicting soil responses and liquefaction potential around breakwaters have become one of the main concerns when design and maintain these marine structures. The traditional models used to analyse the soil responses and liquefaction potential in the neighbourhood of breakwaters were mostly limited to two-dimensional (2D) frameworks, in which only the middle cross-section of the breakwaters under perpendicular waves can be investigated. However, the natural environment is three-dimensional (3D) that involves much more complicated fluid-seabed-structure interactions, which requires a 3D model. What’s more, most of the existing models assumed the seabed foundation as poro-elastic medium, which only the oscillatory soil responses and momentary liquefaction can be studied. Nevertheless, the residual soil responses and liquefaction within the poro-elastoplastic soil are more significant and can cause more severe damage to the marine structure foundations. Another deficiency of the traditional models is the lack of advanced Computational Fluid Dynamic (CFD) model to accurately simulate more realistic conditions, for example, including the interactions of ocean currents. According to the gaps in previous literature, the main objective of this thesis is defined as numerically predicting the soil responses and examining the breakwater foundation stability (i.e., liquefaction potential) under combined waves and currents loading within both poroelastic and poro-elastoplastic seabed foundation from both two- and three-dimensional perspectives for different engineering conditions. One of the main novel contributions of this study is to develop the integrated numerical model that make up for the deficiency of the fluid-seabed-structure interactions problems mentioned above: the wider application ranges including complicated 3D situations; the consideration of poro-elastoplastic soil behaviour and corresponding soil liquefaction; the inclusion of an advanced flow model to precisely predict the hydrodynamic behaviour around the structures. In the future, the models can be further developed and applied to practical engineering analyses, providing preliminary results for the design of the projects. The integrated numerical model consists of the flow sub-model, the seabed sub-model and the coupling module between two sub-models. The flow model is developed based on the Finite Volume Method (FVM) by solving the Volume-Averaged Reynolds Averaged Navier-Stokes (VARANS) equations for simulating the two incompressible phases (i.e., water and air) inside and outside the porous medium. The seabed model is governed by the dynamic Biot’s equations known as the u− p approximations, in which the relative displacements of pore fluid to soil particles are ignored and the acceleration of pore fluid and solid particles is included. Two constitutive models: poro-elastic model for oscillatory soil responses and momentary liquefaction; and poro-elastoplastic model for residual soil responses and residual liquefaction, are incorporated into the seabed model. An integration module is developed between flow sub-model and seabed sub-model through pressure continuity on the common faces. A set of validation works have been done to prove the capability of simulating the fluid-seabed-breakwater interactions in an accurate way. By adopting the integrated numerical model, three numerical studies have been conducted in this thesis, including one 2D study (soil responses around submerged breakwaters with Bragg reflection) and two 3D studies (seabed foundation stability around breakwaters at river mouth; seabed foundation stability around offshore detached breakwaters). A series of results, including the hydrodynamic properties of flow domain, variation of pore pressure, effective stresses and soil displacements, and characteristics of soil liquefaction within both poro-elastic and poro-elastoplastic seabed foundation have been obtained. Numerical results revealed that the construction of breakwaters can dramatically change the flow pattern and stress state in the vicinity, which will further affect the assessment of foundation stability. Besides, compared to the poro-elastic seabed foundation, the liquefaction is much easier to occur in the poro-elastoplastic seabed foundation and usually will develop to a much more significant level, which can cause critical failure of the structures. Furthermore, the effects of wave characteristics and soil properties on the breakwater foundation stability have been examined through parametric studies: the soil liquefaction is more serious within the loosely deposited seabed with poor drainage conditions under large wave height and wave period. It was also found that the currents have remarkable effects on foundation stability that aggravate with the increase of currents velocity.