Finite element algorithms for dynamic analysis of geotechnical problems

Research Doctorate - Doctor of Philosophy (PhD)The objective of this study is to document the development of a computational procedure for the analysis of coupled geotechnical problems involving finite deformation, inertia effects and changing boundary conditions. The procedure involves new finite element (FE) algorithms that were formulated and implemented into SNAC—a FE code developed by the geomechanics group at the University of Newcastle, Australia. The numerical scheme was then utilised to analyse some important offshore geotechnical problems. The first development concerns the implementation of the governing equations of two-phase saturated porous media in a mixed form, allowing predictions of solid displacement, pore fluid pressure and Darcy velocity. The generalised-α method was chosen to integrate the governing equations in the time domain. The formulation was extended to consider geometrical nonlinearity within the framework of the Arbitrary Lagrangian–Eulerian approach. Suitable absorbing boundary conditions were also incorporated to model the radiation of bulk waves towards infinity at the truncated FE mesh boundaries. Some closedform solutions were also developed, which are suitable to verify the implementation of dynamic consolidation algorithms. The second development involves the formulation and implementation of a high-order contact algorithm for solid–fluid mixtures accounting for large deformations and inertia effects. The contact algorithm is based on a mortar segment-to-segment approach formulated for cases of frictionless and frictional interfaces. The node-to-segment approach was also employed to compare and highlight the merits of the mortar method when dealing with dynamic coupled problems. The computational procedure was evaluated by modelling some numerical exercises and comparing the predicted results with alternative numerical and analytical solutions where possible. In the last part of the thesis, the computational framework was employed to successfully model the problems of dynamically penetrating anchors and offshore pipeline-seabed interactions. The analysis of dynamically penetrating anchors comprises the simulation of the penetration process and consolidation of the soil surrounding the penetrometer. The analysis of the pipeline-seabed interaction involves the simulation of the laying process and the largeamplitude lateral motion of the pipe