The emphasis of this dissertation is on using numerical and plasticity based methods to study two main areas of offshore geotechnics. The first part of this dissertation focuses on the undrained behavior of deeply embedded anchor plates under combined shear and torsion. Plate anchors are increasingly being used instead of typical foundation systems to anchor offshore floating platforms to sustain uplift operating forces. However extreme loading cases would create general loading conditions involving six degrees of freedom. The focus of my research was to evaluate the bearing capacity of plate anchors under two-way horizontal and torsional loading and to study the decreasing effect of torsional moment on the horizontal bearing capacity of these foundations. The study takes advantage of several approaches: Numerical simulation (two and three dimensional finite element analysis) Evaluating and modification of the available plasticity solutions Developing equations for three degree-of-freedom yield locus surfaces The same methodology is applied to evaluate the response of shallow foundations for subsea infrastructure subjected to significant eccentric horizontal loads. The second part of this study focuses on offshore geohazards. Coastal communities and the offshore industry can be impacted directly by geohazards, such as submarine slope failures, or by tsunamis generated by the failed mass movements. This study aims at evaluating the triggering mechanisms of submarine landslide under cyclic wave and earthquake loading. A simple effective stress elasto-plastic model with a minimal number of parameters accounting for monotonic and cyclic response of fine-grained material is developed. The new constitutive soil model could be used to simulate case histories and conduct parametric study to evaluate the effect of slope inclination angle, the earthquake loading with different PGA, frequency content, and duration, as well as various deposition rates to simulate different over pressure levels. This study will generate more insight on the static and cyclic behavior of submarine slopes and influencing factors on their triggering mechanisms using more comprehensive and realistic modeling tools. Several objectives are defined: Developing an appropriate constitutive formulation, Evaluating the constitutive model and material parameters for available databases.

Trabecular bone is a porous composite material which consists of a mineral phase (mainly hydroxyapatite), organic phase (mostly type I collagen) and water assembled into a complex, hierarchical structure. In biomechanical modelling, its mechanical response to loads is generally assumed to be instantaneous, i.e. it is treated as a time-independent material. It is, however, recognised that the response of trabecular bone to loads is time-dependent. Study of this time-dependent behaviour is important in several contexts such as: to understand energy dissipation ability of bone; to understand the age-related non-traumatic fractures; to predict implant loosening due to cyclic loading; to understand progressive vertebral deformity; and for pre-clinical evaluation of total joint replacement. To investigate time-dependent behaviour, bovine trabecular bone samples were subjected to compressive loading, creep, unloading and recovery at multiple load levels (corresponding to apparent strain of 2,000-25,000 με). The results show that: the time-dependent behaviour of trabecular bone comprises of both recoverable and irrecoverable strains; the strain response is nonlinearly related to applied load levels; and the response is associated with bone volume fraction. It was found that bone with low porosity demonstrates elastic stiffening followed by elastic softening, while elastic softening is demonstrated by porous bone at relatively low loads. Linear, nonlinear viscoelastic and nonlinear viscoelastic-viscoplastic constitutive models were developed to predict trabecular bone's time-dependent behaviour. Nonlinear viscoelastic constitutive model was found to predict the recovery behaviour well, while nonlinear viscoelastic-viscoplastic model predicts the full creep-recovery behaviour reasonably well. Depending on the requirements all these models can be used to incorporate time-dependent behaviour in finite element models. To evaluate the contribution of the key constituents of trabecular bone and its microstructure, tests were conducted on demineralised and deproteinised samples. Reversed cyclic loading experiments (tension to compression) were conducted on demineralised trabecular bone samples. It was found that demineralised bone exhibits asymmetric mechanical response - elastic stiffening in tension and softening in compression. This tension to compression transition was found to be smooth. Tensile multiple-load-creep-unload-recovery experiments on demineralised trabecular samples show irrecoverable strain (or residual strain) even at the low stress levels. Demineralised trabecular bone samples demonstrate elastic stiffening with increasing load levels in tension, and their time-dependent behaviour is nonlinear with respect to applied loads . Nonlinear viscoelastic constitutive model was developed which can predict its recovery behaviour well. Experiments on deproteinised samples showed that their modulus and strength are reasonably well related to bone volume fraction. The study considers an application of time-dependent behaviour of trabecular bone. Time-dependent properties are assigned to trabecular bone in a bone-screw system, in which the screw is subjected to cyclic loading. It is found that separation between bone and the screw at the interface can increase with increasing number of cycles which can accentuate loosening. The relative larger deformation occurs when this system to be loaded at the higher loading frequency. The deformation at the bone-screw interface is related to trabecular bone's bone volume fraction; screws in a more porous bone are at a higher risk of loosening.