PhD Studentship: The role of fabric anisotropy on cyclic loading of offshore soils: a grain-scale investigation

Renewable offshore wind energy plays a critical role on meeting net zero targets, which leads to the need for novel design and construction solutions for offshore wind structures that are subjected to dynamic loading conditions (wind, waves, earthquakes, tsunamis) in the harsh offshore environment. Those loads are transferred to the marine soil deposits around the structure foundation, and thus, the degradation of the soil’s strength and stiffness in time will dictate the structures’ lifespan. This degradation is not yet fully understood in a way that can be quantified with confidence.

This project proposes use of an existing bespoke miniature triaxial shearing experimental apparatus, that will be able to apply cyclic loading on miniature sand samples, while the motion of each of their grains will be observed with the use of existing X-Ray Computed Tomography facilities at Durham University. Grain-scale phenomena during this complex loading condition will be systematically analysed for the first time and new correlations between the evolution of soil’s microstructure and the degradation of their strength and stiffness will be generated. Those data will then be used M development of digital twin samples, that will be analysed computationally using the Level Set Discrete Element Method (LS-DEM).

Granular soils exhibit highly anisotropic mechanical behaviour, mainly due to the non-spherical shape of their grains, with preferred orientations when forming assemblies, often called the orientation of the material’s fabric. The orientation of fabric with respect to the orientation of the applied load determines the effect of fabric anisotropy on the material’s strength and stiffness, like the role of the orientation of fibres in a loaded composite material.

During loading, the effect on stiffness and strength of soils has been successfully studied macroscopically for monotonic loading. The macroscopic observations of the effects, in combination with a qualitative description of fabric evolution with the use of Discrete Element numerical modelling on idealised granular materials, has led to significant theoretical advances on the mathematical description of fabric anisotropy for monotonic loading. However, the quantification of fabric anisotropy during cyclic loading is much more complicated and still an open research question.

The goal of the project is to shed light in unexplained phenomena that happens in the micro-scale during the repetitive cyclic loading that offshore soils exhibit next to offshore foundations during complex environmental loading conditions. It will form the foundations for new theories in offshore soil mechanics, specifically considering the complexities of cyclic loading.

Training and Skills

You will benefit from a taught programme, giving you a broad understanding of the breadth and depth of current and emerging offshore wind sector needs. This begins with an intensive six-month programme at the University of Hull for the new student, drawing on the expertise and facilities of our academic partners. It is supplemented by Continuing Professional Development (CPD), which is embedded throughout your 4-year research scholarship.

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