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

Research projects

Project Description:

This Research Project is part of the Aura CDT’s Offshore wind energy geotechnics Cluster.

More than 100 countries worldwide have proposed net zero targets in the next few decades. Renewable offshore wind energy plays a critical role on meeting these targets, and this 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 has not been fully understood and cannot be quantified with confidence, mainly due to the complex grain dynamics at the micro scale and the nature of the repetitive cyclic environmental loads.

This project proposes use of an existing bespoke miniature triaxial shearing experimental apparatus, that unlike traditional triaxial testing devices, 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 for the development of digital twin samples, that will be analysed computationally using the Level Set Discrete Element Method (LS-DEM). The latter is a micromechanical computational method that allows the simulation of grain-dynamics in particles with realistic shapes, extracted from the XRCT images. Both the experimental and computational output data will set the foundation for the development of new theories and mathematical models in soil mechanics and help engineers to create new design solutions for the acceleration of renewable energy infrastructure exploitation, towards more sustainable and resilient societies.

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, fabric anisotropy evolves due to the continuously changing orientations of grains, and 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.

To achieve this research goal, the grain-scale quantification of fabric evolution during cyclic loading, must be thoroughly studied. Microfocus X-Ray Computed Tomography (μCT) has been extensively used for grain-scale experiments in soils, and more recently for quantitative measurements of fabric evolution. Conventional testing setups (e.g., the triaxial apparatus) have been miniaturised to be able to image large numbers of grains at high resolution, and such a device is available in Durham University. The quantification of fabric evolution during cyclic loading is not extensively investigated yet and the alternation of loading from triaxial compression to triaxial extension during cyclic loading is the main key triggering of the anisotropic behaviour of the soil. Finally, by using the XRCT images to develop digital twin samples with an open-source implementation of the LS-DEM method in YADE, grain-scale simulations can complement the experimental data, where the experiments will be used to validate the numerical results. This combination forms a complete characterisation, both experimental and computation, of the evolving properties of the material in cyclic loading.

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.

For an informal discussion, call +44 (0) 1482 463331
or contact