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

Project Description:

This project is open to applicants until 16 May 2024. The successful candidate will begin their study in September 2024.

This Research Project is part of the EPSRC CDT in Offshore Wind Sustainability and Resilience’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.

Training & Skills

Student will be trained to use the XRCT facility at Durham University. They will be trained in scientific computing and high-performance computing by the Advanced Research Computing unit at Durham. Finally, you will develop skills on scientific writing, research presentation and communication.

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 intake, drawing on the expertise and facilities of all four academic partners. It is supplemented by Continuing Professional Development (CPD), which is embedded throughout your 4-year research scholarship.

Further Queries

If you would like more information about this project, please let us know by emailing auracdt@hull.ac.uk.

Entry Requirements

If you have received or expect to achieve before starting your PhD programme a First-class Honours degree, or a 2:1 Honours degree and a Masters, or a Distinction at Masters level a degree (or the international equivalents) in Civil Engineering, Mechanical Engineering, Applied Mathematics, Applied Physics, or any other Engineering or Physics related degree, we would like to hear from you.

If your first language is not English, or you require a Student Visa to study, you will be required to provide evidence of your English language proficiency level that meets the requirements of the Aura CDT’s academic partners. This course requires academic IELTS 7.0 overall, with no less than 6.0 in each skill. Please contact auracdt@hull.ac.uk for further guidance or questions.

Funding

The CDT is funded by the EPSRC, allowing us to provide scholarships that cover fees plus a stipend set at the UKRI nationally agreed rates, £19,237 per annum at 2024/25 rates (subject to progress).

Eligibility

Our funded Doctoral Scholarships are available to UK Students. In addition, we have a number of Scholarships that are open to International Students. Research council funding for postgraduate research has residence requirements. To be considered a Home student, and therefore eligible for a full award, a student must have no restrictions on how long they can stay in the UK and have been ordinarily resident in the UK for at least 3 years prior to the start of the scholarship (with some further constraint regarding residence for education). For full eligibility information, please refer to the EPSRC website.

How to Apply

Please note, you may only apply for ONE project offered through the EPSRC CDT in Offshore Wind Energy Sustainability and Resilience.

Applications are open until 16 May 2024.

Applications to this project are made via the Durham University admissions system. If you have not applied to Durham University before, you will need to set up an account to enable you to track the progress of your application and upload supporting documents.

Follow this link to apply for CDT projects at Durham University:
https://studyatdurham.microsoftcrmportals.com/en-US/

For CDT projects based at Durham University you need to select “PhD Engineering” as your course and “H1A201” as your course code. Please make sure you select “October” intake – although note that the PhD will actually start in September, with the 6-month taught programme, based at the University of Hull.

With your application, you need to upload copies of the following supporting evidence:

• Complete transcripts (and final degree certificate(s) where possible). If your qualification documents are not in English, you will need to supply copies of your original language documents as well as their official translation into English.
• Your Curriculum Vitae (CV).
• A completed Supplementary Application Form (upload when asked for your Personal Statement).

Guidance on completing your Supplementary Application Form:

The EPSRC CDT in Offshore Wind Energy Sustainability and Resilience is committed to generating a diverse and inclusive training programme. As part of our inclusive practices, the Centre adopts a process of assessing applicants’ experience, skills and attributes independently of personal details. To enable us to do this, we ask you, as the applicant, to complete the Supplementary Application Form, omitting the following personally identifying information from the form – name(s), ethnic group, nationality, age, gender, religion, disability, sexual orientation. The form is then used by the Panel to assess and select applicants for interview. The form asks for details of your education, training and employment history as well as some specific questions about your motivations and research experience and interests. It is very important that you do not include the personally identifying information specified.

Completing the form

Applicants must:

• Remove references to: Name(s), ethnic group, nationality, age, gender, religion, disability, sexual orientation
• Complete all sections of the form in font and size Calibri 11pt

Please download the Supplementary Application Form here.

Uploading the form

When you have completed the form, please save it as a pdf format and labelled as follows:

Last name_first name PhD application form

Upload the form as part of your application documents through the Durham University student application portal, when asked to add your Personal Statement. The form replaces the Personal Statement and so you do not need to complete the Personal Statement section.
Our support team will then process the form removing your name and allocating you a number prior to your application being assessed.

Interviews

Interviews will be held during June and will be conducted by a panel of academics from Durham University.
For an informal discussion please contact auracdt@hull.ac.uk