Continuing the study of soil behaviour and constitutive modelling: interview with Associate Geotechnical Engineer Farid Khayyer

Farid Khayyer

For more than 14 years, Associate Geotechnical Engineer Farid Khayyer has been ‘digging deeper’. Coinciding with his PHD studies with the University of South Australia, Farid presented his insights into research and study on soil fabric and numerical modelling at the 20th International Conference on Soil Mechanics and Geotechnical Engineering 2022, in Sydney Australia.

Here Farid demonstrates a Leading with Science® approach to continuing the study of soil behaviour and constitutive modelling.

As a Geotechnical Engineer, tell us how you commenced your career?

I started my career as a geotechnical engineer working in Tehran. I worked on the Tehran Metro project, the largest rapid transit metro system in the Middle East, for five years, primarily in tunnelling. So, it was very early on in my career I learned a lot about soil and structure interaction. Before I relocated to Australia, I started and ran my own soil improvement consultancy firm, which specialised in ground / soil improvement.

I started with Tetra Tech Coffey about 5 years ago, working in South Australia on a major Defence project, then relocated to Melbourne to support on several major road and rail infrastructure projects because of my past experience.

Along with supporting my clients at Tetra Tech, I am currently completing my PhD in Geotechnics with the University of South Australia, the focus of which is on numerical modelling.

Tell us a bit more about your research and this paper you  presented?

The technical term for it is discrete element modelling, and I’m doing my PhD under supervision of Dr. Mizanur Rahman and Dr. Rajibul Karim at the University of South Australia.

Dr. Rahman is a Professor in Geotechnics at University of South Australia, and he has brought together a group of likeminded academics to perform research, and it is exciting to be part of this team. I am really proud to have been invited by the scientific committee to present my paper at the 20th International Conference on Soil Mechanics and Geotechnical Engineering 2022 in Sydney, Australia this year.

The title of my paper is the Fabric anisotropy of granular soils: a Discrete Element Method (DEM) study and my presentation focus aims to share with my industry colleagues some of the research of the constitutive model to capture that the impact of the soil fabric on the behaviour of the soil mass.

Basically, the research is the missing link from micro to the macro-scale behaviour of soil.

In basic principles, imagine the soil is a mix of particles. When they are next to each other it works like a mass. Owing to this discrete nature, soil is a very complex material to describe mathematically using continuum mechanics. An alternative approach is to consider the granular material as an assemblage of particles, where the physical processes that govern the constitutive behaviour can be understood more rigorously starting from the grain scale level.

DEM is a powerful tool that offers a unique opportunity to obtain complete quantitative information on all microscopic features of an assembly of particles and simulate macroscopic behaviour of soil.

My study allows a three-dimensional DEM to explore the soil properties by taking the critical role of soil fabric into consideration.

What has prompted this research?

If we can start modelling the soil from the particle level, to some extent you would be able to capture the complexity, and this gives you a better understanding of the soil behaviour and enables us to apply this constitutive model in computer software.

Right now, we don’t have such things. In the future I expect a combination of techniques would be incorporated into a single computer software based on a soil model that is related to discrete element method.

Capturing soil stress or soil strain is very well known in the geotechnical profession because it has been studied over the past 50 or 60 years. But for soil fabric it is something that has not yet been explored fully.

Our professional community has identified this problem for almost a decade now. This is the next evolution of understanding soil behaviour. It really is bringing engineers, science, and data together. I am very excited to be supporting this research.

Why is this so important and what positive impacts will it deliver?

Right now, we have some software that can model the soil. Each of these programs have their own limitations and sometimes the results are indicative and may not be accurate enough (more particularly for weak or problematic soils).

Accuracy and certainty in design is important these days, because if you are more accurate you can be more efficient.

In engineering we usually consider a factor of safety in design to mitigate the risk associated to this uncertainty; however, the factor of safety in geotechnical engineering is typically greater than other engineering disciplines, primarily due to the complexity of soil behaviour. So, if someone can introduce a more accurate soil model, they might be able to improve the design efficiency.

This research will enable us to make our models and design more efficient and sustainable, resulting in massive savings in the design.  It will also have further benefits as we get further into research areas such as numerical modelling of microbial induced carbonate precipitation (MICP) mechanism, also known as Bio-cementation.

One of the great things about working at Tetra Tech is having access to technical experts. We have Dr Reena Hora here at Tetra Tech, who specialises in these areas of research. I feel very lucky that I get to work alongside such knowledgeable and collaborative colleagues that I can discuss my research and ideas.

What excites you about the future of our industry?

Recently I have been involved in a number of major road upgrade projects here in Victoria, Australia. Sustainability has been a requirement in the design process, so it really is encouraging to see when government and our clients have an appetite for more sustainable solutions. It is exciting that we, as engineers, can incorporate products or solutions in our designs and put forward our ideas.

However, as designers and engineers, we also need to be able to model those things. So, I want to say that the industry is very keen in supporting this idea that brings this knowledge into practice.

We are on a mission for science to catch up to where we want to be, where our sustainability goals are. What fuels me to continue my research is that the industry has the appetite to adopt and explore sustainable approaches where we once may not have done so much. But the science must still back it up.

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