Insight into geotechnical numerical modelling - What is numerical modelling in geotechnical engineering and how do we do it correctly?

What is numerical modelling?

Numerical modelling of soil is where we try to mimic the behaviour of natural soils using mathematical models which are programmed into computer software. We model the spatial distribution of soil using a finite element mesh and enable the computer to calculate and reach equilibrium of external forces, internal stresses and strains within the soil for a given external displacement or loading scenario. If our mathematical model of the soil, the constitutive model, can mimic the soil behaviour correctly, then it will react to the external loads and displacements in the same way that a real soil will react and hence we are able to predict what will happen in reality before we actually apply loads and displacements in the field.

Figure 1: Example of DEM geotechnical numerical modelling for slope failure (Gong et al, 2024)

You only get out what you put in

Early days of Numerical Modelling (decades ago) often required lengthy run times due to limited computing power . When the results were obtained, you knew that the results would be most likely acceptable as a lot of effort went into input modelling verification . Nowadays, the software are very user friendly with accessible fast computing power and hence It is simple to build a model and run . However, the results may not be fully valid or accurate. The issues lie with the lack of understanding of the input to the model, boundary conditions and use of appropriate soil constitutive model.

What we must understand is that any numerical model or finite element analysis will only behave in a way that we have allowed it to behave and it will not exhibit the correct or real soil behaviour if our constitutive mode and boundary conditions does not enable or allow it to. So, we control what the numerical analysis can and cannot do without realising it.

Numerical modelling in geotechnics is not straight-forward. Soil is like no other engineering material. In fact, some may say that soil can never be modelled accurately.  Soil is not like steel, where you can design the required strength. What nature gives you, is what you need to work with. Ground engineering professionals are skilled at finding the best design solution for your projects, using the results of numerical analysis.

Best practice

A warning. Not all models are the same. If two ground engineers model the same design scenario, very different outcomes can be realised. This is not only in terms of the dimensions adopted, but also extends to the input parameters, the loads and the soil behaviour adopted!

Due to our experience in the use of numerical software, we do have some suggestions to guide practitioners:

• The design team should have an expert at numerical modelling, to lead the modelling team. This is usually someone with a PhD level in the fundamentals of soil mechanics and has detailed understanding of finite element or finite difference numerical modelling. Please note that just running finite element software for years does not automatically make someone an expert.

• The user needs to have good knowledge of boundary conditions, soil constitutive models and software limitations.

• There are different numerical modelling techniques in existence including Finite Element Method, Discrete Element Method, Finite Difference Method etc. An appreciation of what the numerical method being used is doing is key to a successful model.

• Within the tools there are several ways of modelling the soil. The model is used to determine such effects as settlement, pile head displacement, cyclic accumulation, tensile capacity etc. There are hundreds of models. The user needs to be capable of determining which model is best to adopt, and most importantly… why.

• The mesh quality is key. Some of the more advanced modelling tools sometimes have a weak point in terms of its mesh generator, careful consideration of this should be done at the meshing stage.

• Due to the current processing power of existing computers, if the model is set up properly, it can run within a reasonable timeframe. In theory, the more time spent on the model preparation, the better the model performance would be.

• Layering should be well considered to be the optimum thickness to ensure a balance between accuracy of a ground profile and the time taken for running the model.

Sometimes we have seen that clients and consultants move to numerical modelling directly at the early stages of a project. In our humble opinion, analytical insight and assessments which provide clear insight into the fundamentals of the problem needs to be the first step. This would help to identify to the most important aspects in the problem to be model numerically. Trying to model every single aspect of a problem at the onset will often lead to long run times and unclear results. Thus, numerical modelling should be used as part of overall engineering together with analytical assessments and past experiences, as opposed to using it as the only source for solution.

Time to set a precedent? At Precedent Consultants, we’ve experts who have spent those gruelling years modelling. Some of our team are distinguished numerical modellers that can solve issues that your project faces. Visit www.precedentconsultants.com to learn more about our services or to schedule a call to discuss your project needs.

Key references

• Gong, B., Zhao, T., Thusyanthan, I., & Tang, C. Modelling rock fracturing by a novel implicit continuous to discontinuous method. Computers and Geotechnics, 166 (2024), 106035.