The University of Pretoria’s unique device to help with geotechnical modelling

A nuanced understanding of soil behaviour under diverse conditions is crucial for creating safe and sustainable infrastructure in civil engineering.

Computer models can be used to simulate or model soil-structure interaction problems, but this is difficult and often inaccurate without a centrifuge.

Civil Engineering Professor SW Jacobz from the University of Pretoria (UP) explained that a fundamental challenge in geotechnical modelling is that it is typically a small-scale representation.

“The problem with a model is that it’s a small thing, and the Earth pressure in the model, if it is a geotechnical problem, will also be a small Earth pressure,” Jacobz said.

He further emphasises that the soil’s behaviour will be inaccurate if the model does not have the correct stress.

“For soil to behave realistically, it is important for you to have the same stress in the model as in real life,” said the Professor.

To achieve this, a model created at a scale of 1:50 needs to be made 50 times heavier to ensure the stresses are correct.

As such, the UP’s Faculty of Engineering, Built Environment and Information Technology (EBIT) houses the largest geotechnical centrifuge in the southern hemisphere.

The centrifuge is a C67 Generation 4 geotechnical centrifuge manufactured by Actidyn, France, in 2011.

The model platform measures 1.0 m x 0.8 m with 1.3 m of head space, and the radius of the model platform is 3 meters.

The centrifuge is equipped with electrical, hydraulic, pneumatic and water sliprings which support a wide range of actuators to be used on models.  

Data is transmitted via a fibre optic link, eliminating noise. Cameras and solid-state computers handle observation, data acquisition, and actuator control.

Officially unveiled on 13 June 2012, it was funded through a collaboration between the National Research Foundation, which pays two-thirds of the cost, and the university, which contributes one-third.

The geotechnical centrifuge accelerates small-scale soil models through high accelerations, thus creating a realistic stress distribution within the model that corresponds to the full-scale situation.

It directly addresses this scaling problem by artificially increasing the weight of the model. “This is unique on the African continent. There’s no other like it on the continent,” said Jacobsz.

How it works

The geotechnical centrifuge works by spinning a small-scale soil model at high speed to generate strong gravitational forces (g-forces).

This process mimics the stress conditions found in real-world, full-scale scenarios.

Professor Jacobsz explains that for a model to behave like actual soil, it must experience the same stress levels.

Since smaller models naturally have lower earth pressures, the centrifuge compensates by effectively making them “heavier.”

• Scaling and Acceleration: If a model is built at 1:50 scale, it needs to experience 50 times Earth’s gravity (50g) for accurate stress conditions. The University’s centrifuge is a 150G-ton machine, meaning it can spin a model weighing up to 1 ton at 150 g.
• Physical Operation: The model sits on a platform inside the centrifuge. As the machine accelerates, the platform swings upward on hinges, ensuring the force always acts perpendicular to the platform—just like in real-world gravity conditions.
• Speed and Radius: The University of Pretoria’s centrifuge has a 3-meter radius from its axis to the model platform. When running at 150g, the model moves along a circular path at around 250 km/h.
• Instrumentation and Remote Operation: Since the centrifuge operates at extreme speeds, all monitoring and data collection are done remotely. The system includes cameras, lighting, and sensors to track model behaviour. Instruments like pressure, displacement, and water pressure sensors provide crucial data.
• Model Container (Strong Box): The models are housed in a strong aluminium box, typically 50 mm thick, with a glass window for observation. These boxes can be divided to hold different model sizes.

Important work

The facility has become a crucial centre for addressing critical geotechnical challenges.

A significant portion of their ongoing work focuses on the stability of tailings dams, which the mining industry has largely supported due to concerns about tailings dam stability.

Government entities have also sought the facility’s expertise, as Professor Jacobsz noted their involvement in the investigation of Jagersfontein Tailings Dam Collapse for the Department of Water and Sanitation.

The centrifuge’s importance is also evident in the wide range of research and collaborations it enables.

UPO collaborated with overseas universities like Durham and Cambridge to study expansive foundations and the University of Western Australia for research on cave mining.

Moreover, the facility has led to the development of innovative technologies, such as unique tensiometers for measuring negative water pressure that are sold internationally.

Other images of the geotechnical centrifuge