Posted on June 15, 2012
Prof Cheryl de la Rey said that mounting the geotechnical centrifuge is an indication of the University of Pretoria’s commitment to its vision of becoming a research-intensive institution as it is set out in the University’s long-term strategic plan (UP 2025). She also added that being a research-intensive university is also about ensuring that graduates produced by the University of Pretoria, especially in the field of civil engineering, are in high demand, both locally and internationally.
Professors Elsabe Kearsley and Schalk Jacobsz of the Department of Civil Engineering successfully applied to the Research Infrastructure Support Programme (RISP) of the National Research Foundation (NRF) for funding to procure a geotechnical centrifuge. The equipment was co-funded by the University of Pretoria.
A geotechnical centrifuge is used to carry out model studies of geotechnical problems. Its purpose is to accelerate small-scale soil models to high accelerations, thus creating a realistic stress distribution within the model that corresponds with the full-scale situation. For example, a model with a scale of 1:50 needs to be accelerated to 50 times the earth’s gravity, or 50G. This is necessary because realistic soil behaviour in the model will only occur at the correct stress level.
The centrifuge will be used for modelling soil-structure interaction problems that are difficult to simulate using computer models. Such problems are common in civil engineering. A good example is the study of the dolomitic sinkholes that often occur south of Pretoria and the West Rand. A research programme is being developed to investigate means by which realistic sinkholes can be created in the centrifuge to enable researchers to study the effects of these sinkholes on various foundation types and structures. Scope also exists to investigate sinkhole rehabilitation methods. New sinkholes open up regularly in the Centurion area and need to be closed quickly to ensure public safety.
The Department of Civil Engineering has developed seismic investigation techniques to measure the small-strain stiffness of soil by means of seismic methods. This is now used routinely by many consulting engineers when they design foundations. However, questions exist regarding the application of the small-strain stiffness in determining the correct compressibility to be used when calculating foundation settlement. A project has just commenced to model the placement and loading of a foundation in the centrifuge during which small-strain stiffness and model foundation settlement will be measured so that the abovementioned questions can be answered.
Urban water distribution systems often rely on large-diameter, thin-walled buried pipelines. The stability of such pipes, when buried in trenches or under fill embankments, also requires further research. A research project is currently underway to investigate the deformations of such a pipeline as a result of the loads imposed on it by the soil and the resulting stability of the soil-pipe system.
During the construction of the Gautrain viaduct through Centurion, large-scale surcharge trials were carried out to measure the compressibility of the dolomitic soils on which the viaduct had to be founded. The back analysis of the settlement measured during these trials was a complex undertaking that required many assumptions to be made. An investigation is currently being conducted to assess the best way to do such an analysis. The behaviour of the surcharge loads will be modelled in the geotechnical centrifuge during which settlement and soil compressibility will be measured and compared. This may have important applications for future projects when construction has to be carried out on dolomitic surfaces.
In addition to applications in the field of civil engineering of which the abovementioned cases are examples, the centrifuge can also be used for research in mining engineering, geology, mechanical engineering and other fields where large accelerations have to be applied to models or components.
As indicated above, the primary purpose of the centrifuge laboratory is to serve as a research facility for the University. This facility will help to attract graduate students interested in carrying out model testing during their master’s and PhD studies. Collaboration with other departments, universities and research institutions will also be expanded. The geotechnical centrifuge will also be available to companies who would like to carry out testing of physical models for commercial purposes.
The University of Pretoria’s centrifuge is currently the largest in the southern hemisphere and only the second one in Africa. The instrument is classified as a 150G-ton instrument, which means that it is capable of accelerating a model weighing up to 1 ton to 150 times the earth’s gravity. The centrifuge’s model platform measures 0,9 m x 0,8 m, with unobstructed headroom of 1,3 m. The radius, measured from the centrifuge axis to the model platform, is 3 m. This means that at an acceleration of 150G, the model can travel in a circular track at 240 km/h.
In addition to the support received from the National Research Foundation to acquire the centrifuge, the University has received remarkable support from industry to create the infrastructure necessary to safely house the instrument. For safety reasons, the centrifuge was installed in a heavily reinforced concrete chamber. The Department of Civil Engineering would like to express sincere gratitude to the following companies for their tremendous support:
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