Current projects involving centrifuge modelling include the following:
Modelling tailings dam liquefaction / flow failures in the centrifuge
The 2019 Bromadinho tailings dam failure and other similar recent events have shown the devastating consequences of a tailings dam failure. Tailings dams are usually designed using drained slope stability analysis. However, it is becoming aparent that it may be necessary to verify the stability of tailings dams under undrained conditions. This typically results in much lower factors of safety which means that mines have to carry out very expensive remedial works, such as the construction of massive stability buttresses, to ensure satisfactor tailings dam stability.
South Africa has hundreds of tailings dams which have been operated satisfactorily for many years and tailings dams failures are very rare. We want to understand the conditions under which flow failure of tailings dams can be triggered and are carrying out model tests in the geotechnical centrifuge to study such failures. The example in the video shows a model slope which in drained instability resulting from a rising water table was the trigger initiating the failure.
The Wind-Africa Project
This project is funded by the Global Challenges Fund of the EPSRC (Engineering and Physical Sciences Researfch Council of the United Kingdom) and is focused on developing guidelines for the design of wind turbined foundation on expansive soils. The project is carried out in collaboration with the Universities of Durham, Cambridge and Khartoum. The project will include the testing of a number of large scale prototype foundation in an expansive clay profile near Steelpoort in the Limpopo province in 2019. The field work will be complemented by a series of model tests in the geotechnical centrifuge at the University of Pretoria with numerical modelling by Durham University and laboratory testing focused at characterising the expansive clay at the University of Cambridge.
The first challenge to overcome in terms of the centrifuge modelling was to devise a way in which an expansive soil profile could be caused to swell in a short space of time to allow the effects of soils expansion on structural elements to be investigated in the physical models. This has been successfully achieved with a swell profile matching that described by Van der Merwe (1964). The development of actuators for the application of cyclic load to foundation elements are currently in process with the bulk of the centrifuge modelling scheduled for 2019.
Cavity propagation leading to sinkhole formation studied by means of centrifuge and discrete element modelling
The process of cavity propagation resulting in sinkhole formation is being studied by means of a series of deep trapdoor experiments using physical models in the geotechnical centrifuge. Tests started with the testing of glass beads to provide data against which discrete element models using the program Blaze-Dem are calibrated. Blaze-Dem was developed by the Dr Nico Wilke and co-workers at the Department of Mechanical Engineering and runs on graphics processors (GPUs) instead of conventional CPUs. This results in a two-order-of-magnitude increase in the speed of analysis, allowing DEM parametric studies of geotechnical problems to become feasible within reasonable time frames. The study is being extended testing physical models of fine and coarse sands of various angularities complemented by DEM analyses modelling particles of various polyhedral shapes. For more information contact Kate Purchase of Prof Jacobsz.
Modelling the formation of sinkholes in residual dolomitic profiles
The mechanism of sinkhole formation from cavity propagation to the surface and potential sinkhole size are being studied by means of centrifuge modelling. A study modelling cavity propagation towards the surface in different soil conditions under different moisture conditions by means of trapdoor experiments and discrete element modelling using Blaze-Dem (developed by the Dept Mechanical Engineering) are in progress. For further information contact Prof Jacobsz. Further advances of this work involve measuring matric suctions with tensiometers in a soil profile during water infiltration modelling pipe leaks.
• The behaviour of Ultra-thin Continuously Reinforced Concrete modelled in the centrifuge (Kearsley et al., 2014)
• Modelling the effect of dewatering-induced differential settlement on statically indetermination portal frames.
• Modelling of large scale surcharge trials used to quantify the mass stiffness of a residual dolomite profile for the Gautrain Project south of Pretoria (Jacobsz, 2014).
• Modelling a soil nail retaining wall along the Gautrain Railway Line in Pretoria (Jacobsz, 2014).
• An MEng study by Andre Archer linking small strain stiffness measured using Continuous Surface Wave (CSW) testing to equivalent foundation stiffness has been completed under the supervision of Prof Heymann. During tests a model foundation was loaded during which load-settlement behaviour was monitored, as well as the shear wave velocity using bender elements.