UP engineer Dr Michael du Plessis is this year’s recipient of the prestigious Rocha Medal, awarded to him by the International Society for Rock Mechanics and Rock Engineering (ISRM) at the 10th Asian Rock Mechanics Symposium in Singapore. 

The ISRM Board was founded in 1962 in Salzburg, Austria, and instituted an annual prize in honour of past president Prof Manuel Rocha who organised the first ISRM Congress. Prof Rocha was also responsible for transforming the international collaboration from amateurish to a fully fledged international scientific association.

The Rocha Medal is intended to stimulate young researchers in the field of rock mechanics, and the bronze medal and cash prize has been awarded annually since 1982 for an outstanding doctoral thesis selected by a committee appointed for the purpose.

Dr Du Plessis completed his doctoral thesis in 2016 through UP’s Department of Mining Engineering, an achievement that makes him an elite researcher internationally in the field of rock engineering, and significantly contributes to the research capacity of the department.

A key figure in South African rock engineering, Dr du Plessis is the Rock Engineering Manager for Lonmin. As the third-largest platinum producer, his role requires him to optimise the business in terms of safety, profitability and long-term sustainability. 

He initiates and participates in many projects to develop new mining methods, systems and technologies, and his PhD study was aimed at developing a better understanding of crush pillar behaviour. This pillar system provides an optimised extraction strategy as mining progresses deeper and benefits this industry in terms of safety and production. This work, therefore, has the potential to impact significantly on the profitability and sustainability of the platinum industry in South Africa.

He conducts further pillar-related research through UP alongside Prof Francois Malan (previous recipient of the Rocha Medal) and Prof John Napier (supervisor and co-supervisor who has collaborated with four recipients of the Rocha Medal). The ongoing research is intended to address shortcomings in understanding the design, application and behaviour of pillar systems in platinum mines. In the past, this has led to the closure of mines due to uneconomical mining or under-design, resulting in mine instability. 

Another of Dr du Plessis’s research interests is the geologic structure contributing to large-scale structurally related instability of mining excavations in platinum mines. He has written several publications to guide the industry based on findings from case studies he has particpated in or supervised. He also provides guidance in terms of ongoing research requirements, and assists with the development of the rock engineering qualifications to ensure operational competence. Over the past 10 years, Dr du Plessis has successfully implemented a rock development pool, delivering qualified rock engineers to the industry. He’s passionate about the advancement of willing individuals and personally supervises their progress.

Abstract to his doctoral thesis: “The design and behaviour of crush pillars on the Merensky Reef”

Crush pillars are used extensively in the platinum mines of South Africa as part of the stope support in intermediate depth tabular mines. Effective crush pillar design ensures that the pillars crush when formed at the mining face. This behaviour occurs when the pillars have a width-to-height ratio of approximately 2:1. Once crushed, the residual stress state of the pillars provides a local support function. In many cases, effective pillar crushing is not achieved, resulting in pillar seismicity. As this mining area produces approximately 70% of the world’s platinum group metals, it is critical that layouts and pillar design are optimised to ensure safety and sustainable production. The objective of the research was to determine the parameters that influence crush pillar behaviour. A limit equilibrium constitutive model was proposed to investigate the behaviour of the pillars. The model, implemented in a displacement discontinuity boundary element code, provides insights into the stress evolution of a crush pillar. The results indicate that the stress on the pillar depends on its position relative to the mining face, size, the impact of geological structures, layout, rock mass parameters and mining depth. 

A comprehensive underground mining trial was conducted to quantify the behaviour of crush pillars. A numerical model was used to back-analyse the pillar behaviour at the underground trial site which consisted of a mined area of approximately 22 000 m2 containing 55 crush pillars. Both the observed and measured behaviour of the crush pillars in the trial site could be replicated by the model. The findings validate the use of the limit equilibrium model to simulate the behaviour of crush pillars on a mine-wide scale.