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Important Dates
Abstract Submission Opens
1 Jul 2017
Abstract Submission Deadline
1 Dec 2017 (Extended to 10 Jan 2018)
Notification of Abstract Acceptance
15 Jan 2018
Full Paper Submission Deadline
16 Apr 2018 (Extended to 15 Jun 2018)
Notification of Full Paper Acceptance
From 16 July 2018
Author Registration
By 17 September 2018

Rocha Medal Award 2018

Michael du PLESSIS
Department of Mining Engineering, University of Pretoria, South Africa

Thu-01 Nov 2018 | 08:30 – 09:15 | Summit 2

"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 provide 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 which 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 provided insights into the stress evolution of a crush pillar. The results indicated 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 validated the use of the limit equilibrium model to simulate the behaviour of crush pillars on a mine-wide scale.

Dr. Michael du Plessis is appointed as 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.
Michael pro-actively initiates and participates in many projects to develop new mining methods, systems and technologies. 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 have a significant impact on the profitability and sustainability of the platinum industry in South Africa
Michael conducts further pillar related research through the University of Pretoria alongside Prof. Francois Malan and Prof. John Napier. The ongoing research is intended to address shortcomings in understanding the design, application and behaviour of pillar systems in the platinum mines. This has in the past led to the closure of mines due to uneconomical mining or under-design, resulting in mine instability.
One of Michael’s other research interests is to study and understand the geologic structure contributing to large scale structurally related instability of mining excavations in the platinum mines. He leads this area of study with numerous publications guiding industry on the learnings from case studies he has personally taken part in or supervised.
Michael is one of the key figures in South African Rock Engineering. He provides guidance to industry regarding ongoing research requirements. He also assists with the development of the Rock Engineering qualification to ensure operational competence. Michael has successfully implemented a Rock Engineering development pool over the past 10 years exiting qualified Rock Engineers to industry. He is passionate about the advancement of willing individuals and personally supervises their progress.