A few questions elicited a wealth of information from Dr Michael Roberts, whose definitive doctoral research yielded the the 95% fallout height criteria. Find out what he had to say about this and other issues.

Dr Michael Roberts PhotoRockTalk: What is your educational background?
Mike: BSc (Hons) Geology, Wits. 1974.
MSc in Structural Geology and Rock Mechanics. Royal School of Mines, Imperial College, London. 1977.
COMREC Number 63.1981
PhD in Mining Engineering, Wits. 1999.

RockTalk: How did your career within the mining industry begin? What got you into Rock Engineering?
Mike: In my second year at Wits, 1971, I was lectured by Dr Nick Gay on rock deformation, stress strain etc. and I have been hooked to this day. In order to specialise in rock engineering in those years, I had to go to the UK where Imperial College, London offered an MSc degree over a year, lectures 9 am to 5 pm every day plus a thesis. I did the Structural Geology and Rock Mechanics option. The thesis was on the state of stress in the granites of the Cornish tin mining district in SW England. I was supported by my wife at this time. She worked in London courtesy of dual British/South African citizenship.

I joined Randfontein Estates Gold Mine, a 650 000 t/m mine in the JCI group, in 1977 and was appointed Rock Engineering Manager in 1981 after getting my Rock Engineering Certificate. I had a staff of ± 12 persons. (Sounds young at 31 years old but Andy Haile was 26 when he was appointed Rock Engineering Manager for Kloof Gold mine).

Randfontein Estates Gold Mine was a moderate depth gold mine, but large portions were mechanised, with seam heights of up to 20 metres and multiseam mining. Crush pillars and elongates replaced grout pack support in the conventional mining sections. Any poor layout was immediately penalised by stress.

After 8 years on the mine, in 1985, I joined the Chamber of Mines Research Organisation and began a career in research that lasted 23 years. I authored or co-authored 54 papers, portions of three rock engineering books and many research reports in that time, and was an NRF rated researcher. All this time I was available as a consultant and undertook a lot of such work in many different commodities and countries. Over a four-year period, I was the Programme Manager in charge of all rock engineering research at COMRO, later MININGTEK, with a staff of between 70 and 90 rock engineers. Difficult to manage, but what talent! I remember Ray Durrheim, Matthew Handley, Andy Haile, Debbie Self, Marianne Macellari, Francois Malan, Arno Daehnke, John Napier, John Ryder and Steve Spottisswoode, among many others.

In 2008 I joined TWP as Head of the Rock Engineering Department with the brief to build up the department, which was done very enjoyably; I was also involved in some very challenging projects and got to further understand the Northern Cape manganese mines. TWP is now known as Worley Parsons TWP, as it was bought by Worley Parsons in early 2013. I formally retired in October 2013, but have been retained by WorleyParsonsTWP as a Rock Engineering Consultant Principal.

RockTalk: Which mining environments have you worked in?
Mike: All underground mining environments are familiar to me, however I have done only a small amount of work on coal. I have always preferred the hard rock mines.

RockTalk: What are some of the highlights of your career
Mike: In the 1990s and up to about 2003, there were teams from COMRO/Miningtek who went into recent rockburst sites in order to understand the physics involved when a rock burst occurred. Recent meant no disturbance at the site except to retrieve those injured. Each one of these investigations was written up. These reports still exist within SIMRAC and I have copies, as does Professor Ray Durrheim. A lot of the understanding gained is in current textbooks.
For about 20 years I was fortunate to travel and observe the world's mining industry first hand. I went underground in India, China, Canada, and Australia many times, as well as in other countries such as Finland and Mexico. It is a truism that all mines can be dark, damp and uncomfortable, independent of their geographical location.

RockTalk: Who or what has motivated you over your career span?
Mike: Tony Jager, Dave Ortlepp and Nick Gay

RockTalk: Would you say the Rock Engineering discipline has changed over the years?
Mike: No, you still have to make the call based on incomplete data and you have to use engineering judgement wisely.

RockTalk: Your PhD work is renowned for developing support design criteria; the 95% fallout criterion for rockfalls and the 95% ejection thickness criterion for rockbursts. What prompted you to do this research? Was it easy to get the mining industry to adopt your criteria?
Mike: I was always interested in support design. When I joined COMRO, it was to head up the research in the support section. With respect to getting the mining industry buy-in, it was easy in a sense that no credible criteria existed before – difficult to believe, but true.

RockTalk: Over the years, the criterion has been misinterpreted and/or abused, what were your original objective and conditions for the criteria?
Mike: The 95% fallout criterion (for rockfalls) and the 95% ejection thickness criterion (for rockbursts) were proposed in my PhD after my analysis of a large rock related fatal accident database developed within the SIMRAC project GAP032. In this study, it was found that the inclusion of 95% of all fatal accident data, actually just excluded the remaining 5% of those fatal accidents where the mining layout had a big impact on the fall of ground or rockburst ejection thickness. As would be expected, the largest falls were often affected or created by the poor mining layout and would thus be seen as controllable by layout modification rather than by support. There was never the intention to design to only 95% of all falls of ground or ejection thicknesses; rather this would represent all possible falls of ground or ejection thicknesses not influenced by layout.

I have noticed the misinterpretation of such data over the years, but perhaps less than perceived. I have always assumed that a certificated rock engineer will recognise that a 95% fallout thickness criterion based on a fatal accident database will be different from a 95% fallout thickness criterion based simply on fallouts that occurred on the mine and would determine his criteria accordingly.
Ultimately, the legally responsible rock engineer must be able to defend his determination of criteria used to design support systems. Don't get this wrong, the ultimate bad scenario can be as bad as cross-examination by a hostile lawyer with an expert witness by his side at a fatality inquiry, leading to negligence charges, so any rock engineer would be foolish to leave himself/herself exposed in this way.

RockTalk: In your opinion, is the level of rock engineering related research these days the same as it was in your heyday? If there are any gaps, what do you think can be done?
Mike: The rock engineering research capability in the mining industry is poor and the CSIR must accept some blame in this respect, around decisions made about eight to 10 years ago.
A big research gap is further understanding of flexural slip thrust faults (domes) in the Bushveld Complex. They have killed untold numbers of people over the years.

RockTalk: What two rock engineering issues do you think remain unresolved in the South African mining industry? How can these issues be addressed?
Mike: How to use Excess Sheer Stress for design credibly; and
What is the physics involved in the path to failure when a rock falls out of the sidewall or hangingwall? We have all seen this, a rock falls out of the hangingwall after two weeks (months) of exposure. What is the physics of time-dependent failure?

RockTalk: What are some of challenges facing younger rock engineers today?
Mike: Don't get railroaded by managers or project managers. Say it how it is; don't compromise technically.

RockTalk: What is the best advice you have ever been given?
Mike: Believe the physics. If the physics says it will collapse or stay stable, it will.

RockTalk: What advice would you give the younger generation rock engineers?
Mike: Get underground at every opportunity and make sure you know all mining methods intimately.

RockTalk: When you're not working, how do you unwind?
Mike: I enjoy reading