Research work has and still is being undertaken to obtain a clear picture of the cause of rockfalls in Australian underground mines. Why? Because it remains one of the mining industry's most pressing planning challenges.
The fact that the first phase of the Australian Centre for Geomechanics (ACG) research project, Towards the Elimination of Rockfall Fatalities, found the highest risk of rockfall injury was posed by small rockfalls related to development activities less than 10m from a working face would probably not come as a surprise to many experienced underground mining professionals.
But the project, completed in 2001, clarified a number of key issues for those working close to (less than 10m) and further away from (more than 50m) active development faces. It also showed the Australian underground metalliferous industry had made step changes in its ground control practices in the late 1990s, resulting in a significant reduction in the number of injuries and fatalities close to the face.
Australian rockfall fatality trends indicate most recent incidents caused by large tonnage falls more than 50m from the face occurred where non-development related tasks were being carried out. Investigation has also shown nearly half of all rockfalls occur when there is no activity taking place and have little or no consequence. At worst, falls of this nature may constitute a minor interruption to production activities that may require something as simple as a short-term change to the mining schedule, enabling an area to be rehabilitated or designated no travelling unless authorised.
When the result of the fall is personal injury or equipment damage, the impact on employee moral and mine productivity can be more considerable. The effort and resources that a mine may need to invest into an employee’s physical recovery, ground rehabilitation, equipment recovery and repair, accident investigation, and measures to prevent a reoccurrence can be both exhaustive and costly. Clearly, if a significant effort is not concentrated on understanding all the factors (both geotechnical and circumstantial) involved in a serious rockfall, the possibility of a similar repeated incident would be detrimental to the operation on a number of fronts.
So, what should be the focus of further research into rockfalls? To date, a substantial amount of effort has been channelled into identifying the hazard itself (stress, poor ground, corrosion of support, etc). Unfortunately, despite this effort, the industry is no closer to accurately predicting when and where a rockfall will occur or more importantly, whether it will cause an injury. Viewing the problem within a risk management framework, it is not only important to understand the hazard, but one also needs to assess how personnel and equipment are exposed to rockfalls. This is the critical link to determine the consequence of rockfalls. In the second phase of the ACG rockfall project, currently in progress, the exposure of people to rockfall hazard has become a strong research focus.
To reflect current practices, the data analysis now only includes rockfalls that have occurred since 1998. A further 301 cases have been added to the database since Phase one for a total of 484 post-1998 cases (from 26 mines). What is this new data telling us? Although the analysis is only preliminary, some important conclusions have emerged.
A significant proportion of the total number of rockfalls still happen away from the face. However, the total area in which these falls can occur is extensive (up to hundreds of kilometres in large operations). Mine personnel are often highly mobile when away from the face – for example, travelling in declines. As a result, the probability of workers experiencing a rockfall as it happens is low. In terms of risk assessment, the hazard is significant but the exposure is low. Unfortunately, when it happens, the consequences can be very severe.
The risk profile of rockfalls close to the face is totally different. Within this small area there are numerous activities occurring. In terms of risk, the hazard is significant and the exposure is also high. Prior to 1998, working near the face of an underground mine in Australia was a very risky occupation, but things have changed.
First, “new” ground support practices do not allow anyone to work under unsupported backs. Moreover, surface support such as shotcrete and mesh is increasingly used in most mines.
So, have we got this problem solved? Well, partly. While there have been very few rockfall fatalities near the face in recent years, there are still many injuries associated with near-face activities every year. Interestingly, it was found that some operations have a large number of rockfalls occurring near the face with no injuries, while others have only few rockfalls of which a significant proportion results in injuries. Is this only the luck of the draw or is there something else influencing the results of rockfalls?
It is believed the key is in detailing how work proceeds near the face, and how people within their activities and work processes get exposed to potential rockfalls, whether it is only for a few minutes during the shift or for longer periods of time. We have called this exercise “process mapping”. By comparing the processes from operations experiencing rockfalls with low proportion or no injuries against the ones that have a higher proportion of their rockfalls causing injuries, we are aiming at isolating the “when and where” rockfalls that hurt people. It is also hoped that this comparison will lead to solutions in terms of modifying current processes to reduce or eliminate exposing people working near the face to potential rockfalls.
Phase two of the ACG Towards the Elimination of Rockfall Fatalities research project commenced in May 2002. The ACG acknowledges the support of the project sponsors: AngloGold Australia, BHP Billiton, MERIWA, Newmont, Placer Dome Asia Pacific, Rio Tinto, WMC Resources and Xstrata.
Meanwhile, ACG has introduced an Australian Rockfall Database CD Rom which is designed to encourage mines across Australia to standardise rockfall data collection and improve the quality of information gathered for all types of rockfall.
ACG said the CD Rom allowed mines to analyse incidents to reduce the risk of rockfall in all areas of their operations. Josephine Ruddle can be contacted via email@example.com for a copy of the Rockfall Database CD Rom.
* Paul Nedin is rockfall research leader and Yves Potvin director of the Australian Centre for Geomechanics.
Australia’s Mining Monthly