Australian rockfall research reveals industry advancement

From the very beginning, rockfalls have remained the principal cause of fatalities associated with mining activities. Despite modern techniques and heavy mechanization, the rockfall problem persists. Recent studies conducted by the Australian Centre for Geomechanics (ACG) have contributed to a better understanding as to how this problem has evolved in Australia during the last decade. More importantly, ACG research projects have identified areas in current mines where the rockfall risk remains elevated, as well as the tasks most at risk for rockfall injuries.

The first ACG research project in rockfall commenced in 2001. With industry support, the Australian Rockfall Research Phase II project expanded the rockfall database and offers a detailed insight into the updated and revised findings. Both projects were co-ordinated by project leader Paul Nedin, Underground Mining Solutions.

The ACG’s Australian Rockfall Research Phase II Database features 795 case histories collected from 29 underground mines. It is one of the most comprehensive datasets ever assembled on the subject. The trend in injuries caused by rockfalls clearly demonstrates that the Australian mining industry made a positive step change between 1997 and 1999, eliminating over 75% of the annual rockfall injuries (see barchart) over the two-year period. Drastic changes in regulations, culture and practices all contributed to deliver this spectacular improvement.

Historical data have shown that more than 80% of the rockfall injuries happened within 10 m of an active development mining face. This is not surprising given the intense activities concentrated in this confined area where freshly blasted rock faces are continuously being exposed. Also of interest is the relatively small size of rockfalls causing injuries, with over 90% of them weighing less than 2 tonnes — a weight that mesh can normally support. This indicates that there may be some deficiency in the coverage and usage of surface support.

Although current mining practices (which promote no personnel exposure under unsupported ground and the systematic installation of rock surface support) did have an important impact on reducing rockfall injuries, statistics show that a remnant risk remains. The industry is still consistently experiencing rockfall injuries near the mining face. More specifically, most of the rockfall injuries in recent years came from rock fragments detaching from the face, the bottom three metres of walls or the last half metre of the roof (where the roof joins with the advancing face). These areas are generally unsupported for practical reasons. The tasks that are most at risk of sustaining rockfall injuries are indeed the ones requiring personnel to work very close to the face, such as charging, cleaning blastholes, marking-up the face, jumbo drilling, off-siding and installing ground support.

A further sustainable reduction of the rockfall injury rate requires further changes in practices near the face. The easy solution, albeit possibly expensive, would be to systematically install surface support on every face and the lower walls. It is possible that new products such as Thin Spray-on Liners (TSLs) can offer a cost-effective alternative, if such an extensive use of surface support is required. Perhaps another solution would be to alter processes to minimize the exposure of workers to the unsupported face. Some innovations would undoubtedly be required to execute normal mining tasks near the face, without exposing mine personnel. This is possibly where future research should concentrate. In the meantime, only the implementation of the most rigorous risk management processes for tasks near active development mining faces will control the number and severity of rockfall injuries.

A significant number of rockfalls occur away from active development mining faces, but statistics show that they rarely cause injuries. The severity of these injuries can, however, be severe. Nearly 180 rockfalls away from the face were reported in the database and only eight caused injuries. The lower rate of injuries away from the face is explained by the lower exposure of personnel, as most drives away from the face are either unattended or involve personnel transiting. Further analysis of the eight cases of injuries away from the face revealed that stope brows, where no surface support is installed, are particularly prone to rockfall injuries.

Historical data have also shown that special tasks that are not part of the normal mining processes can occasionally cause severe rockfall injuries and even fatalities. For example, the rehabilitation of damaged areas, the development or repair of orebins, orepasses or other parts of the mine infrastructure, which are different from the usual horizontal development mining, may pose “new” risks that are not adequately addressed by normal procedures.

The mining industry achieved a spectacular reduction in the number of rockfall injuries in the late 1990s by changing some of the mining practices near active mining faces. It must now continue the progress towards the minimization of rockfall injuries. A remnant risk exists near the face and a constant rate of rockfall injuries have been observed since the early 2000s. Further reduction in the injuries will be possible only if further changes in practices are developed. These changes could involve the systematic installation of surface support on the face and sidewalls, or the development of new processes and equipment to remove workers from exposure to the face and walls. The application of stringent risk management will assist in maintaining the current level of safety and possibly achieve incremental improvements.

The ACG acknowledges the support of Australian Centre for Geomechanics Australian Rockfall Research Phase II project sponsors: BHP Billiton, Kalgoorlie Consolidated Gold Mines Pty. Ltd., MPI Mines, Newmont Australia Ltd., Placer Dome Asia Pacific, Perilya Ltd., Rio Tinto, Sons of Gwalia, WMC Resources Ltd., Xstrata and MERIWA.

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Dr. Yves Potvin is director of the Australian Centre for Geomechanics.