Although several research programmes have attempted to predict rock bursts, none have been wholly successful. Maurice Jones reports.
Rock busts, or sudden explosive spalling of solid rock into mining excavations, are most often associated with the world’s deepest mines, such as the South African gold mines of Witwatersrand, where it is estimated around 20 miners a year are killed as a consequence, although procedures are improving.
However rock bursts have also occurred when any highly stressed rock mass is disturbed by new excavation, or perhaps by a seismic event nearby. The likelihood of hazardous ground in the South African gold mines is known to be generally aggravated by the presence of intrusions such as dykes, major faults and other structural anomalies. More ‘conventional’ falls of ground (FOG) can also be triggered by seismic events elsewhere.
The website of the South African Association of Societies for Occupational Safety and Health lists many publications on rock-burst, and there is also an A Code of Practice to combat rock-burst and rock-fall accidents in mines.
Related phenomena can include outbursts of high-pressure gas (usually methane in coal mines), also moving large amounts of rock, and sometimes inrushes, also called inundations, due to the presence of water under pressure behind a rock barrier that is inadequate to hold it back in place.
The obvious, but probably oversimplified solution, is not to excavate near any such hazardous areas, and safety regulations normally cover minimum distances required between excavations and known bodies of water. However, the extent of the hazard may be unknown or it is decided that excavation is vital to the continuance of mining activity. When mine management is aware of a general, but non-specific, rock burst hazard, precautions can be taken to relieve stress in the rock and/or to ensure adequate support or rock entrapment to prevent violent collapse. Regulations or mine inspectors may specify minimum pillar sizes so that supporting rock within a mining district may not be overloaded to a likely rock-burst.
Anglo Gold Ashanti, who operates the world’s deepest mine of Mponeng, says that prediction is not possible. Instead Mponeng operates a programme of ‘destressing’ virgin rock by smaller section ‘pilot’ excavations in new developments or, in production panels, the drilling and blasting of some holes ahead of the normal depth of round for the face.
Ground-probing radar has demonstrated that this has a beneficial effect in transferring stresses away from the stope face, and prevents the accumulation of strain energy ahead of the working face, reducing the potential for face damage. The University of Utah, US, is researching into both whether earthquakes trigger mine collapses and/or does mining trigger earthquakes.
The next level of prevention is to install more support measures, especially using materials that can cope with the high stresses involved, either through total resistance or, with some rockbolts, control of the forces involved to slow down their effects, thus making the area safer for escape, rescue or more preventative measures.
Normet has recently introduced a new type of energy-absorbing rock-bolt to the South African market with its D-Bolt. It was developed by Prof Charlie C Li at the Norwegian University of Science and Technology (NTNU), Trondheim, and was being marketed by the university commercial spin-off, Dynamic Rock Support. Normet acquired DRS early this year.
Some specialist consultants have set up operations dedicated to rock burst prevention. One such is DMT of Germany. DMT offers advice, assessment of mining situations, the planning and performance of preventative measures, and training of specialist staff. It also operates a stand-by emergency service.
Geobrugg of Switzerland specialises in solutions to geohazards covering both underground and slope stabilisation on the surface. Its approach includes the TECCO mesh using high-tensile steel wire. It can be installed using rolls carried by drill jumbos using its MESHA handler to achieve up to 30 per cent faster protection than welded wire mesh.
Video 1: Demonstration of the installation of Geobrugg TECCO steel wire mesh for rock coverage without manual handling by using the Rock Australia MESHA installation handler on a drill jumbo
Video 2: Features and installation procedure of Sandvik Mining’s new mechanical dynamic (MD) rockbolt recently launched into Australia for a high-performance alternative to resin bolts, split sets and some cable bolting