For hard rock mining, blasting is considered as the most efficient and economical method used for fragmenting rocks masses. Nonetheless, in reality only 20-30% of the available energy is used for rocks fragmentation and displacement, while the rest is wasted in the form of ground vibration, air blast, noise and fly-rocks. Ground vibration and air blast are a matter of great concern as they could result in damage to existing surface structures and generate nuisances to the receptors in the vicinity of mines.
Ground Vibration
Humans begin to perceive ground vibration at around 0.12 mm/s PPV, a level significantly lower than the vibration level where damage may start to occur. The longer a vibration of a given peak velocity lasts; the more disturbing people will find it. In addition, the longer a vibration lasts, the greater the probability of it causing damage, all other things being equal. It should be noted that there is no correlation between vibration complaints and the ground vibration level, as people may start to complain about vibration even at very low levels.
Chiappetta (2000) and Griffin (1996) defined ground vibration levels for different frequencies as defined in table 1.
Table 1: Human response to ground vibration
Effects on humans | Ground vibration Level (mm/s) | Comment (FRA, 2012) |
Imperceptible | 0.025 – 0.076 | The vibration level ± 270 – 130 m from a vibratory roller |
Barely perceptible | 0.076 – 0.254 | The vibration level ± 130 – 58 m from a vibratory roller |
Distinctly perceptible | 0.254 – 0.762 | The vibration level ± 58 – 28 m from a vibratory roller |
Strongly perceptible | 0.762 – 2.540 | The vibration level ± 28 – 13 m from a vibratory roller |
Disturbing | 2.540 – 7.620 | The vibration level closer than 13 m from a vibratory roller |
Very disturbing | 7.620 – 25.400 | The vibration level right next to a vibratory roller |
Vibration damage probability, as with many other quantities in science, roughly follows an S-shaped “sigmoid curve”, as a function of vibration intensity. Over a range of low vibration intensities, no houses are damaged. At these low intensities, people may be able to feel the vibration, even though no visible damage is done. At the highest vibration velocities (intensities), virtually all structures experiencing the vibration can visibly be damaged. Essentially all the people feeling such a high intensity vibration will be made distinctly uncomfortable by it. This report uses a limit of 2.54 mm/s as a potentially disturbing and 7.6 mm/s very disturbing.
Air blast concerns
Air blasts can cause discomfort to persons and, at high levels, damage to structures. At very high levels, it may even cause injury to people. Air blasts could also interact with structures and create secondary noises which people detect, raising their concern about the blasting activity. While rare, window breakage may be the result of an air blast. Air blast levels that may result in damage were estimated by Persson (1994) and Oriard (2002) and is defined in Table 2.
Table 2: Air blast levels that may result in damage or complaints
Descriptor | Acoustic level (dB) |
Air pressure from an 11 m/s wind gust. | 110 |
Annoyance threshold in Australia. Mildly unpleasant. | 115 |
Recommended limit in Australia for sensitive sites. | 120 |
Resonant response of large surfaces (roofs, ceilings). Complaints start. | 130 |
Limit for human irritability, USBM and OSMRE limit. | 134 |
Some windows break. | 150 |
Most windows break. | 170 |
Structural Damage. | 180 |
Fly-rock concerns
Fly rock is a significant danger to people, equipment and structures with damage due to this being undeniable. Mines therefore go through significant effort to ensure that the risks from fly rock are absolutely minimized due to the potential penalties to the mine if fly-rock complaints are registered. These penalties may be institutional consequences (regulatory directives, fines, legal action) and monetary compensation. As such there should be no risk of fly rock at structures or where people or animals may congregate. This is the main reason for the 500 m exclusion zone around blasting activities.
Reference
Tendele Mining (Pty) Ltd, blasting impact assessment for the development of an Anthracite Mine as part of the Somkhele Mine Expansion Project North-west of Mtubatuba, KwaZulu-Natal.