Vibration

Blast vibration - what is it?

In any construction activity, mining or quarrying, when rock is encountered that is too hard to excavate, explosives are used to break the rock into fragments that can be easily dug and transported. These blasts cause vibrations that travel through the ground away from the blast rather like waves when a stone is thrown into a pond. This is referred to as blast vibration.

Humans are very sensitive to vibration and can detect levels as low as 0.15mm/second, although they are poorly equipped to distinguish between the different intensities. How they notice and respond to vibration varies greatly from person to person.

How are blast vibrations measured?

The amount of vibration can be measured in velocity (millimetres of movement per second) and is directly related to the size of the blast and the distance from the blast – the closer to the blast the generally the greater the vibration. The maximum blast vibration levels permitted at Waihi’s mining operations are specified in the consent conditions. These limits are lower than all other known international standards for blast vibration.

Fixed monitoring stations are located at strategic sites. A mobile vibration monitor is also used to collect additional data.

Information from every blast is recorded and becomes a reference for future blasts.

Blast vibration

Levels set by the consent conditions are for personal comfort and are well below the levels known to cause either superficial damage, such as cracking of plaster, or structural damage to building elements.

Blasting

The peak particle velocity (vector sum) shall be no more than:
Development blasts:
5mm/s for 95% of the monitored events
2mm/s on average
Development blasts shall have a total duration of not more than 12 seconds.

Production blasts:
5mm/s for 95% of the monitored events
3mm/s on average
Production blasts shall have a total duration of not more than 9 seconds.

No underground blast event shall have a duration of more than 18 seconds.

Martha Pit blasts:
5mm/s for 95% of the monitored events.

Maintenance/safety blasts:
1mm/s for all maintenance/safety blast events
Maintenance/safety blasts shall have a total duration of not more than 2 seconds, other than for maintenance to retain tunnels, shafts and accessways in Correnso and SUPA.

Compliance with the 95% and average limits shall be measured over a six month rolling period.

Blast vibration control methods

The Company uses various techniques to limit blast vibrations; each blast design can be modified by:

  • number of blast holes
  • weight of explosive
  • amount of stemming
  • delay timing.