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.
How are blast vibrations measured?
The amount of vibration can be measured in 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 greater the vibration. The maximum blast vibration levels permitted at Martha Mine 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 response
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.
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 any of the building elements.
The following table describes a range of activities and the vibration levels they produce.
| Household activity |
|
| jumping |
|
| heel drop |
|
| nail hammering |
|
| walking |
|
| shutting door |
|
| sliding door |
|
| During initial construction | 10mm/s peak particle velocity within the frequency range 3 to 12Hz |
| 12 months after the construction phase is completed. | 5mm/s vector sum velocity ( equivalent to 4mm/s peak particle velocity) |
Common construction activities often induce levels of vibration exceeding those induced by the blasting activities. Properties within 20 metres of pile driving or hydraulic hammering are commonly exposed to vibration up to five mm/s. Road compactors also induce several mm/s of vibration at five metres. Traffic vibration up to seven mm/s has been measured at a distance of five metres, with trains known to produce more than three mm/s at a distance of 25 metres.
Protecting the Pumphouse
The maximum blast vibration levels set in the consent conditions were established to ensure that deterioration of the pumphouse structure does not occur as a result of vibrations from blasting. The company will monitor and control the levels to ensure that these conditions are complied with. Consent conditions also require that an annual structural survey of the building be conducted and report compiled for the Hauraki District Council.
Blast vibration control methods
Blasting is an essential activity at Martha Mine because most of the rock is too hard to be excavated without being fractured first. The Company uses various techniques to limit blast vibrations. The pit is divided into zones which reflect the sensitivity of nearby residences and different geological conditions. Within each zone blast design can be modified by:
- number of blast holes
- weight of explosive
- amount of stemming
- delay timing
Some mines use as much as 1000kg of explosive in each drill hole. At Martha the charge has been reduced to as little as 1.6 kilograms in areas considered more vibration sensitive. It is believed that no other open pit mines routinely use less than 15kg of explosive per blast hole.
By considering blast vibration during mine planning, measures have been implemented to reduce potential impacts. Much of the uppermost parts of the mine extension are in softer, weathered rock and can be removed by excavator. Harder rock will require blasting. Vibration levels have reduced as the pit has deepened. These levels will increase as blasting takes place on the upper levels but must remain within the consent conditions. As the pit deepens the areas being blasted will become increasingly further away from residences and businesses so the effects will decrease as mining proceeds.
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