Overview

‘Vibration’ is the term used to describe oscillation, reciprocation or other periodic motion of a body forced from equilibrium. A low level of vibration is a normal feature of the environment, and is typically not perceptible to most people. When this low background level is exceeded, vibration can cause annoyance and adverse reactions from the community.

In the mining industry, vibration is experienced and/or generated by many items of plant and equipment. The major source of vibration emission, which can be of sufficient strength to cause community concern, is blasting. This is described as an ‘impulsive’ vibration source, characterised by brief (typically less than one second) period of vibration that significantly exceeds the background level.

Conveyors, processing plant and other plant and equipment also emit vibration, but in a more steady state manner (that is, with relatively small fluctuations in amplitude over the operating period). Vibration emitted from this equipment is not typically perceptible at distances greater than 20 metres from the source. However, excessive vibration can create additional noise that can affect receptors significant distances away as noise, rather than vibration.

Also of significance is the vibration of machinery such as crushing and grinding circuits and vibrating screens, which emit very low-frequency airborne sound energy. Though not always strictly ‘vibration’, the low frequency sound waves can result in what appears to be vibration in a structure at the receptor.

Unwanted effects of vibration due to mining activities include:

  • nuisance or annoyance from both vibration and noise levels
  • fatigue, nausea and other health effects
  • injury to people
  • damage to sensitive equipment
  • damage to structures (including rock strength in mines).

Complaints regarding vibrations in residential situations often arise when the vibration magnitudes are only slightly greater than the perceptible levels. In practically all cases, the magnitudes at which vibration is first perceptible are such that there is no possibility of fatigue or other vibration-induced symptoms.

However, because people are able to ‘feel’ very low levels of vibration, they often overestimate the risk of damage associated with vibration in buildings. This is especially true when the source of the vibration is outside the building, and not within their control. Conversely, people will often readily accept much higher levels of vibration from familiar sources such as wind, domestic appliances and people walking on floors and slamming doors.

Ground vibration from blasting is the radiation of mechanical energy within a rock mass or soil. It comprises various vibration phases travelling at different velocities. These phases are reflected, refracted, attenuated and scattered within the rock mass or soil, so that the resulting ground vibration at any particular location will have a complex character with various peaks and frequency content. Typically, higher frequencies are attenuated rapidly, meaning that at close distances to the source such frequencies will be present in greater proportion than at far distances from the source.

The magnitude of the ground vibration, together with the ground vibration frequency, is commonly used to define damage criteria. The choice of the appropriate damage criterion may require consideration of the frequencies arising from the blast. Studies and experience show that well-designed and controlled blasts are unlikely to create ground vibrations of a magnitude that causes damage. Particular structures such as tall buildings, or abnormal ground conditions such as water-logged ground, should be carefully considered in a specialist study.

Cracks in buildings may be attributable to causes other than ground vibration, including ground or foundation movements (settlement and swell) associated with reactive clay soils during periods of prolonged dry or wet weather. Seismic events from underground mine activity rarely affect surface structures. Where there have been cases of damage it has been associated with some caving operation or pillar collapse in conjunction with unusual geological conditions. There is usually a band of strong and brittle material that acts to collect and transmit the vibration to some zone at the surface where the hard band outcrops or comes near to the surface.

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