3.12 Closure and rehabilitation phase

Noise impacts are likely to be significantly reduced during the closure and rehabilitation phase, compared with the ‘normal’ operations of a mine. However, noise impacts from closure and rehabilitation cannot be ignored, as earthmoving equipment remains operational and often operates in exposed locations as the final landform is created. Any environmental management plans for the mine should remain in operation for closure and rehabilitation to allow for ongoing noise monitoring and community consultation as required.

CASE STUDY: Effective monitoring to understand a noise problem

One of the major dump stations for a large open-cut coal mine was located close to a residential property. The homestead for the property was located approximately 1,200 metres north-east of the dump station, on elevated land with a clear line of sight to the dump station.

The current owners had resided on the property since June 2000. Complaints had been received since the dump station was upgraded in mid-2002.

Ambient noise logging at the site showed low background levels typical of rural areas, but no clear pattern of elevated noise levels in relation to dump station activities. There was no clear correlation of high noise levels with high coal throughput, nor low noise levels with low coal throughput. This made it difficult to understand the extent and nature of the problem.

Measurements during the daytime period typically showed that mine noise was inaudible, and natural noises such as insect and bird noise dominated. Measurements were repeated during the night time period to understand the conditions during the times when complaints were typically made.

This demonstrated that noise levels at night increased by 5 dBA to 6 dBA compared to daytime levels, due to the presence of adverse weather conditions such as temperature inversions. Background noise levels were very low. As a result, noise emission from the dump station dominated the ambient noise environment.

The major noise sources audible were:

  • engine and track noise from dozers
  • conveyor noise
  • clangs from coal dropping through the rill tower
  • noise from trucks unloading at the dump station.

The coal stockpile acted as a noise barrier for the mobile equipment operating behind the stockpile. It was found that noise levels from this equipment could increase by up to 12 dBA when the stockpile was low.

Methods were proposed to reduce these noise levels, including:

  • replacing tracked dozers with rubber-tyre dozers
  • controlling the size of the coal to reduce the impact noise on the rill tower
  • shielding the overland conveyor
  • maintaining coal stockpiles at a high level during night-time periods.

In the end, the mine operator helped to relocate the homestead to the opposite side of the property, away from the dump station. Noise emission from the dump station was completely inaudible at this location, even with background noise levels of 18 dBA.

This case study demonstrates that noise logging alone is often insufficient to properly characterise a noise problem and determine a solution. Background levels can vary considerably with seasons, which, coupled with variability in the noise source and weather effects, can make it difficult to draw conclusions from the logging. Sometimes there is no substitute for attended nighttime measurements under the ‘typical worst case’ operating conditions.

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Dump station showing the effect of coal stockpiles. Soruce: Emma Charlton, AECOM.

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