3.9 Planning phase

Good planning is essential to mitigate noise impacts which might otherwise affect the surrounding community or the natural environment. Optimising the way in which an exploration program is conducted and the way the mine is designed from the very earliest phase, with the assistance of an acoustic specialist, can minimise impacts and assist in meeting community expectations.

The first step in implementing leading practice for a new project, or the redevelopment of an existing project, is to ensure the appropriate expertise is available in the team that will conduct the environmental assessment that examines the proposal in detail and identifies all the potential sources of noise.

The stages of work in the planning phase can be broadly categorised as follows:

  • monitoring background or ambient noise within any potentially affected community
  • setting noise criteria and design goals for assessing adverse impacts, including on-site and off-site noise (the mandatory regulatory criteria vary slightly between jurisdictions; more information is available from the environment protection authority in each state)
  • predicting noise levels for a number of future scenarios, including on-site and off-site (transportation) scenarios—this typically involves developing a comprehensive computer model.

Where the environmental assessment shows that the noise criteria will be exceeded, there is a requirement to design feasible and reasonable mitigation measures that will enable the impacts to be effectively reduced. Where this is not possible it is likely that the acquisition of properties will be necessary.

3.9.1 Background or ambient noise monitoring

As part of the environmental assessment process for any project there is normally a requirement to understand and measure the existing ambient noise environment. Monitoring normally takes the form of collecting measurements using an unattended, automatic noise logger. The monitoring should be conducted over a sufficient time period to reflect the true and repeated conditions that are typically experienced in the area, and should not be unduly influenced by seasonal variations due to temperature inversions, winds, insects and so on. In practice, continuous monitoring is conducted for a minimum of one week at representative surrounding residences or other noise-sensitive receivers (such as schools or churches), ideally prior to the mine becoming operational or while the mine is not operating.

The information obtained from these measurements is normally used to set criteria for the project. The most important measure is the background noise level (the LA90), which is normally measured in 15-minute periods.

Meteorological conditions can significantly influence noise levels. Steady wind, for instance, generally causes an increase in background noise levels due to movement of trees. Strong winds and rain can lead to falsely inflated noise level measurements. To enable periods of adverse weather to be identified, a weather station should be set up to continuously monitor wind speed and direction and rainfall. Noise data should then be filtered to account for periods of weather conditions that had an influence over the recorded noise results.

Some residences surrounding new mine sites already experience noise from road traffic, rail lines, other existing mines or other sources of intrusive noise. In these situations, in addition to unattended monitoring there may also be a need to do attended noise monitoring to understand the existing noise levels and estimate the contribution from each source. These measurements may also provide a way of validating the noise prediction method to be used in assessing noise from the project. Often measurements may be done at one or two representative properties in order to validate any predictions.

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Example of environmental noise logger in operation. Source: Wilkinson Murray.

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Example of chart output from noise monitor. Source: Wilkinson Murray.

3.9.2 Regulations

Operational and construction noise

Noise guidelines vary across Australia. Generally, they comprise two aspects: a control on the emergence of mine noise above the background level, and/or an absolute level which varies between daytime, evening and night.

In very quiet areas the regulatory restriction is normally determined based on the emergence above background noise criterion, while in areas with existing industrial or road traffic noise it may be the absolute or ‘amenity’ criterion which is most stringent. The ‘emergence’ criterion is for mine noise (typically measured over a 15-minute period as either an Leq or L10) to not exceed the background noise level (normally measured as an L90) by more than 5 dBA. If the mine noise has ‘unpleasant’ characteristics (such as tonality or impulsiveness) at the receiver, it is necessary to add a correction factor to the measured or predicted mine noise.

To prevent successive developments from causing ‘background creep’, planning levels are often set below the existing background noise level to ensure that the cumulative effect does not result in the background noise environment exceeding an acceptable level.

In addition to these criteria, there is also the requirement to consider the potential for sleep disturbance at night time. This could occur due to noisy crashes and bangs from shunting wagons, or the first load being dumped in an empty haul truck. The assessment normally considers an emergence above the background level, but addresses the short-term maximum noise level rather than an ‘average’ Leq or L10.

Transportation noise

Rather than needing to achieve a ‘background +’ criteria, the approach for transportation noise sources often just nominates an absolute limit to be achieved, based on an hourly limit or 24-hour limit. Where the existing traffic noise levels are already quite high and in excess of the absolute limit, the normal approach is to set criteria to limit any further increases in noise.

Airblast noise

Australian Standard AS 2187.2–2006 Explosives—storage and use—use of explosives provides recommended limits for the control of cosmetic damage to structures. A limit of 133 dB(linear) is recommended as a safe level that will prevent structural/architectural damage from airblast. The standard further notes that different limits may need to be developed for service structures such as pipelines, powerlines and cables located above ground.

The standard’s criteria are designed to assess the risk of structural damage. They are not appropriate for assessing human reactions to airblast. The guidelines for these vary around Australia; however, limits in the order of 110 dB(linear) to 120 dB(linear) are typically recommended. These should be used for buildings that will remain occupied during blasting. Higher limits apply to unoccupied buildings.

3.9.3 Modelling future mining scenarios

Predicting noise emissions from mining projects is usually conducted using environmental noise modelling software. There are a variety of noise prediction software packages available; so long as they use industry-recognised algorithms, they should be acceptable. The ability to handle different meteorological conditions is also very important. The acoustic specialist will normally have a preference, and mine management should understand which model they are proposing to use and how output from that model will relate to noise levels in the community.

A noise model will require three types of information:

  • ground topographic data to represent the mine footprint in several stages of its life—this includes the depth of pits, and the location and gradient of haul roads
  • locations of all plant and equipment and estimates of their noise generation— this is like an aerial photo taken at a time representative of a ‘typical worst case’ operational scenario (not an absolute worst case), describing where equipment would be and what noise it would be generating (this will probably be revisited several times during the modelling)
  • data on meteorological conditions over several years, from a weather station on or near the site.

Noise models can be particularly useful in determining the ranking of noise sources on site and, therefore, changes in noise contribution from a mine as a result of various operational scenarios or noise mitigation measures. As a planning tool they may provide data on ‘average’ noise levels expected at receivers, sufficient to allow planning type decisions.

Of course, at this stage of the project there is no alternative to using a predictive model, but the mine management must understand the limitations of any noise model. At the start of a planning process, a noise model cannot predict with a high degree of accuracy (1–2dBA) what the noise level will be at a particular residence over a 15-minute period for a specific operational scenario. Over time (several years), with enough validation during the detail design phase and the operational phases, the noise model should evolve such that it becomes very site specific and more accurate.

Noise models are only as good as the information entered. Most are developed using empirical data based on measurements conducted in various parts of the world over the past 30–40 years. They are poorly developed with respect to meteorological conditions and assume that only one ‘set of conditions’ exists along the noise path from source to receiver. This clearly does not occur in practice, hence a range of measured noise levels would be expected for a ‘set’ of wind speed and direction or temperature inversion factors. Models can represent the best fi t or average of measured data. It is important to realise that the noise levels can vary by 5 dBA and even up to 10 dBA under different meteorological conditions.

3.9.4 Mitigation measures and acquisitions

In the planning phase, as well as identifying ‘in principle’ measures to reduce noise at source, the project proponent will often need to consider acquiring some properties.

On- site control measures that have been used successfully by companies employing leading practice include:

  • selecting low-noise plant
  • optimising mine layout to shield noise-generating plant and haul roads
  • applying additional silencing measures for fixed and mobile plant and ventilation fans
  • installing acoustic enclosures around process plant
  • using ‘smart alarms’ to minimise complaints regarding vehicle reversing alarms
  • minimising tonal components or impulsive or intermittent characteristics of noise
  • strategically designing bund walls for acoustical screening
  • incorporating buffer zones and landscaped setbacks.
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