There are inherent problems in limiting noise levels from minerals workings because most of the work is carried out in the open, and the mineral can only be worked where it occurs. There are however, a number of good practices that should be adopted to minimise the noise impact of a site. Information on these methods is provided in Chapter 8 of BS 5228-1:2009 445 (with separate sections for opencast coal and other surface minerals).
As with all potential environmental impacts, it is better to control the noise at source, than to try to reduce it over its transmission pathway or at the receptor. It is at the planning stage where there is most scope for mitigating noise issues. This includes hours of operation (non-operation removes the noise at source) which will be specified in the planning conditions and often includes Saturday mornings. Working hours also needs to take into account the arrival and departure of HGVs, particularly if there is an asphalt plant on site, which may be loading vehicles through the night or early morning.
|If the noise is not reduced sufficiently at source, then the noise reaching the noise-sensitive area can be reduced by increasing the distance between source and receiver (not always possible) or introducing noise reduction barriers. Close proximity working shown in Photograph 13 is not advised (it was an unoccupied test house)!|
Photograph 13. Noisy? Yes, but no-one was living in this research house
Good noise control for a site is dependent on noise issues being fully assessed and considered at the planning stage of the site. Once the site is operational, it is much more difficult to significantly change the noise generation from the site without significant cost and timescale implications.
Issues to be taken into account at the planning stage should include the use of existing natural features and the location of screening mounds (Photograph 14). The positioning of fences, maintenance compounds, pumps and any other plant operated at night, and haul road location both on site and from the site to the public highway, should also be considered. It might be possible to design the site such that the direction of operation reduces the noise at sensitive locations, although other factors such as geotechnical stability may determine the mine plan.
Photograph 14. Screening using natural (left) and artificial (right) bunds
Photograph 15. Mobile crushing and screening plant next to a blasted face
Perhaps the biggest issue to be considered is the type and location of processing plant. For operational and financing reasons, there has been a considerable increase in the number of mobile processing units being used. They are usually situated closer to the working face (Photograph 15), which means they are set deeper in the quarry, and therefore less likely to be a source of problem noise. Indeed, the reduction in noise can be a major factor in the decision about whether to use mobile crushing and screening units.
|If a fixed plant is chosen on a new site, it is impossible to locate at anywhere other than on the surface. However, when an existing site is being developed or extended, it may be possible to relocate the processing plant "in-pit". There are a number of examples of this (Photograph 16), with significant benefits to the noise and dust emitted, but relocating plant is an extremely expensive operation, and would probably only be possible if it was operationally necessary in terms of replacing old plant, or opening up reserves.|
Photograph 16. Fixed processing plant located "in-pit"
It is standard practice that an operator will need to submit details of predicted noise levels for the operation of the site. Prediction models are capable of making reasonably accurate predictions of future noise levels, and planning conditions will take these into consideration, subject to the background noise levels and overall limits.
Choice of equipment
There may be significant noise benefits that can be achieved by choice of equipment. This may occur if a quieter method of working is an option for the site. One example of this is the use of conveyors to transport material as compared with dump trucks. However, there are a large number of operational issues to be balanced against noise considerations, and it is unlikely to be possible for sites always to use the quietest working method.
In the past, there have been significant differences in noise levels generated by different makes of similar sized plant, but progress has been made on limiting plant noise through EC Directives. One of the first of these directives in 1986 covered noise from plant such as excavators, dozers and loaders 157. More recently, a wider range of plant has been covered by the EC Directive 'Noise emission in the environment by equipment for use outdoors' 158. The EC Directive was implemented into UK law in 2001 L0179. These Directives have set noise limits in terms of maximum sound power level as measured under specified conditions. This has meant that noise output from certain types of plant is now regulated, and the Directives are now imposing lower sound power levels. . Manufacturers must display the appropriate information on their products (shown in Photograph 17 as a value of 106 LWA for this Volvo L120E wheeled loader)
Photograph 17. Sound power level rating displayed for this Volvo wheeled loader
In highly sensitive areas, it may be possible to fit additional noise reduction features onto equipment; items like noise absorbent panelling to the sides of engine compartments, or rubber lining in dump truck bodies.
Noise-sensitive properties will experience noise from most if not all items of equipment operating on a site. At most sites where there will be a large number of different noise sources operational, a large reduction in noise levels for one item of plant may not be noticed in the overall site noise. It is only generally if sites are working at night, or there are fixed items of plant close to a noise-sensitive property, that individual items of plant can become dominant. This highlights the need for effective noise reduction measures to be put in place for all sources.
Vehicle reversing alarms
One specific item of equipment that can cause complaints is vehicle reversing alarms. These are provided for safety reasons for the workforce, and need to generate a certain level of noise to achieve this. They are a particular problem on the overburden mounds which project higher than the original ground level on many opencast coal sites. However, there are now more options such as directional and adjustable systems, which can help to minimise the noise impact, and developments are being made all the time in this important area of health and safety.
Operation and maintenance of plant
In order to ensure that noise emissions from mineral sites remain acceptable, it is of fundamental importance that the equipment on site is well maintained. Significant increases in noise levels from items of plant can be generated by small defects in silencers or acoustic enclosures not being used as designed. Poor maintenance often leads to the generation of annoying noises, e.g. squeaking bearings, un-silenced exhausts, which will lead to more complaints than would be expected from the overall increase in noise levels. A summary of the practical measures in the choice and use of plant to reduce noise is given in Table 3.
Table 3. Practical measures to reduce noise from plant.
|Adopt a buying policy that includes consideration of noise for all new items of plant.|
|Avoid unnecessary revving of engines and switch off equipment when not required.|
|Ensure plant and vehicles are properly maintained, check silencers and bearings.|
|If the noise is directional, point the source away from noise-sensitive locations.|
|Keep internal haul routes well maintained and avoid steep gradients.|
|Use enclosures for noisy plant such as pumps or generators.|
|Use rubber linings in, for example, chutes and dumpers to reduce impact noise.|
|Minimise drop height of materials.|
|Limit the use of particularly noisy plant or vehicles.|
|Start up plant and vehicles sequentially rather than together.|
|Ensure the plant and vehicles are operated with noise control hoods closed.|
|Keep lorry tailgates closed where possible.|
|Use any appropriate acoustic treatment equipment.|
Once the working method and equipment for the site have been chosen, acoustic screening with a noise barrier is the main method of noise control that can be implemented on the site. A noise barrier is any large object that blocks the line of sight between the noise source and the receiver, and its performance depends on the factors listed in Table 4 L0089. Some operations, such as processing of the mineral and maintenance operations, may possibly be carried out within buildings, but most of the operations on site can only be screened rather than enclosed.
Table 4. Factors affecting sound barrier performance (from ALSF Review)
|Barrier geometry (height, length, and cross-sectional form).|
|Presence of sound absorbing materials on the barriers.|
|Aperture size in the barrier.|
|Form and frequency spectrum of the noise source.|
|Source and receiver locations relative to the barrier position.|
|Topographic and acoustic impedance of the ground between the source and receiver positions.|
|Local weather conditions, particularly wind speed and direction.|
|Presence of nearby reflecting/scattering objects.|
Consideration of acoustic screening must take place at the site planning stage. The primary method of screening operations is to use baffle mounds or noise fences. Baffle mounds (Photograph 18) use material such as soils and overburden that has to be removed to allow access to the mineral. They are generally located on the site boundary and are usually designed to provide screening for noise-sensitive properties. There are sometimes practical limitations on screening all working areas, particularly on sites on hillsides. In some situations, there may be difficulties in providing a mound on the site boundary, so a noise fence can be used instead (Photograph 19).
Photograph 18. Soil baffle mound on left of picture
Photograph 19. Acoustic fence on site boundary
The formation and removal of baffle mounds is often the noisiest operation on a site, but the mounds will generally provide a reduction of 5-10dB(A) in noise levels generated by the site. A 10dB(A) reduction is the equivalent of halving the loudness of the site noise. As working progresses below ground level, the amount of screening automatically increases and the noise level decreases. Therefore, operational noise levels are normally at their highest at the start of quarrying operations, although some shallow sand and gravel operations might never be hidden from particular receptors.
The ALSF Review of Noise L0089 looks at the use of plantations and vegetation to reduce noise levels, and cites a number of studies, most of which focus on industrial sites and traffic noise. The review notes that vegetation belts in the range of tens of metres thick are required to attain significant noise attenuation. If the site can be designed to make use of existing vegetation, then this may be helpful, but planting new trees would not have any beneficial effect on noise for many years.
More localised acoustic screening can be provided for specific fixed location items of plant, particularly if the plant is working 24 hours per day. Items such as pumps and compressors can be provided with localised screening to reduce noise emissions, and in cases like this, the screening should be as close to the source as possible. It can also be used alongside site access roads where this is close to sensitive properties.
Tables 5 and 6 give final brief overviews of the good practice that can be undertaken by regulators and operators respectively, in relation to the control of environmental noise associated with mineral workings.
Table 5. Summary of good practice for Mineral Planning Authorities
|Consider the ambient noise, planning policies and the duration of the noise.|
|Discuss any limits and monitoring with the local Environmental Health Officer (or equivalent).|
Consider the need to agree or specify planning conditions relating to:
Occasionally, planning conditions will be required for particular activities:
Table 6. Summary of good practice for operators
|Discuss noise in advance with the MPA and demonstrate in their application that proposed conditions can be met.|
Plan ahead and make sure that:
|Ensure that management has the will to run the site as quietly as possible.|
|Check the noise characteristics of plant before use and periodically thereafter; where appropriate retro-fit noisy plant, ensure good operation and maintenance.|
Make no unnecessary noise and reduce noise emissions, e.g.:
As a last resort, reduce the propagation of noise, by the use of:
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