The Mayor's Ambient Noise Strategy - Greater London Authority

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The Mayor’s Ambient Noise Strategy Mayor of London 101 Lack of resources and divisions of responsibility have made it hard to achieve in the UK. Changes in the national railway industry provide a chance for reappraisal. 4B.6 Noise generated from rolling contact between wheel and rail is the dominant source for electric trains, and in many situations, particularly at higher speeds, for diesel trains. Noise generation depends not just on speed, but on the condition, particularly smoothness, of the wheel and railhead surfaces. Wheels with cast-iron tread brakes are typically 8 dB(A) noisier in rolling than disc-braked wheels. Corrugations, with a pitch typically between 30mm and 80mm (but considerably longer on London Underground), can form along the running surface of the rail. These can be removed by railhead grinding, provided the corrugation is not too deep. Rail corrugation can increase nearby noise by 10 to 20 dB(A) - subjectively twice or four times as loud. This is larger than noise reductions typically achievable by noise barriers. The prediction method used in the UK for assessing new or additional railways 1 , assumes track in good condition. Across a sample of the UK rail network, actual noise levels may be averagely 5 dB(A) higher, but in some locations up to 20 dB(A) higher. Noise levels typically change over track maintenance and replacement cycles. 4B.7 On the national rail system, rail regrinding has usually been done only for safety reasons, rather than noise. The benefits of appropriate types of rail grinding for noise control are increasingly recognised. Additional grinders have been, or will soon be, bought into use. By 2006, a full preventative regime for gauge corner cracking will be introduced, which could give significant benefits for environmental noise. Automated systems for identifying railhead condition, wheel flats, and other problems, are increasingly available. Systems have been installed at a number of locations which identify trains that could damage the infrastructure by causing excessive forces (for example, due to wheel flats or out-of-round wheels). Such trains, which would also be noisy, could be withdrawn from service until the problem is rectified. Some sections of track prove susceptible to more rapid corrugation re-growth than others. This is for complex reasons, many of them locally specific and not currently predictable. It may not be cost-effective to require grinding to a set standard everywhere. On London Underground, rail grinding is already carried out for acoustic reasons and different standards may be needed depending on the sensitivity of the site. Further research is needed to establish cost-effective monitoring and maintenance regimes in different situations. New contracts, franchises and other arrangements should not be so structured as to make improved noise management artificially difficult or expensive.
102 Mayor of London The Mayor’s Ambient Noise Strategy 4B.8 Some fundamental design issues can be particularly intractable. High levels of tonal noise (see glossary) can occur on tight radius curves. The piercing sound can be highly intrusive and wheels can be damaged, generating more noise. Ways of reducing ‘wheel squeal’ include railhead or wheel lubrication, water sprays, and special coatings to rail running surfaces. Resilient wheels, such as used on many modern tram systems, are primarily effective in reducing squeal rather than normal rolling noise. 4B.9 Continuously welded rail is quieter than traditional jointed track. On national rail, much jointed track has already been replaced with continuously welded rail, giving noise benefits of perhaps 2 dB L Aeq . Long welded rail (maximum 700 metres, as opposed to continuous welded) has been used on parts of the Underground. Quieter rail types are under development, including tuned damping (see glossary) of the rail to reduce noise radiation. Rail fastenings and mounting systems include proprietary systems designed to offer groundborne noise or vibration reductions. Maintaining good track quality includes ensuring railway sleepers are firmly bedded in the ballast. The open structure of coarse aggregate ballast gives noise absorption (of perhaps 2 dB) compared with slab track, in which rails are attached to a solid concrete slab. Under-ballast mats - a resilient mat placed below ballast - can reduce groundborne noise and vibration as can floating slab track, although cost effectiveness needs to be considered. Space constraints may also preclude use of floating slab track. 4B.10 The mechanisms by which noise and vibration propagate from underground sections of track to buildings are complex. They include the type of tunnel, type of property, distance, ground conditions (including groundwater), and type of rail fixing. Reducing vibration at the wheel-rail interface is a typical first area for optimisation. policy 19 The Mayor will urge the Government to allocate clear responsibilities, and to consider suitable funding mechanisms, for the rail industry to develop cost-effective quieter railway track technologies and management systems. Research to establish the parameters for cost-effective good practice should be carried out urgently, taking account of any guidance provided by the European Commission-funded ‘STAIRRS’ project. Quieter rolling stock and operation 4B.11 New railway rolling stock, particularly that used in moderate speed suburban operations, is generally quieter. The replacement of treadbraked rail vehicles with modern stock using disc brakes has typically reduced noise. Traditional iron block tread brakes, still fitted to many vehicles, cause roughening of the wheel treads. Other composite brake
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