Construction noise permit for rock breaking near flats
A contractor is preparing a construction noise permit application for six months of sheet-pile driving and rock breaking for a footbridge foundation. The nearest residential block is 18 m from the site boundary and has windows facing the works; a second block is 55 m away. The accepted noise level for the area is 67 dB(A) for the day period. The contractor's consultant predicts 84 dB(A) at the nearest façade from a hydraulic breaker, and the contractor proposes a 3 m high plywood barrier erected 2 m from the breaker, claiming 12 dB(A) insertion loss. The programme requires rock breaking from 07:00 to 19:00, six days a week. Residents have already complained about vibration. The contractor asks you to confirm that the barrier is 'sufficient' and to support the construction noise permit application.
1. Evaluate the barrier claim and the predicted noise level. What technical factors determine the real insertion loss?
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Barrier claim. A 12 dB insertion loss is an idealised or laboratory figure, not a prediction to rely on. Insertion loss depends on the barrier's surface mass, its height relative to the source–receiver line of sight, its distance from source and receiver, gaps at the base or sides, reflections from other surfaces, and the frequency spectrum. A 3 m plywood barrier 2 m from the breaker may be close to the source, which is good, but the nearest block is 18 m away and has upper floors. The line of sight from a breaker at about 1 m height to a window 10 m up passes over a 3 m barrier at only about 2 m from the source. The barrier therefore provides little diffraction protection to upper floors and is vulnerable to leakage. Field performance is often 5–10 dB, not 12. The arithmetic is decisive: 84 dB(A) minus the claimed 12 dB(A) is 72 dB(A), still 5 dB(A) above the accepted noise level of 67 dB(A). Even a perfect 17 dB reduction would only just meet the accepted level, and meeting the accepted level is not a guarantee that the permit will be issued.
2. What alternative or additional mitigation would you require, and how would you sequence it?
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Required mitigation. First, source control: use a quieter method or plant-a hydraulic crusher instead of a breaker where geotechnical conditions allow, silenced plant, electric equipment, and maintenance of mufflers. Second, containment: a proprietary acoustic enclosure or a barrier close to the source with sealed joints, returns at the ends, and a height that blocks the line of sight to upper floors; verify with measurements, not certificates. Third, layout and scheduling: move breaking away from the nearest boundary, limit simultaneous activities, and avoid the most sensitive hours. Fourth, management: real-time noise monitoring at the façade, a complaint hotline, vibration monitoring and condition surveys for the residents. Fifth, if the permit is granted, comply strictly with its conditions and keep records. I would sequence source control first because it reduces noise at all receptors; barriers are secondary. I would require a trial with measurements before the six-month programme is committed.
3. The contractor asks you to sign a letter saying the works will comply with the accepted noise level. What is your position?
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Signing a compliance letter. I would not sign a letter saying the works 'will comply' with the accepted noise level. No one can guarantee future construction noise, and the prediction already shows an exceedance even with the proposed barrier. What I can sign is a factual letter describing the predicted levels, the assumptions, the mitigation proposed and the residual risk. If the contractor wants support for the permit application, the application should present the mitigation and monitoring honestly. Signing an assurance that is not supported by the numbers would be misleading and could damage the profession's credibility. I would also advise the contractor that a construction noise permit is not a right; the authority can refuse it or impose conditions, and non-compliance is an offence.