Noise Propagation along Construction Tunnels

Tunnel excavation is a major construction stage for subway establishment. In Hong Kong, construction work can only be carried out during 0700-1900 hours unless assessment-based Construction Noise Permit (CNP) is granted from the Authority. With CNP, tunnel construction becomes continuous 24-hour operation to achieve a tight construction programme. In the CNP assessment, noise attenuation of tunnel is not regarded as mitigation measure unless in-situ measurement report is provided. In reality, noise attenuation of long tunnel with rough surface is a natural, cost-free and effective mitigation measure to reduce the noise impact from tunneling machines. This paper presents noise attenuation measurements of various construction tunnels in Hong Kong and establish an empirical prediction method for various construction tunnels. As most of the noisy construction plants like rock drill and hydraulic breaker are operated deep inside a long tunnel, those noise can only propagate along the tunnel and radiate from the tunnel portal openings. Current predictive models for noise attenuation along ventilation ducts or long corridor have been developed for a few decades, while that for construction tunnels is yet to be developed for increasing demand of 24-hour tunnel construction. This paper presents various measurement results of noise attenuation along different types of tunnels and comparing with the model predictions. Noise attenuation of 1200m long tunnel with concrete linings and 400m long tunnel with shotcrete linings were measured to be 51dB and 39dB. The required noise attenuation of 1200m and 400m long tunnel is 46dB(A) and 35dB(A) respectively to achieve the statutory criteria. With these tunnel noise attenuation measurement results, the 24-hour drill and blast excavation was allowed to conduct at tunnel section deeper than 1200m and 400m, and the 24-hour drill and blast excavation was improved from 6 to 9 blasts per week so that the tunnel excavation schedule were shortened.