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Acoustic monitoring of the structural and operational conditions of a live underground siphon via matched field processing 881
A new experimental facility was developed at the University of Bradford to study the application of acoustic methods to monitor the evolution of blockages in or damage of an underground drainage siphon. The acoustic part of this facility consists of one underwater speaker, one reference hydrophone and three measurement hydrophones installed in the opposite legs of the siphon. The spectral composition of the acoustical signal received on three measurement hydrophones was analysed and used in matched field processing to determine the degree of correlation between the acoustic field in the siphon under different operational and structural conditions. The effects of the water level, bubbles and surrounding environment have been studied. The results suggest that the acoustical signal is very sensitive to the changed conditions in the siphon. A progressive increase in the amount of the deposited sediment results in a progressive increase in the acoustic attenuation. It can be detected by measuring the sound pressure level in a selected frequency band. Wall damage results in the change of the acoustic pressure in the low frequency band and reduced correlation between the damage and undamaged condition data. It can be detected by applying a matched field processing algorithm.
Hydraulic siphons are used extensively in underground drainage systems underneath railway embankments and roads to remove the excess of water during and after a rainfall event. These are critical assets where failure has the potential to cause the road or railway to flood with a loss of its structural stability and transport capacity. Some form of continuous remote monitoring would be of great benefit in proving the on-going safety of the assets and to give early warning of failure to enable remedial works to be planned and implemented in a timely manner and to mitigate the impact of failure by allowing time to take appropriate mitigation measures. Such a system would reduce significantly or eliminate completely the need for regular CCTV survey of the siphons. This would lead to substantial cost savings, since CCTV survey of the siphons is expensive and may prove to be impossible if the siphons cannot be pumped dry.
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