Effects of shear coupling on the acoustic characteristics of torsional waves in buried fluid-filled pipes

Waves that propagate at low frequencies in buried pipes are of interest for the detection of pipe fracture and other potential modes of pipe failure. Currently there is a demand for methods which can detect and locate fracture events as they occur and, if possible, remotely determine the mode of failure. Of particular interest is spiral fracture, which may commonly occur in various pipe systems under a variety of different circumstances. This paper therefore focuses on torsional wave motion in buried pipes. The effects of shear coupling on the characteristics of wave propagation and attenuation are analysed for a compact pipe/soil interface for which there is no relative motion between the pipe wall and the surrounding soil. A simple analytical expression is derived for the torsional wavenumber from which both the wavespeed and wave attenuation can be obtained. Example results are presented for both a cast iron pipe and a MDPE pipe buried in two typical soil types. It is found that the predominant effect of shear coupling is to add stiffness and damping due to shear wave radiation. The wave also becomes highly dispersive at very low frequencies, leading to a significant change in the wavespeed compared with the in-vacuo case. Whether the torsional wavespeed increases or decreases from that in an in-vacuo pipe, depends on both frequency and properties of the pipe and the soil, which determines the relatively ratio of the contributions of shear wave radiation and the pipe wall mass. In general, the shear coupling results in a larger wavespeed, apart from at the extreme low frequencies, for a metal pipe and a smaller wavespeed for a plastic pipe. Future wavenumber measurements are being undertaken on the dedicated pipe rig to corroborate the theoretical predictions.