Detection of cavities around concrete sewage pipelines using impact-echo method

Impact-echo method has been successfully used to detect flaws in concrete and to evaluate the state of grout bonding in tunnels. It has not yet been used to detect underground cavities formed by leakage of pipelines, which is becoming a major problem in many urban environments. We perform model and field tests to evaluate whether the impact-echo method can be used to detect cavities around concrete sewage pipelines. Fourier spectrum was used to determine the resonant frequency and the short time Fourier transform was used to determine the temporal variation of the resonant frequency component. Results show that the presence of cavity cannot be reliably predicted from the wave time history or Fourier spectrum. The spectrogram is demonstrated to be a better indicator for detecting the presence of a cavity. The cavity induces a longer tail in the spectrogram because of a higher impedance contrast and a lower attenuation. We propose a new quantitative index based on the resonant frequency and the spectrogram, denoted as the sustained duration. Both model and field tests demonstrate that the cavity in wet soils can be successfully detected using the proposed index.