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Detection of Cavities around Sewer Pipelines using Impact-echo Method

Detection of Cavities around Sewer Pipelines using Impact-echo Method
Jaemo Kang
Issue Date
In recent years, the phenomenon of ground subsidence in major cities has become a major social problem resulting in damage to infrastructures and threatening people’s safety. A survey released by the City of Seoul has showed that more than 80% of the ground subsidence occurred mainly around sewer pipelines. Since sewer pipes tend to rapidly deteriorate, a technical guideline is needed to effectively order replacement or rehabilitation by evaluating the soundness of the sewer pipelines and identifying any common occurrences of danger. Traditionally, Closed-circuit television (CCTV) image analysis are used for the condition assessment of sewer pipelines. Although this method may intuitively evaluate the soundness, it depends entirely on the judgment of surveyor and can not verify underground subsidence risk around sewer. Ground penetrating radar (GPR) and electrical resistivity survey are mainly used to investigate the cavity, which is a trigger of ground subsidence. It is difficult to detect cavities around the sewer pipe because this survey method is performed on the surface and the survey distance is relatively short. In this research, the impact-echo method was proposed as a measure to effectively detect cavities around sewer pipelines. The impact-echo method has been successfully used to detect flaws in concrete and to evaluate the state of grout bonding in tunnels. But, it has not yet been used to detect underground cavities. However, some studies show that the impact-echo method can be used to find the voids inside or the other side of the structure. It is considered that the impact-echo method can be conducted simple test procedure and small equipments, so it can be used to in-pipe investigation for detect cavities. Model and field tests was performed to evaluate whether the impact-echo method can be used to detect cavities around concrete sewer pipelines, and moreover a new impact device was developed to apply a consistent high frequency impact load on concrete pipes, thereby increasing the reliability of the test procedure. Fourier spectrum (FFT) was used to determine the resonance frequency and the short time Fourier transform (STFT) was used to determine the temporal variation of the resonance 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. Finally, a new quantitative index, denoted as the sustained duration, is proposed to detect the cavity around sewage pipelines. The duration that the normalized z-axis value lasts more than a threshold amplitude is calculated and defined as the sustained duration. Both model and field tests demonstrate that the cavity in wet soils can be successfully detected using the proposed index.
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