Research on signal processing of segment-grout defect in tunnel based on impact-echo method

In shield tunneling construction, grout of precast concrete segments is widely applied to various geological conditions. However, the connection performance between the grout and segments after the grout cannot be determined by destructive means, such as coring. The impact-echo method is used extensively in nondestructive testing of civil engineering structures. Nevertheless, there are inadequate researches on determining defects in segment-grout structure. In this paper, the segment-grout structure was simulated experimentally and artificial defects were introduced in the grout. The Short-time Fourier Transform (STFT) and Wavelet Transform (WT) of signals obtained by the impact-echo method were carried out to analyze the time-frequency domain features of the impact-echo signals with and without defects in the grout. On this basis, defects in the grout layer were determined. Finally, energies of wavelet packet were calculated and characteristic relative energy indexes of wavelet packet at different positions were detained. It's found that the values at positions without defects are relatively low. It's demonstrated that the signal analysis of impact-echo method has to combine time-frequency domain information and energy distribution for detection of defects in the grout layer. Wavelet transform and energy analysis methods are effective means to detect defects in the grout layer.