Abstract: It has been a long concerned issue that a variety of measurement techniques is applied to detect the internal structure of underground medium in geotechnical engineering. The theoretical basis of the impact imaging method is to take characteristics of the elastic wave propagating through different medium surfaces. Thus, it has the substantial potential to produce the resolution for target medium and the feasibility for the detection of buried low-velocity layers. In this study, the principle of impacted image detection method was clarified. For the horizontal layered medium model with a low-speed interlayer, a series of response waveform data in each separate case was obtained by using finite difference numerical simulation. Then the corresponding amplitude, dominant frequency energy and time frequency of response waveform were analyzed. Furthermore, their change characteristics were obtained. In addition, the validity and the accuracy of the impacted image detection method were verified. Based on the results of numerical cases, the impact imaging method was applied to detect the grouting quality of the immersed tube tunnel in Haihe Binhai, Tianjin. According to the measurement of waveform data collected on-site, we investigated the visualization of the response waveform, frequency spectrum, time frequency and the comparison of the change characteristics of elastic wave before and after the filling. A comprehensive evaluation was finally realized for the grouting effect on the immersed tunnel baseboard and a satisfied detection result was also achieved.

Key words: elastic wave, multi-layered medium, impact imaging method, response waveform, grouting effect