Application of Impact Imaging Method on Nondestructive Detection of Void Defects in Sandwich-Structured Immersed Tunnel

Sandwich-structured immersed tunnel is an essential component in the field of cross-sea tunnel structural engineering which has a complicated internal structure. The void defects between the contacting surface of the steel and concrete reduce the carrying capacity of the structure. There are presently no effective, rapid, and accurate methods for detecting the casting quality of steel-shell concrete. Based on the propagation law of elastic stress waves in different media and the theory of a near-source wavefield, a prototype experiment on the nondestructive detection of void defects in sandwich-structured immersed tunnel was conducted. The experiment firstly determined the detection criterion for the impact imaging method through the features of the waveform data collected from 2 compartments (12 opening positions) of the full-scale model and then used the detection criterion to perform blind tests on the other 12 compartments of the full-scale model. The blind tests' results of 72 opening positions showed that the prediction precision is 87.5% and thereby verify the applicability and accuracy of the detection criterion of the impact imaging method.