Soundness evaluation of a tunnel concrete lining by using the hammer impact-induced sound wave

A number of tunnels have been constructed in Korea in the last century. The aging of tunnel with time involves huge amount of maintenance works, which are directly related to the safety of tunnel. One of the key components in maintaining tunnel is how effectively we can find defects in the tunnel concrete lining. As an inspection tool, several non-destructive testing techniques have been often applied to the detection of crack and cavity in the tunnel concrete lining, including GPR, SASW, ultra sonic test, AE method, etc. However, most of these techniques require sophisticated measuring techniques as well as complicated analyses; unless well trained engineers are fully involved in testing from beginning to end, the results of the tests are often questionable. In order to overcome these problems, a sound-wave technique is adopted as an inspection tool to examine defects of the tunnel concrete lining. The input signal (i.e., sound wave signal) is induced by hammer impact. Data acquisition system is carefully designed, and the wavelet transform technique is used to analyze the received signal in time-frequency domain. To quantify the state of concrete lining, the soundness index is newly defined which can be obtained by manipulating the results of ridge analysis. The applicability of this technique is verified through model tests. Model tests indicated that it is possible to evaluate the existence of cavities as well as the existence of crack zones using the proposed technique. Proposed scheme can be utilized as a diagnosis tool which can easily be performed before performing the further complicated technique.