Study on microtremor spectrum properties of tunnel lining

A large number of tunnels in the world have been in service for several decades, and their health conditions deteriorate with the increasing service ages. Therefore, the effective inspection methods are essential to assess the health conditions of tunnels. The method of visual inspection has been commonly utilized to investigate the integrity of lining concrete; however, this method is incapable of providing sufficient information about the inner state of the lining concrete, such as the existence of cracks and cavities. Since the microtremor characteristics of a concrete structure are significantly affected by its damage degree, this feature may serve as an effective method to investigate the health conditions of lining concrete. In this study, in-situ microtremor measurements were performed on three spans of a road tunnel with varying damage degrees, and the microtremors were analyzed at the frequency domain (i.e., power spectral density(PSD)) and time-frequency domain (i.e., the short-time Fourier transform (STFT)). This study suggests that the health condition can be evaluated by comparing the changes of frequency peaks in PSD and STFT, and the damages existing in the concrete cause a larger damping ratio. Therefore, as an initial step, this study gives clear evidence that the microtremor intensity characteristics have strong relation with the health conditions of lining concrete, which has the potential to be used in engineering practices.