Nowadays, aged tunnels keep continuously increasing all over the world, which require effective inspection methods to assess their health conditions. In this study, both in situ acceleration wave measurements and numerical simulations were carried out to study the microtremor characteristics of mountain tunnel lining. Power spectrum density (PSD) of signals was calculated and peak frequencies were identified using the peak-picking method. Discontinuous contacts between rock masses and lining concrete were simplified as weak interfaces with low stiffness, which play the role of elastic supporting during dynamic calculation. Influences of voids, rock type and concrete type on their peak frequencies were evaluated. The results of the numerical analysis show that the normal stiffness of rock-concrete interface has strong correlation with the magnitude of peak frequency. The frequency response of tunnel lining is affected by the presence of voids located around tunnel circumference behind lining. The first peak frequency is dependent on the void size and location. The larger the void size and location angle, the greater the decrease of the first peak frequency. The peak frequency also decreases as the strength of concrete decreases, and is not affected by the change of properties of unweathered rock masses. Additional frequency modes can be identified when voids are not located on the central axis of tunnel. The first peak frequency variation can be considered to be intimately linked with the stress state of the tunnel lining influenced by the existence of voids.
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