Numerical study on the evaluation of tunnel shotcrete using the Impact-Echo method coupled with Fourier transform and short-time Fourier transform

This study presents a simple but robust evaluation technique of the bonding state of shotcrete applied to tunnels using Impact-Echo (IE) method and thus explores the effect of the ground types, thickness of the shotcrete, undulating surfaces and impact sources on system vibration through numerical simulations based on the finite element method. All the signals obtained from numerical simulation for various conditions are analyzed at the frequency domain using Fourier transform (FT) and at the time–frequency domain using short-time Fourier transform (STFT). As the bonding condition worsens, the resonant frequency (fn) and the resonance duration (TR) increase and the geometric damping ratio (DG) decreases. The results of a comprehensive numerical analysis suggest that the shape of the contour in the time–frequency domain, the level of fn, and the TR and DG values can be used as effective indicators for the evaluation of the shotcrete thickness, ground type, and bonding state. Finally, this paper presents a procedure for evaluating the tunnel shotcrete state, the shotcrete thickness, and the ground type with the IE method coupled with Fourier transform and short-time Fourier transform. Limited field tests were also performed to verify the proposed evaluation chart for tunnel shotcrete states.