Bonding state evaluation of tunnel shotcrete applied onto hard rocks using the impact-echo method

Abstract

Shotcrete is an important primary support for tunnelling in rock. The bonding state of shotcrete on excavated rock surfaces is a core issue in the safe construction and maintenance of tunnels. Although shotcrete may be applied well initially onto excavated rock surfaces, it is affected by blasting, rock deformation, and shrinkage and can debond from the excavated surface, causing problems such as corrosion, buckling, fracturing and the creation of internal voids. This study suggests an effective non-destructive evaluation method of the tunnel shotcrete bonding state applied onto hard rocks using the impact-echo (IE) method. Various bonding conditions (i.e., fully bonded, debonded, and void) were simulated using numerical and experimental models. The signals obtained from the numerical and experimental IE tests were analyzed at the time domain, frequency domain, and time-frequency domain (i.e., the short-time Fourier transform). Both the numerical and experimental results suggest that the bonding state of shotcrete can be evaluated through changes in the resonance frequency and geometric damping ratio in a frequency domain analysis, and through changes in the contour shape and correlation coefficient in a time-frequency analysis. Limited field tests were also performed for practical applications. (c) 2009 Elsevier Ltd. All rights reserved.