Health Monitoring of Tunnel Shotcrete Lining Using Nondestructive Testing Methods

Health monitoring of shotcrete on excavated rock surfaces is a crucial issue when considering the short-term and long-term stability of tunnels. The state of the back-surface of shotcrete is investigated using impact-echo (IE) techniques and ground penetration radar (GPR). A laboratory-scale experimental study was performed as a field application in this study. Three different bonding conditions (i.e., void, de-bonded, or fully bonded) were experimentally modeled. Signals obtained from the IE tests were analyzed at the time-frequency domain. Using an integrated active signal-processing technique coupled with a Short-Time Fourier Transform (STFT) analysis, the bonding state of shotcrete can be effectively evaluated compared to GPR. The IE method coupled with the Short-Time Fourier Transformation shows better performance than GPR for monitoring the quality of shotcrete. Limited field tests were also performed to verify the evaluation chart for the health monitoring of tunnel shotcrete linings.