Evaluation of rock bolt integrity using Fourier and wavelet transforms

As rock bolts have become one of the main support systems in tunnels and underground structures, the integrity of the rock bolts affects the safety of these types of structures. The purpose of this study is the evaluation of rock bolt integrity using Fourier and wavelet transforms of guided ultrasonic waves. Rock bolt specimens with various defect ratios are embedded into a large scale concrete block in the laboratory and in the rock mass in the field. Guided waves are then generated by a piezo disk element and measured by an acoustic emission sensor. The captured signals are analyzed in the frequency domain using the Fourier transform, and in the time-frequency domain using the wavelet transform based on a Gabor wavelet. The magnitude spectrum obtained from the Fourier transform shows that a portion of high frequency content increases with an increase in the defect ratio. The peak values in the time-frequency domain represent the interval of travel time of each echo. The energy and phase velocities of the guided waves increase with the defect ratio. This study shows that the magnitude spectrum ratio and the energy and phase velocities may be indicators for the evaluation of rock bolt integrity.