Investigating the unbonded sections of fully grouted rock bolts using a microphone

In fully grouted rock bolts, unbonded sections may exist at any location within holes. This study aims to investigate the integrity of grouted rock bolts using a microphone and rock-bolt model tests. Rock bolts having different unbonded lengths are prepared in a concrete block. Longitudinal and flexural vibrations are generated on the rebar by applying axial and transverse hammer impacts on the rock-bolt head. The acoustic wave from each vibration is evaluated using a microphone, and the frequency responses to the vibrations are analyzed. The results show that as the unbonded length increases, the predominant frequency of the longitudinal vibrations decreases and the flexibility of the flexural vibrations increases. This study demonstrates that the integrity of grouted rock bolts without accessories can be evaluated by microphones and frequency–response analyses.