Detection of the unbonded sections of fully grouted rock bolts using acoustic waves

Unbonded sections may occur at various points along the length of fully grouted rock bolts. This research explores the condition assessment of these bolts through acoustic wave analyses and controlled model experiments. Rock bolts with varying lengths of unbonded sections were embedded in a concrete block. Hammer strikes were applied to the bolt heads to induce longitudinal vibrations in the rebars. The resulting acoustic signals were measured by a microphone, and their frequency characteristics were subsequently analyzed. It was observed that a longer unbonded section corresponded to a reduction in the dominant frequency of the longitudinal mode. This study demonstrates that the structural soundness of fully grouted rock bolts can be effectively assessed using microphone measurements and analysis of vibration frequency responses.