Influence of bond defects on the pullout response of grouted rockbolts: Analytical study and experimental validation

Rockbolts have been proven to be an effective and efficient technique to strengthen and retrofit rock masses in civil engineering. The stress transfer mechanism at the bolt-grout interface has been recognized as one of the key factors affecting the reinforcement effectiveness. In practice, bond defects are commonly observed at the interface owing to poor grouting or environmental deterioration during service life, which leads to the degradation of the bonding performance and even unexpected structural failure in severe cases. This paper develops an analytical model to predict the full-range pullout response of grouted rockbolts with bond defects. The model involves closed-form solutions for the load-displacement curve, stress distribution and peak load and takes into account the influences of the defect length and location, embedded length, and interfacial friction. The proposed solution presents a notable advantage in practical design as it highlights the key behavioural differences between short and long rockbolts and enables a theoretical estimation of the effective embedment length without experimental calibrations. An experimental study on the influence of interfacial bond defects on the rockbolt pullout behavior is designed, considering different defect lengths, borehole diameters, and bond lengths. After the developed model is verified with the self-conducted experimental results and the data collated from the literature, a sensitive analysis was conducted to quantify the relation between bond defects and structural responses. It is found that the proposed model can predict the pullout response of grouted rockbolts with bond defects with reasonable accuracy. The results also indicate that for rockbolts with an embedded length slightly greater than Leff, increasing defect length from 10%Leff to 30%Leff can result in a load capacity reduction of exceeding 10% due to the increased effective embedment length. Defects located close to the loaded end, within high stress zones, have a more pronounced impact on the load response and the effective embedment length compared to those positioned near the free end in lower stress zones. The present study can provide guidelines for practical design when a bond defect is ducted during inspection, enabling a more accurate performance evaluation of grouted rockbolt systems.