Factors Affecting Pre-Tension and Load Carrying Capacity in Rockbolts - A Review of Fastener Design

Recent studies into rock bolting parameters have suggested that increasing nut length has a positive influence on the pretension achieved during rockbolt installation. A literature survey on bolts and fasteners was undertaken, as well as a laboratory testing program to examine this suggestion. The testing program examined M24 nuts ranging from 1D to 1.5D (24 mm – 36 mm). These nuts were used to tension a rockbolt using an instrumented drill rig to measure torque, and a through-hole load cell to measure pretension. It was found that nut length has no impact on the pretension values achieved during rockbolt installation. The largest factor affecting pretension values was found to be the levels of friction acting between bearing surfaces. Overcoming this friction is estimated to account for 85-90% of the applied torque from the drill motor.