The holding mechanism of under-reamed rockbolts differs from that of conventional rockbolts, in which the bonding or the friction along the element-rock interface provides the holding capacity. The under-reamed end is kinematically blocked by the surrounding rock mass and can provide a greater holding capacity, especially in soft rock, whereas the strength of a soft rock frequently controls the bonding strength of the element-rock interface. Both an experimental study and numerical analyses were performed to examine the holding mechanisms of model under-reamed rockbolts subjected to direct pull out loading and pretensioning. When subjected to direct pull out loading, the holding capacity originates from the capacity of the rock resisting tensile fracture. Failure is characterized by the formation of a smooth, conical region bounded by a tensile crack, which subsequently separates from the surrounding rock. Correspondingly, the holding capacity is related to the tensile strength of the rock, bolt length and size of the under-reamed end. When subjected to pre-tensioning, the holding mechanism is provided by the ability of the rock to form two conical zones between the faceplate and the under-reamed end, and to prevent subsequent indentation of the two cones. Major factors influencing the holding capacity of under-reamed bolts include the size of the under-reamed end, bolt length and properties of the rock.
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