Behaviour of yielding mechanical hybrid rockbolts under complex loading conditions

The original “hybrid bolt” was a pragmatic solution to address installation issues in the use of resin grouted rockbolts in heavily fractured ground and the inadequate capacity of friction rock stabilisers. The original hybrid rockbolt involved installing a resin rebar in a friction rock stabiliser. The development of Yielding Mechanical Hybrid Rockbolts has been driven by efforts to eliminate the use of resin in heavily fractured ground. A typical Yielding Mechanical Hybrid Rockbolt consists of a steel tendon mechanically anchored within a friction unit. The bolt is percussion driven by the rock drill and the mechanical anchor is subsequently activated using rotation. This installation process improves the rockbolt resilience to hole closures and avoids issues associated with the use of resin. Yielding Mechanical Hybrid Rockbolts are used in both squeezing and rockburst prone ground conditions.

This paper addresses a significant knowledge gap related to the behaviour of Yielding Mechanical Hybrid Rockbolts under multiple quasi-static conditions. A comprehensive experimental program investigated the complete load displacement performance of Yielding Mechanical Hybrid Rockbolts under axial, shear, and a combination of tensile and shear loads. It was observed that the load capacity increases from axial (0°), combined axial and shear (30°), combined axial and shear (60°), and pure shear (90°). The displacement capacity, however, decreases under the same testing conditions. The results are consistent but there is a slightly greater variability as the loading angle increases from 0° to 90°. It was observed that beyond 60° loading angle there is greater variability in the results as the influence of the shear component manifests itself in greater disintegration of the concrete blocks at the shear interface.