Testing and modelling of an energy absorbing rock bolt

Dynamic loads on underground structures, such as tunnels, may be caused by blasting during nearby construction work or bomb detonations on the surface above e.g. a rock shelter. These loads give rise to vibrations that propagate as stress waves through the rock, possibly causing damage to structures and installations or even causing fragments or pieces of rock to be thrown out from the rock surface. A common and important energy absorbing component in rock support is steel rock bolts. A new type of energy absorbing rock bolt has been developed and dynamically tested. The bolt consists of a smooth steel bar with impressions made over a section of its length to provide required anchorage when fully grouted. When subjected to a dynamic load, the adhesive bond between bar and surrounding grout is lost and the bolt is able to absorb energy by plastic lengthening. The dynamic tests were performed as free-fall tests of rock bolts in concrete cylinders. As a follow up, the problem has been analysed using numerical stress wave propagation models. A statically, or quasi-statically, loaded rock bolt shows evenly distributed elongation. The tests performed with dynamically loaded rock bolts show that the distribution of plastic strain along the length of a fully grouted bolt is not constant. For the tested type of rock bolt to absorb a reasonable amount of energy, the permanent plastic strain must be allowed to propagate an appropriate distance along the bolt. This propagation is governed by the impulse time and the shape of the dynamic stress-strain curve of the bolt material.