Impact testing for rockbolt design in rockburst conditions

Abstract

Weight-drop tests on steel rods simulating the impact loading of mechanical bolts by rock blocks ejected during rockbursts were performed in the laboratory to study the dynamic response of these bolts. The steel rods were loaded to deform either elastically or plastically. Cushions of rubber and wood and a slip mechanism were investigated as a means to reduce the elastic stress or plastic strain in steel rods. It is found that the elastic stress or the plastic strain can be determined using the energy balance approach. While the stress can be used as the controlling parameter for rockbolt design within the elastic range, the plastic strain parameter must be employed to exploit the energy absorbing potential of the bolt if plastic deformation of the bolt is acceptable. Rockbolt design examples are given to illustrate the concepts of elastic and plastic designs. It is found that soft cushions placed between mesh and head plate can be employed to absorb wave energy and protect weak connections in a rockbolt-mesh support system, effective slip mechanisms must be designed for fully grouted bolts, and frictional bolts are ideal for excavation walls. Finally, it is pointed out that if the accumulation of plastic strains is anticipated due to repeated seismic loadings, additional support elements must be installed with time.