Radial stiffness of rock bolt samples and required thickness of the steel tube in impact tests

Steel tube is used to simulate rock mass in laboratory impact tests of rock bolts. The steel tube must be equivalent to the borehole in the rock mass in radial stiffness in order to obtain valid test results. The stiffness of the steel tube is directly associated with the thickness of the steel tube. The importance of the tube thickness has not been paid enough attention in the laboratory tests of rock bolts at present. This technical note derives a formula for calculating the required thickness of the steel tube used for laboratory tests of rock bolts. It also provides a formula for calculating the stiffness of the steel tube under plane stress which is the realistic loading condition of rock bolts in the field. The analysis reveals that the grout in the tube hole does not play a role in the calculation of the thickness of the steel tube. The required thickness of the steel tube is solely dependent on the Young's moduli and Poisson's ratios of the tube steel and the rock mass to be simulated.