Determination of dynamic rock strength to assess blasting efficiency

Dynamic tests using the Split Hopkinson Pressure Bar test (SHPB) were conducted on granite specimens in order to study the efficiency of the drill and blast method and the environmental impact of the excavation of a tunnel in urban areas. The tests were performed at different strain rates of loading corresponding to variable energy levels. Compared to the static uniaxial strength, the dynamic strength of the rock was found to be much higher. The dynamic strength clearly increases with the strain rate of the loading but decreases with the duration of load application. The same conclusions can be drawn for the Young's modulus. Using the experimental results, the strength parameters of the rock were back calibrated using a Finite Element model of the test; preliminary numerical simulations of SHPB tests gave good results. These results can therefore be used to correlate the blasting load to the damage observed in the tunnel during excavation.