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.
It has long been known that rock materials behave quite differently under dynamic loadings compared to static loading. Recent studies (Bohloli 1997, Zhou et al. 2012) showed that the Unconfined Compressive Strength (UCS) of rock increases when the loading pulse value increases.
In the present paper, the results of an experimental program performed on Lavasan granite in dynamic loading are presented and compared with static loading tests results (Pellet et al. 2011). This program was conceived to assess the efficiency of the drill and blast excavation method as well as to evaluate the environmental impact of tunnel excavation in an urban area.
The Split Hopkinson Pressure Bar test (SHPB) was developed by Kolsky (1949) as a modification of the Hopkinson pressure bar test (Hopkinson 1914). Several studies have been carried out with this equipment on different materials (Forquin et al. 2010, Johnson 2010, Kaiser 1998,Weimin and Jinyu 2009).
The SHPB system is composed of two axial bars (input bar and output bar) and a striker launched by a gas gun. Figure 1 shows a general view of SHPB while Figure 2 presents a schematic of the device.
A short cylindrical specimen is installed between the two main bars. The impact between the striker and the input bar generates a compressive wave (loading wave and unloading waves) which is the output to the specimen.