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Abstract 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. 1 Introduction 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. 2 Experimental Program 2.1 Experimental set up for SHPB test 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.
- Asia (0.46)
- North America > United States (0.28)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Igneous Rock > Granite (0.46)
Abstract Lemaitre's viscoplastic model is widely used to model the long-term behavior of rocks, especially argillaceous rocks that are being considered as host rocks for radioactive waste repositories. However, this model has marked disadvantages, such as pressure independency and no volumetric viscoplastic strain, which greatly limit its ability to reproduce the complex phenomena in underground structures. In view of these, based on thermodynamic principles, Lemaitre's phenomenological viscoplastic model is extended in this paper. Two equivalent surfaces are introduced in the p-q stress plane in order to consider the contribution of hydrostatic stress on the viscoplastic strain and damage respectively. The inherent concept is that the stress points located on the same equivalent surface share the same intensity for both viscoplastic strain and damage. Moreover, in this model, the non-associated flow rule is assumed and a hyperbolic viscoplastic potential is used to reproduce volumetric viscoplastic deformation. The time dependent damage evolution law is also modified to rely on inelastic strain. In order to verify the constitutive model and to study the newly added parameters, several numerical procedures were performed corresponding to quasi-static tests, creep tests and relaxation tests. Finally, a detailed discussion about the influences of these parameters on both compression and tension cases is presented. 1 Introduction Lemaitre's viscoplastic model is widely used to characterize the long-term behavior of argillite (Souley et al. 2011) as well as to model the time dependent evolution of the Excavation Damage Zone (EDZ) around underground openings (Pellet et al. 2009). However this constitutive model assumes that the hydrostatic stress has no influence on the viscoplastic behavior and it imposes no volumetric viscoplastic strain. These are obvious drawbacks in its ability to model geomaterial behavior. In this paper, an extension of Lemaitre's model that will overcome these drawbacks is presented. Based on thermodynamic principles, two equivalent surfaces are introduced to take into account the influence of hydrostatic stress on both the evolution of viscoplastic strain and damage. In addition, the volumetric viscoplastic strain is introduced by using a hyperbolic viscoplastic potential. The time dependent damage evolution law is also modified to be dependent on inelastic strain. This model is implemented into the finite element software ABAQUS 6.10, and detailed studies of the new parameters are performed by modeling quasi-static tests, creep tests and relaxation tests.