Dynamic Mechanical Properties of Veined Rocks Under Confinement

Abstract

As global mineral demand grows, mining deepens, and major risks arise from extreme geomechanical conditions surrounding the underground infrastructure. Among these risks, strainburst corresponds to complex events characterized by the spontaneous release of energy at the limits of excavations triggered by dynamic disturbances such as blasting and seismicity. To study this phenomenon, we report uniaxial and biaxial confinement tests using a Triaxial Hopkinson bar (Tri-HB) system, and a high-speed camera (200,000 frames per second) to understand the role of dynamic loads and high strain rates (10-102 s-1) in the mechanical behavior and fracture patterns of veined rocks. The rock samples are coming from one of the largest underground mines in the world: El Teniente (Chile). Our results show an increase in elastic modulus after increasing σ2 pre-stress. In addition, fracture propagation is controlled by veinlets that are oriented favorably with respect to the direction of the dynamic load, and two types of post-peak curves, I and II, are observed. While type I is related to fractures for which the sample does not lose the load capacity in a biaxial confinement state, type II is associated to fragmentation and expulsion of fragments from the free faces of the tested samples. In addition, two rockburst-related phenomena are observed: slab buckling, and expulsion of fragments at high speeds. Our results suggest that fractures propagate through the matrix when the mechanical properties show a strong dependence on the strain rate, while they are restricted to the veinlets when this dependence is less evident. If a dynamic pulse generates deformation rates between 10 to 102 s-1, stiffness increases, and therefore energy dissipation as well. Combined with the proneness to block formation in veined rocks, these are conditions that favor the occurrence of strainburst and could increase the severity of this phenomenon in underground excavations.

Keywords: Veinlets, Triaxial Hopkinson bar, Dynamic failure, Strainburst assessment