Dynamic mechanical properties of veined rocks under biaxial confinement

Abstract

As the global demand for minerals continues to rise, mining operations deepen, and major risks arise from extreme geomechanical conditions. One risk of major concern is strainburst, a complex event characterized by the spontaneous release of energy at the limits of excavations triggered by dynamic disturbances such as blasting and induced seismicity. To study this phenomenon, we ran coupled static-dynamic tests performed under uniaxial and biaxial confinement in a Triaxial Hopkinson bar system. We used veined-rock samples from one of the largest underground mines in the world: El Teniente (Chile). Additionally, a high-speed camera was employed to study the role of dynamic loads and high strain rates (10-102 s−1) on the fracture pattern propagation. Our results show an increase in elastic modulus after increasing σ2 pre-stress. Two types of post-peak curves: I and II, were identified. 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 with the fragmentation and expulsion of fragments from the free faces of the tested samples. In addition, we observed that fracture propagation is controlled by veinlets that exhibit a favorable orientation in relation to the direction of the dynamic load. Moreover, two rockburst-related phenomena are observed: slab buckling, and rapid fragment ejection. Our findings suggest that fracture propagation occurs through the rock matrix when the mechanical properties exhibit a strong dependence on the strain rate. When this dependence is less evident, they are restricted to the veinlets. If a dynamic pulse generates deformation rates between 10 to 102 s−1, stiffness increases, and therefore energy dissipation increases as well. When combined with the predisposition of veined rocks to form blocks, these conditions create favorable environments for strainburst occurrences and may exacerbate the severity of this phenomenon in underground excavations.