The role of veinlets in the unconfined behavior of El Teniente Mine rock samples: Implications for mining-induced rockmass failure.

El Teniente, located in central Chile, is the largest underground copper mine in the world, with ore extraction reaching depths of up to one thousand meters below the surface. The primary ore body is hosted in volcanic rocks of basic composition, extensively intersected by a stockwork of closely spaced veins. At present, the combination of the regional stress regime, localized stress concentrations around excavations, and the heterogeneous nature of the veined rock mass poses significant challenges to operational safety, particularly when considering an extension of the mine’s productive life. Discontinuities, ranging from large-scale faults to small-scale veins, are closely linked to failure processes in mine excavations. In highly homogeneous, intact rock, spalling failure in tunnels is typically initiated at about 50% of the rock’s uniaxial compressive strength (UCS). This relationship is not consistently observed in veined rocks, where standard UCS tests often fail to account for the mechanical influence of discontinuities during deformation. In this study, an experimental setup that integrates four strain gauges, acoustic emission (AE) monitoring, and digital image correlation (DIC) during uniaxial compressive strength (UCS) testing was developed. The method was applied to 20 mm-diameter cylindrical rock specimens from El Teniente to investigate the role of sulfide-rich veins in rock deformation. Samples were selected to minimize the occurrence of multiple veins, containing instead a single primary vein (< 4 mm thick) oriented between 0° and 90° relative to the axial loading direction. Particular emphasis was placed on strain partitioning, with DIC employed to obtain full-field strain measurements, enabling the quantification of strain differences between the rock matrix and the veins. Experimental results indicated that veins accommodated greater strains than the surrounding rock matrix during both the elastic and plastic regimes. All the samples shown a rotation of the local stress tensor on the vein when reaching the onset of dilatancy. Veins oriented between 0 to 40° yielded before the bulk material, with yield onset occurring at 50-60% of the UCS. These findings suggest that precursory shear strain within favorably oriented veins, evidenced by the onset of AE activity, dilatancy and DIC, may play a critical role in initiating rockmass failure at excavation boundaries.