Evolution of Stress-Induced Damage and Anisotropy Prior to Rock Failure in the Main Lithologies of El Teniente Mine, Central Chile

Abstract The expansion toward deep mining poses new geomechanical challenges. To understand the deformation and seismic behavior of rocks under conditions at mining relevant depths and under elevated stress prior to failure, holistic characterization of stress-induced anisotropy should be performed. To do so, we designed a suite of triaxial tests under confining pressures of 25 MPa, at ambient temperature conditions, on 100 mm × 40 mm cylinders of five representative lithologies of the El Teniente underground mine: a sericitic breccia, a veined andesite, a tonalite, diorite, and finally a dacite. We contemporaneously recorded the output of acoustic emissions (AE: a laboratory analog to earthquake activity) and performed active seismic surveys to understand the evolution of compressional wave velocities in six different directions. Both the mechanical and seismic responses differed between the lithologies. The tonalite exhibited the highest strength, while the breccia was the most compliant of the samples. The tonalite, dacite, and veined andesite samples all exhibit extremely brittle behavior with very little acoustic-emission output prior to failure. However, while much of the deformation prior to failure, in these rocks, was apparently accommodated with little AE output and predominantly elastically, we noted directionally dependent changes in P-wave velocity (Vp) at up to 73% of the failure stress. Similar directional changes in Vp were not observed prior to failure in the veined andesite sample. When accurately characterized, seismic anisotropy related to stress-induced deformation prior to failure may aid in the design of predictive monitoring strategies in deep mining environments.