Underground mining, Finite elements, Stope Project, Dilution.

This thesis has the general objective of quantitatively analyzing the effect of time on the instability and unplanned dilution of the stopes from the underground mining method of openings by sublevels, using numerical modeling, via the finite element method. The Sublevel Open Stope underground mining method is one of the most widespread in underground mining, where unplanned dilution is one of the most important parameters to be considered in ore recovery. The unplanned dilution is directly linked to the stability of the excavations, requiring good management and control for an adequate design of the stopes. The prediction of unplanned dilution consists of numerically determining the post-dismantling displacements that occurred on the walls of the stopes during the time they remain exposed without applying any type of support. The numerical approach adopted here consists of applying an elasto viscoplastic model with formulation for the finite element method, with the ability to simulate the behavior of the rock mass dependent on time, around of the stopes. In the simulation, the finite element code CODE BRIGTH with Mohr-Coulomb plastification criterion and associated type plasticity and the bilinear law of softening were used, where cohesion and effective friction angle are reduced linearly with the occurrence of deviating plastic deformations. The viscoplastic model is Perzyna's, with a structure based on plasticity theory and viscous effects only in the plastic regime. To assess the applicability of the model, a synthetic case and two real cases were analyzed, totaling 22 simulation scenarios. The models represent two-dimensional sections perpendicular to the openings, located in the center of the stope, where displacements were obtained through wall instabilities indicators. The synthetic case was elaborated to understand the behavior of the instability indicators over time, in which the evolution of the indicators was observed as a function of the input parameters of the model, noting that it can be easily calibrated to known rheological conditions of the rock mass. The second case was elaborated from a real situation of displacement that occurred on the roof of an stope of the Zinc Mine of Nexa, with a record of the evolution of displacements carried out by mining in a period of seven months. Its simulation showed some limits of applicability of the model. The third case concerns the opening of the stope of block 9140 located at elevation 420 of the Vazante mine, 268.5 meters from the surface, with a measured dilution of 43.3% on the hangingwall the stope. The results obtained by the simulation were quite coherent with the actual dilution data occurred in the stope. Two instability indicators were analyzed, the regions of low confinement and the horizontal displacements, presenting dilution values very close to the real values, with an increase in the unplanned dilution of 2.2% for each month of exposure. According to the results of the simulations, the elasto-viscoplastic model used proved to be consistent in the proposal to capture the deformations over time in the underground mining stopes, with easy adaptation of the parameters, coherence with real data and theories of the rheological behavior of the rocks. Being a good tool in evaluating the stability of the stope walls over time