A Risk-Based Pillar Design Approach for Improving Safety in Underground Stone Mines

The collapse of a mine pillar is a catastrophic event with great consequences for a mining operation. These events are not uncommon, and have been reported to produce air blasts able to knock down, seriously injury or kill miners; cause cascade pillar failures which involve the collapse of neighboring pillars; produce surface subsidence; and sterilize valuable reserves. In spite of the low probability of occurrence for a pillar collapse in comparison to other ground control instability issues, these consequences make these events high risk. Therefore, the design of these structures should be considered from a risk perspective rather than from a factor-of-safety deterministic approach, as it has been traditionally done. Discontinuities are one of the main failure drivers in underground stone pillars. Regardless of this, traditional pillar strength equations do not consider the effect of these. Recently, the NIOSH pillar strength equation introduced a Large Discontinuity Factor that acknowledges the effect of discontinuities in pillar strength. However, this parameter only considers "averaged" parameters in a deterministic way, failing to account for the spatial variability of fracture networks. This work presents a risk-based pillar design framework that enables to characterize the effect of discontinuities in pillar strength, as well as account for the possible range of stresses that will be acting on pillars. The proposed method was evaluated in an underground dipping stone mine. Discontinuities were characterized by integrating Laser Scanning and virtual discontinuity mapping. Information obtained from the discontinuity mapping process was used to generate discrete fracture networks (DFNs) for each discontinuity set. The Discrete Element Modeling Software 3DEC was used along with the DFNs to simulate fractured rock pillars. Different fractured pillar strength modeling approaches were evaluated, and the most adequate in terms of pillar strength values, failure mechanisms representation, and processing times, was selected. The selected model was tested stochastically, and these results were used to characterize pillar strength variability due to the presence of discontinuities. Pillar stress distributions were estimated using an stochastic finite volume continuous numerical model that accounted for the dipping nature of the deposit and the case study mine design. A pillar probability of failure baseline was defined by contrasting resulting pillar strength and stress distributions using the reliability method. Results from this design framework provide additional decision-making tools to prevent pillar failure from the design stages by reducing the uncertainty. The proposed method enables the integration of pillar design into the risk analysis framework of the mining operation, ultimately improving safety by preventing future pillar collapses.