A Study of the Key Factors Associated with a Massive Pillar Failure in an Underground Limestone Mine Using Numerical Models

Several massive ground collapses occurred in the past decade in US underground stone mines where surface subsidence occurred in most of them. The occurrence of a massive ground collapse in stone mines places the mine workers at great risk. The National Institute for Occupational Safety and Health (NIOSH) initiated a project to better understand the causes of these massive collapses and reduce/eliminate their future occurrence. Previously the authors investigated one of these massive collapses and it was found that pillar failure was the most likely cause for the collapse. In this study, the authors further investigated the impact of pillar benching, karst cavities, and geological discontinuities on the potential for pillar failure. 3D LiDAR scans were utilized to determine the extent, location, and dimensions of the karst cavities. Stress and deformation analysis were conducted using 3DEC models to gain more insight about the impact of floor benching and poor rock mass quality on stone pillar stability. Poor rock mass quality can be attributed to the existence of karst cavities, small joint spacing, and poor joint conditions at the study mine. Based on the numerical modeling results, floor benching in association with linear karst cavity and geological discontinuities in pillars can create a large uncontrolled displacement, sliding along pre-existing joints, and induce tensile stresses near the center of the pillar that may result in a brittle pillar failure. INTRODUCTION Between 2015 and 2021, five massive ground collapses occurred in underground room-and-pillar stone mines in the United States (Evanek et al., 2022 and Rumbaugh et al., 2022). The depth of cover at these collapses was less than 600 ft (∼183 m). These events pose a significant risk of ground failure and air blast that may result in injuries, fatalities, destruction of ventilation controls, and a potential surface subsidence (Evanek et al., 2022). In general, massive collapses can be attributed to roof or pillar failure. An example of massive roof failure is the collapse that occurred at the Lake Lynn Experimental Mine (Iannacchione et al. 2007), while the collapse that occurred at Whitney mine was an example of pillar failure (Esterhuizen et al., 2018).