A Case Study of the Collapse of Slender Pillars Affected by Through-Going Discontinuities at a Limestone Mine in Pennsylvania

Abstract

The sudden collapse of approximately 3 Ha of room-and-pillar workings at a limestone mine in southwestern Pennsylvania in 2015 resulted in an air blast that injured three mine workers. Subsequent investigations showed that an area encompassing 35 pillars had collapsed. The pillars were 9-10 m wide and up to 18 m high. A notable geologic feature is the through-going joints that dip at 50-80° and can extend from the roof to the floor of the pillars. These structures are thought to have weakened the pillars well below the strength that is predicted by empirical equations for hard-rock pillar design. This paper presents the relevant geotechnical data related to the collapsed area and numerical model results that were used to estimate the pillar loading underneath the variable topography, and compares the pillar loads to some established hard-rock pillar strength equations. The outcome is also compared to a strength equation that was developed specifically for limestone mines in which the negative impact of large angular discontinuities is explicitly accounted for. The results show that established hard-rock pillar strength equations do not adequately account for the impact of large through-going discontinuities on the strength of slender pillars. The equations would have significantly overestimated the strength of the pillars at the case study mine. The critical state of the workings would have been predicted correctly by the limestone pillar strength equation that accounts for the large discontinuities.