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

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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.

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(photo by A. T. Iannacchione)

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Acknowledgements

The case study presented in this paper was conducted as part of the mission of NIOSH and MSHA to improve the safety of mineworkers. The authors wish to acknowledge the cooperation of the mining company in providing safe access to the mine after the collapse event and providing relevant geologic data and mine maps for the analysis. The thoughtful suggestions and comments of the reviewers, as well as their commitment to the rock engineering discipline, are acknowledged with thanks. This paper was published in an abbreviated form in the proceedings of the 52nd US Rock Mechanics Symposium, Seattle, Washington, USA, 17–20 June 2018, as paper no. 18-363.

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Authors and Affiliations

  1. NIOSH Pittsburgh Mining Research Division, 626 Cochran’s Mill Road, Pittsburgh, PA, USA

    Gabriel S. Esterhuizen & Michael M. Murphy

  2. MSHA Technical Support Group, Triadelphia, VW, USA

    Paul L. Tyrna

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  1. Gabriel S. Esterhuizen
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  2. Paul L. Tyrna
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  3. Michael M. Murphy
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Correspondence to Gabriel S. Esterhuizen.

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Esterhuizen, G.S., Tyrna, P.L. & Murphy, M.M. A Case Study of the Collapse of Slender Pillars Affected by Through-Going Discontinuities at a Limestone Mine in Pennsylvania. Rock Mech Rock Eng 52, 4941–4952 (2019). https://doi.org/10.1007/s00603-019-01959-6

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