THE INFLUENCE OF GEOLOGIC PROVINCE ON PILLAR STABILITY IN UNDERGROUND LIMESTONE MINES

Four massive pillar collapses occurred between 2015 and 2021 in benched areas at underground room-and-pillar limestone mines in the eastern U.S., including three in Pennsylvania and one in Tennessee. Massive collapses, in which individual pillars experience failure and shed their attributed load onto neighboring, metastable pillars with the result of domino-style cascading collapse, had been rare events. In all cases, post-collapse assessment revealed that failed pillars did not meet the 1.8 Factor of Safety or 0.8 width-to-height ratio recommended by NIOSH research encapsulated in their S-Pillar software, which is based on an empirical database of successes and failures.

The massive collapses also shared the commonality of being situated in the Appalachian Highlands geologic province, in limestone formations that had been subjected to deformation during the Pennsylvanian- to Permian-aged Alleghenian Orogeny. Deformation resulted in the development of persistent joint sets characterized by a Geological Strength Index between 55 and 70. In contrast, limestone mines in formations of similar age that also do not meet S-Pillar criteria have not experienced collapse but are situated in the less deformed Interior Plains geologic province, where jointing is less well developed and where the rock mass is characterized by Geological Strength Index values over 80.

We present the results of pillar strength calculations that incorporate the Geological Strength Index from several mines that experienced massive collapse, as well as those that did not, to demonstrate the negative effect that a jointed rock mass has on pillar stability. We suggest that pillar designs, particularly those in which benching will be incorporated, should include a term for rock mass quality. Calculated pillar strengths incorporating the Geological Strength Index are substantially lower than those based on simply using the Unconfined Compressive Strength obtained from testing intact specimens and may overestimate pillar strength.