Mining Publication: The State-of-the-Art in Coal Pillar Design

By 1980, the U.S. mining community had reached a broad consensus regarding coal pillar design. The pillar load could be estimated from the tributary area theory, and the pillar strength from empirical formulas and laboratory coal strength testing. Then the growth of longwall mining required new thinking. Recently, powerful design methods have emerged from analysis of large data bases of real-world pillar successes and failures. These include the Analysis of Retreat Mining Pillar Stability (ARMPS), the Analysis of Longwall Pillar Stability (ALPS), the Mark-Bieniawski rectangular pillar strength formula, and guidelines for preventing massive pillar collapses. Sophisticated numerical models have also helped transform the pillar design landscape. In the process, our understanding of pillar mechanics has been greatly enriched. A new paradigm divides pillar failure into three categories: Slender pillars (w/h<3.0), which are subject to sudden collapse; squat pillars (w/h>10), which are dominated by entry failure (rib, roof, or floor) and coal bumps, and; intermediate, in which pillar squeezes seem to be the most common failure mode.