Mining Publication: Roof Instability: What Reportable Noninjury Roof Falls in Underground Coal Mines Can Tell Us

Ground falls have historically been responsible for nearly 50% of all fatalities in underground bituminous coal mines (Mark, Pappas & Barczak, 2009). Roof bolting in coal mines began on a modest scale in the early 1950s. Since then, the design of bolts, grout systems, accessories and installation techniques has improved significantly. Currently, roof bolting is the primary means of supporting mine roof in room-and-pillar operations in the bituminous coal sector. Although roof support systems have improved greatly, roof falls continue to occur in bolted areas. Design and maintenance of adequate support systems is essential for ensuring ground stability and preventing roof falls. (Note: The term roof falls is used synonymously with falls of roof and falls in this article.)

Coal mine operators must report two types of roof falls to MSHA: 1) falls causing injury to workers; and 2) noninjury falls in active areas that impair ventilation, impede passage of miners or extend at least to the anchorage zone of roof bolts. Most injury-causing roof falls involve falls of small chunks of roof rock from the immediate roof beam (Robertson & Hinshaw, 2002). MSHA (2010) data indicate that noninjury roof falls are often large and could comprise an entire intersection or extend to an entire pillar length.

The large body of noninjury roof fall reports provides an opportunity to gain insight into the characteristics of roof falls in coal mines. The results will help direct research and development for improving roof support systems. This study was designed to identify geological contributors to roof falls, the relationship between length of bolts and the height of roof fall cavities, and the distribution of roof falls in different coalbeds and mining regions in the U.S. This study included data from approximately 11,600 noninjury roof fall incidents reported to MSHA by more than 800 mines from 1999 through 2008.