Research laboratory NIOSH, near Pittsburgh, conducted a comprehensive analysis of groundfall injuries and reportable noninjury roof falls, using U.S. Mine Safety and Health Administration (MSHA) data from 1995 through 2001. The study considered both longwall and bord and pillar statistics in an attempt to identify trends.
“The effects of groundfall incidents are extensive, ranging from the economic loss of equipment and production to the fatal and nonfatal injuries that result in lasting physical and financial impairments suffered by the victims and the victim’s family,” researchers said.
“In addition, the mining industry is severely impacted by these injury incidents, as well as the thousands of noninjury incidents that damage equipment, halt production or disrupts ventilation.”
(Noninjury incidents are required to be reported if the roof fall occurs in the active workings of the mine and extends above the roof bolt anchorage, impairs ventilation or impedes passage.)
The study found that despite longwall mining accounting for 49% of the underground production and 40% of the hours worked, it accounted for only 19% of the reportable noninjury roof falls.
During the seven-year study period, 58 longwall mines produced almost as the same coal tonnage as 775 room-and-pillar mines. However, room-and-pillar mines accounted for 78% of the total roof-fall injuries and 81% of the total noninjury roof falls. Therefore, the roof-fall-injury incidence rate for room-and-pillar mines was more than double the longwall roof-fall rate.
The researchers believe the significantly lower longwall roof-fall rate may be related to the continuous roof protection provided by longwall faces supports, and to the greater percentage of workers in outby support functions in a typical longwall mine.
Rib injury rates were however, similar for both mining methods, believed to reflect the fact that thicker seams are mined using longwalls. Some 75% of longwall production comes from seams that were more than 1.5m thick, while just 30% of room-and-pillar production comes from thick seams.
The study also identified higher groundfall rates occurring in different geographic regions where there were numerous problematic coal beds, such as in the Illinois Basin. This appears to explain why longwall mines in western Kentucky and Illinois/ Indiana had injury and noninjury roof-fall rates significantly higher than the national average.
Researchers believe this correlates to other studies that have shown that the compressive strengths of typical roof rocks in the Illinois Basin are much lower than those in the central Appalachian coalfields.
In another NIOSH study the strength of roof rocks in the Illinois Basin was compared with that of rocks from southern West Virginia. It concluded that for each of the three major roof rock types (shale, siltstone and sandstone), the strength of the Illinois Basin variety was less than 50% that of the same rock in West Virginia
The data also indicated that weaker geology affects mines in the northern Appalachian coal beds, specifically the Sewickely, Redstone, Pittsburgh, Lower Freeport, Upper Kittanning and Upper Freeport coal beds.
However, despite the weak roof, the data shows that longwall mines in this region maintained an admirable record with noninjury roof-fall rates lower than the national average.
“It seems likely that the narrow entries and immediate roof bolting used in gateroad development at Pittsburgh seam longwalls may have contributed to the low noninjury roof-fall rate,” researchers said.
Small bord and pillar mines with less than 50 workers did have a significantly higher fatality rate. More than 50% of the fatal groundfalls between 1995 and 2001 occurred in mines of this size. The study postulates that perhaps the smaller mines have less expertise and capital to deal with complex ground-control problems.
The study also identified the impact of seasonal change on roof-fall rates. Between July and September, the chance of these occurring nationwide was 50% higher, possibly because of changes in humidity levels that may be causing shale mine roofs to deteriorate.
This article based on a technical paper published in "Mining Engineering" in September 2003, by NIOSH civil engineer, Deno Pappas and Section Chief, Rock Mechanics, Christopher Mark. Original paper entitled "Profile of groundfall incidents in underground coal mines". Rewritten with permission.