By Morgan M. Sears, Ph.D., mining engineer, ground control branch, NIOSH; Ihsan B. Tulu, Ph.D., associate service fellow, NIOSH, Pittsburgh Mining Research Division; and Gabriel Esterhuizen, Ph.D., senior scientist, NIOSH, Pittsburgh Mining Research Division
In 2015, 40 longwall mines provided nearly 60% of the U.S. coal production from underground mining methods. This represents a substantial, yet gradual increase from just under 50% over the last 5 years. As a result of this increased production share, the percentage of ground fall-related fatalities in longwall mines has also increased when compared to all U.S. underground coal mines. Additionally, about 80% of ground fall-related fatalities have occurred in areas where the roof was supported.
In an attempt to better understand the status quo of current U.S. longwall support practices, a sample of 21 longwall mines were visited, representing about 40% of the currently active longwall mines in four of the five major U.S. longwall producing regions. The resulting data was obtained from a wide variety of overburden depths, geologic conditions, mining heights, ground conditions, support practices, and gateroad configurations.
Two of the observed mines had options for cribless tailgates using cable bolts/trusses while the remaining mines used some form of standing support. The majority of mines using standing support employed can type supports (37%), while the rest used pumpable cribs or traditional wood cribs/posts. Standing support densities in the tailgate ranged from 0.05 MPa to 0.18 MPa and up to as much as 0.35 MPa in the No. 2 entry. Both entries averaged a support density of 0.12 MPa.
The data collected is reported using both qualitative and quantitative methods. The results from the research update previous efforts in classifying mining accidents and injuries as well as current support practices. This data provides a necessary background for future research aimed at further reduction of ground fall accidents and injuries. Tailgate support was typically standing support with the exception of one observed mine, which was currently using a cribless tailgate configuration when the depth of cover allowed. The remaining mines used some combination of pumpable cribs (31%), cans (or other preformed cementitious support, 38%), wood cribs in both the four- and nine-point configurations (19%), and propsetters (or other yielding, wood props, 13%).
Tailgate conditions, overall, were typically good and discussions with personnel revealed relatively low rates of unsatisfactory performance. It was fairly common, particularly in the east to see more than one type of support in a single gateroad and more than one type of support used in the recently mined panels. Additionally, the use or lack of cable bolts in the gate entry depends mostly on the orientation and magnitude of the regional and local horizontal stress field. This has a tendency to be more problematic in weak roof conditions in the headgate rather than the tailgate due to the inability to install standing support in the belt entry. However, standard design guidelines and practices are still relatively lacking with mine operators tending to rely on past experiences to guide future designs. Future research, involving numerical modeling of the support/rock interaction as well and field instrumentation, data collection, and support testing will be required to advance the science of gateroad support design for use in future mining operation.