Researchers from the Spokane Research Laboratory of the National Institute for Occupational Safety and Health have used the Particle Flow Code (PFC2D) computer program to determine whether detecting large voids in surge piles is feasible. The method is to determine profil e volume--the volume calculated by determining the profile of the top surface-of the pile near a drawpoint, monitor the volume drawn, and compare the two volumes in real time to detecthe formation of voids. If void formation could be detected early, a warning could alert workers of hazardous conditions. Six numerical experiments were run to simulate various conditions. Results show that when no material was added to the pile, voids could be detected by comparing real-time curves of volume scanned and volume drawn. Slopes and differences of the curves were used to detect a void. When material flowed into the area from a stacker tube, void detection was difficult. Prior knowledge of flow curve slopes and an additional indicator of flow from a stacker tube may be necessary to detect voids. An indicator of the presence of a bulldozer in the area is necessary to help identify material added to the pile.
The Mine Safety and Health Administration (MSHA) issued Program Information Bulletin P99-1 on January 13, 1999, to alert the coal mining industry about hazards associated with coal surge piles and the large number of fatalities associated with surge pile accidents since 1980 (17 fatalities from 13 accidents). Twelve of these 13 accidents occurred when a void formed in the surge pile during drawdown and collapsed. Ten of the victims were bulldozer operators.
Iverson and others (2001) performed a fault-tree analysis .to determine the most probable root cause(s) of bulldozers falling into voids. Although several conditions must be present before a bulldozer can fall into a void, two necessary conditions are the formation of a void and the presence of a bulldozer over the draw cone area. Bulldozers must work near draw cones, and rules and safeguards must be in place to prevent operators from driving bulldozers over draw cones. However, if a void could be detected early on, operators could be warned to stay far away from feeder areas until the bridge over a void collapses.
While the risk of being drawn into a cone during drawdown operations is significant, once personnel or equipment are engulfed, rescue operations may be even more hazardous. Rescue operations involve sending a worker down into the cone in a suspended basket or cherry picker or cutting a path into the cone from the side, both processes that send a second equipment operator beneath hazardous slopes or cut walls.