A survey of roof and pillar conditions in underground limestone mines in the United States has revealed that bench mining of the floor between pillars can cause instability in the pillars at the perimeter of the benched area. Increased loading of these pillars was observed at several mines. Large inclined geological structures that are exposed in pillar ribs were observed to contribute to pillar instability. The paper describes a study that was carried out using numerical models to assess the effects of bench mining on pillar load and stability. It was found that the benched pillars shed load onto the surrounding pillars owing to their reduced stiffness. The pillars at the perimeter of the benching area will start shedding load as soon as benching increases the height of one side of the pillar. A case study is described which shows the effects of bench mining on limestone mine pillars.
Underground limestone mines in the U.S. make use of the room and pillar method of mining. Bench mining of the floor between the pillars is sometimes carried out to improve utilization of the reserves. Figure 1 shows an area where bench mining of the floor is underway. The National Institute for Occupational Safety and Health (NIOSH), Pittsburgh Research Laboratory is investigating pillar and roof stability in U.S. limestone mines. Observations of pillar conditions carried out as part of this study revealed that the condition of pillars around the perimeter of bench mining operations can be worse than elsewhere in the mine. Unstable pillar ribs represent a rock fall hazard to mine personnel. Over the decade from 1996 through 2005, ground falls were the cause of 36% of fatalities and 12% of all injury related lost workdays in underground limestone mines (Mine Safety and Health Administration, 2006).