Observations and evaluation of floor benching effects on pillar stability in U.S. limestone mines

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.