ABSTRACT
Sooner or later virtually every seam in the Appalachian coal region will experience ground control problems due to multi-seam interaction. Mining of ultra-close seams which lie within 25 feet vertically of each other, such as where seams have split, gives rise to many of these problems. Columnization of pillars is the most common practice in pillar layout for roof stability, yet analyses demonstrate that columnization results in a very high load concentration in the pillars. On the other hand, while offsetting pillars may decrease stress concentrations in the pillars, considerably higher shear stresses can be generated in the lower seam roof. Differing interaction mechanisms in ultra-close seam mining are governed by the special characteristics of such mining operations. The innerburden is the dominant structural component in all stability considerations and the structural integrity of the innerburden is primarily governed by the relative positions of upper and lower pillars.
