International Rock Mechanics Symposium, Montreaux, 1979
Authors: J. F. T. Agapito (Consultant), M. P. Hardy (University of Minnesota), and J. Page (Atlantic Richfield)
Tabular ore deposits form a major portion of the reserves of the coal, potash, trona, and limestone that are mined by underground methods, while some copper and lead-zinc deposits are also tabular in shape. Oil shale mining represents a new addition to this group. Mining of tabular, extensive, ore bodies becomes difficult when the thickness of the ore body increases. For thin seams, longwall methods or room-and-pillar methods results in good extraction ratios, but as the seam thickness increases, longwall methods are not practical (with present mining machinery) and for room-and-pillar methods extraction ratios decline because pillar strengths decrease with increased height. Alternative mine layouts must be found to mine thick seams with high extraction. This paper details two such possible mining methods and discusses the major factors in the selection of roof spans and pillar sizes that control the overall extraction ratios. These two methods are referred to as the ‘Lane and Pillar’ and the ‘Rib, Slab and Fill method’. The lane and pillar method involves mining long parallel lanes between barrier pillars then pillaring by mining cross cuts on the retreat to leave ‘small’ yielding pillars. Because miners do not need access to the yielding pillar area, these pillars can be designed with a low safety factor. Design for this method includes not only room span and rib pillar dimension selection for the first pass mining, but also estimation of yielding pillar ‘post failure’ characteristics and ultimate load carrying capacity. Analysis has been performed using the displacement discontinuity model to illustrate the influence of the post failure characteristic of the yielding pillars on the safety of the mining method. The rib, slab and fill method is similar to the above method but here high extraction is gained on the retreat by slabbing the rib pillars. Additional extraction is attained because backfill is introduced to the lanes after the slabbing operation. This fill acts to improve the post failure load-bearing characteristics of the rib pillar, thereby limiting the ultimate room closure. Primary mining is always adjacent to undisturbed ore under very stable conditions. Slabbing is in the adjacent room between the primary mining and a backfilled room. Analysis of this mining method depends heavily on the assumed characteristics of the pillar behavior. The fill acts only to improve the behavior of the pillar after the peak load has been reached. Analysis using the displacement discontinuity method is presented to demonstrate the significance of this pillar-fill interaction on the overall allowable extraction ratio.