Inter-panel barrier pillars are relatively wide pillars between longwall panels and may be used in deep coal mines, say, below 700 meters. Narrow pillars are favored for resource recovery and operational efficiency, while wide pillars are favored for safety and stability. Finite element analysis in two and three dimensions provides useful guidance in determining a satisfactory pillar width. At a deep coal mine (828 m depth) in central Utah where a two-entry system is used, a width of 150 m is adequate in consideration of stratigraphy, rock properties, premining stress, panel and pillar geometry and the excavation sequence. Three sets of four borehole pressure cell measurements at the study site were of some value in assessing model results.
This paper presents an analysis of inter-panel barrier pillar width in deep, underground coal mines. Analysis is based on the popular and powerful finite element (FE) method. Inter-panel barrier pillars may be used in conjunction with longwall mining at depths where conventional side-by-side panel excavation is difficult. Wider pillars provide better isolation and therefore reduction in mined panel interactions that increase stress, while narrower pillars increase resource recovery and reduce development entry lengths and therefore costs. Inter-panel barrier pillars are usually many times wider than they are high, although pillar width depends on panel width. Longwall panel widths are usually greater than about 250 m (750 ft). A barrier pillar in a 3 m (10 ft) thick seam mined full height may be over 120 m (400 ft) wide. Two- and three-entry systems are commonly used for longwall panel development in the coal mines of central Utah. Figure 1 illustrates a two-entry longwall panel with the possibility of side-by-side panel mining or the use of inter-panel barrier pillars. In side-by-side mining, the previous head gate becomes the next tail gate. Design of chain pillars in these access roads is an important feature of the overall system as Gilbride and Hardy explain . In either system the chain pillars do not survive panel excavation. Indeed, significant supplemental support in the development entries is usually required, frequently in the form of "cans", thin-walled steel cylinders filled with particulate material . Wire mesh or chain-link may be strung along the supplemental support to control slough from ribs.
Fig. 1. Two-entry longwall panel development with an adjacent next panel or inter-panel barrier pillar. (available in full paper)
Barrier pillar safety and economy involve more than width, of course. Of particular interest are possible modes of barrier pillar failure that range from sudden, violent expulsion of coal from pillar ribs into development entries to slow yielding of ribs with maintenance of a strong, confined pillar core. Gas pressure and strata interfaces at seam top and bottom may be of importance in abetting or inhibiting release of horizontal confinement that appears essential to pillar stability. Gas trapped between impermeable strata in roof and floor may also contribute to excavation instability. Mine seismicity is associated with pillar, roof and floor instabilities. Mining may induce movement on existing faults, while fault slip may induce additional stress about mine excavations.