Coal pillar strength estimation from failed and stable cases

A new pillar strength equation is proposed for all practical values of width-height ratio based on 23 unstable and 20 stable pillar cases. Strength data of a number of coal seams are used to show that the in situ large-scale strength is more likely to be affected by depth of cover than the laboratory small-specimen strength. The latter is therefore incorporated in the new equation. Performance of some of the better known pillar strength formulae is compared with the new equation against the case studies. It is proposed that the safety factor should be changed with depth of cover and width-height ratio of pillars. Safety factor values are accordingly recommended for stowed and unstowed pillar arrays and chain pillars. An alternative simpler equation is also proposed for slender pillars