Document Type

Conference Paper

Publication Date


Publication Details

Guy Reed and Russell Frith, An assessment of coal pillar system stability criteria based on a mechanistic evaluation of the interaction between coal pillars and the overburden, in Naj Aziz and Bob Kininmonth (eds.), Proceedings of the 17th Coal Operators' Conference, Mining Engineering, University of Wollongong, 8-10 February 2017, 77-89.


Coal pillar design has historically been based on assigning a design Factor of Safety (FoS) or Stability Factor (SF) to coal pillars according to their estimated strength and the assumed overburden load acting upon them. Acceptable FoS values have been assigned based on past mining experience and at least one methodology includes the determination of a statistical link between FoS and Probability of Failure (PoF). The role of pillar width: height (w/h) ratio has long been established as having a material influence on both the strength of a coal pillar and also its potential mode of failure. However, there has been significant professional disagreement on using both FoS and w/h ratio as part of a combined pillar system stability criterion as compared to using FoS in isolation. The argument being that as w/h ratio is intrinsic to pillar strength, which in turn is intrinsic to FoS, it makes no sense to include w/h ratio twice in the stability assessment. At face value this logic is sound. However, this paper will argue and attempt to demonstrate that there is a valid technical reason to bring the w/h ratio into system stability criteria (other than its influence on pillar strength), this relating to the post-failure stiffness of the pillar, as has been measured in situ, and its interaction with overburden stiffness. By bringing overburden stiffness into pillar system stability considerations, two issues become of direct relevance. The first is the width: depth (W/H) ratio of the panel, in particular whether it is sub-critical or super-critical from a surface subsidence perspective. As a minimum, this directly relates to the accuracy of the pillar loading assumption of full tributary area loading. The second relates to a re-evaluation of pillar FoS based on whether the pillar is in an elastic or non-elastic (i.e. post-yield) state in its as-designed condition, this being relevant to maintaining overburden stiffness at the highest possible level. The significance of the model being presented is the potential to maximise both reserve recovery and mining efficiencies without any discernible increase in geotechnical risk, particularly in thick seam and higher cover depth mining situations. At a time when mining economics are at best marginal, the ability to remove unnecessary design conservatism without negatively impacting those catastrophic risks that relate to global mine stability, should be of interest to all mine operators and is an important topic for discussion amongst the geotechnical fraternity.