Brittle-ductile transition in coal pillar failures

Factor of Safety (FoS) is the most widely used empirical criterion in coal pillar design. The increased vertical stresses associated with deep mining are resulting in pillar sizes much larger than those used in the development of the empirical strength formula. It is not uncommon to see pillars with over 50 m solid widths based on the established design criteria, which reduces overall reserve recovery and development productivity. Previous research revealed that pillars will transform from brittle to ductile failure with strain-hardening behaviour when their w/h ratio exceeds a certain critical value. However, the actual failure mechanics of pillars with different w/h ratios have largely been understudied. This paper discusses whether a coal pillar with a critical w/h ratio and reduced FoS can achieve functionality to allow more flexibility in pillar design for mine layout optimization. Using the more advanced laboratory testing and monitoring equipment that is now available, the failure mechanics of coal samples with different dimensions are investigated through uniaxial compression tests. The w/h value at which in situ pillar behaviour would be expected to transform from brittle to ductile is then considered and discussed further, including the research program that would enable the findings to be safely applied in deep underground mines.