The limitations of the current pillar strength formulas, empirical design charts, and continuum numerical modelling is that these methods do not consider discontinuity driven failure mechanisms (i.e. jointing at critical angles). Strength formulas have been very effective within the coal industry because they have a better representation of the rock mass strength (sample sizes > standard sizes) and large databases of case histories providing sufficient statistical representation of both stable and failed cases. Additionally, coal pillars generally have larger width to height ratio (>1) which decreases the influence of through going structure. This is opposite for hard rock mine where the width to height ratio is generally <2 and as small as 0.3, were structure can have a major influence on the stability. Additionally, the small size of hard rock unconfined compressive strength samples does not represent the strength of the rock mass, without additional considerations. Therefore, an improved strength formula is proposed that includes the influences of rock mass strength and discontinuities. This proposed strength formula, and its associated factors (length effect, and discontinuity dip and properties) can be used to create site specific empirical design charts.

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