Bore-and-fill mining was implemented at a pilot site in Tau Tona mine's shaft pillar on 97 level. Observations, scan data and measurements from this site were used to calibrate an inelastic numerical model of bore-and-fill mining within the carbon leader reef (CLR).

Borehole breakout was found to occur around holes at the site as they were bored. A strike-parallel exposure showed that fracturing within the carbon leader stratum extended far beyond the breakout (up to 1.2 m from the hole sidewall). This observation was confirmed by borehole camera observations in a hole drilled on reef through a bored-and-filled hole.

A Mohr-Coulomb strain-softening model was calibrated against the observed fracturing. It was found that was necessary to include interfaces corresponding with CLR contacts to replicate the extent of fracturing that was observed. The effect of hole position on breakout was quantified by analyzing hole scan data. Models with holes at various positions, together with this data, were used to calibrate the upper and lower CLR contact properties. A novel, displacement-based method for simulating breakout was developed for use on Carbon Leader models. Though the breakout distributions were not precisely matched, the modelled values lay within the extremes that were observed. Trends and other features observed underground were also replicated.

Modelled stresses and measured stress histories showed similar responses to the boring of nearby holes, but the magnitudes differed by varying degrees in each stress component. The modelled sub-vertical stress changes were generally lower than those measured, though both were greater than those obtained from a virgin stress measurement done in the fill. It was noted that either measured value could be obtained by simply shifting the measurement position in the model. The strike stresses correlated reasonably well. Dip stresses were very different: measured tensile stresses greater than the fill strength suggest that these measurements were erroneous.

The result of this work is a calibrated plane strain model which can be used to study the effects of geology, mining sequence and fill properties on the rockmass response to bore- and-fill mining.

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