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Figure 1: Pillar rating system for underground mine in the US

Pillar rating Pillar condition

1 No indication of stress induced fracturing. Intact pillar

2 Spalling on pillar corners, minor spalling of pillar walls. Fractures oriented sub-parallel to walls and are short relative to pillar height

3 Increased pillar spalling. Fractures on pillar walls more numerous and continuous. Fractures sub-parallel to pillar walls and lengths are less than pillar height

4 Continuous, sub-parallel, open fractures along pillar walls. Some diagonal fractures (start of hour-glassing). Fracture lengths greater than half of pillar height

5 Continuous, sub-parallel, open fractures along pillar walls. Well developed diagonal fractures (classic hour-glassing). Fracture lengths greater than half of pillar height

6 Failed pillar, may have minimal residual load carrying capacity and giving support to the stope back. Extreme hour-glass shape or major blocks fallen out

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