ABSTRACT:

A crush pillar support system is routinely applied in the intermediate depth gold and platinum mines of South Africa. The support provided by the pillars depends on the residual strength of the pillars in a failed state. As a pillar is formed, it undergoes various stages of deformation. A crush pillar could pose a seismic risk if it does not fail in close proximity to the mining face. An underground trial mining site was established on Lonmin Platinum to study the behaviour of Merensky Reef crush pillars. The visual observations provided information regarding the fracture sequence of the pillars. Many similarities exist between pillars designed to fail (crush pillars) and other types of pillars failing as a result of various factors in other mines across the world. Various types of measurement was conducted which was used to validate the observed behaviour. The measurements were also compared to a limit equilibrium model used to predict the behaviour of the pillars.

1 UNDERGROUND TRIAL

Crush pillars are designed to ensure that the pillars fail when formed at the mining face. The expected behaviour of the pillars is typically achieved when the pillars have a width to height ratio of approximately 2:1. Once crushed, the residual stress state of the pillars provides a local support function. An underground crush pillar trial site was established towards the end of 2012 on Lonmin's K3 shaft at a depth of approximately 782 mbs (meters below surface). The objective of the trial was to obtain a better understanding of the entire pillar system and the associated risks. The Merensky Reef, dipping at approximately 8 degrees, was mined at an average mining height of approximately 1.3 musing conventional drill and blast techniques. The crush pillars were planned to be 2 m wide and 4 m long (w:h of approximately 1.5). Mining of the trial site was completed by the end of May 2015.

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