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Large hangingwall instabilities have been a problem at Tau Lekoa mine since stoping commenced in 1991. Crush pillars are used to prevent these by limiting spans. Spans between the crush pillars are designed to limit the potential fallout height to a thickness that is controllable by means of internal stope support. The design is based on the empirical relationship between the span and fallout height which is poorly correlated and observed thicknesses often deviate significantly from the design prediction. This paper presents results from a statistical back analysis of 81 large instabilities and includes various geotechnical and mining aspects. Probabilistic keyblock analyses have been conducted to assess the hazard associated with different pillar spans, mining directions and support systems.
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