Severe instability in an underground hard-rock bord and pillar mine situated on the Great Dyke in Zimbabwe necessitated a thorough revision of the existing design. Local geotechnical conditions characterized by undulating, subhorizontal and intersecting shear structures, variable in thickness and relative location within the hangingwall, orebody (pillars), and footwall, were only partially understood prior to the instability event. As a result, a change in the behaviour of the respective horizons was encountered, with an unfavourable outcome.

A revision of the pillar design therefore became necessary. The revised design took a holistic approach, beginning with a re-evaluation of the local rock mass characteristics, a revision of the empirical design, and application of both linear elastic and nonlinear numerical analysis to test and optimize the final outcome. The resulting design is considered to be robust, with the provision of continual evaluation and monitoring of geotechnical conditions concurrent with production activities to test and validate the design.

This paper presents a high-level overview of the investigative process that was undertaken to provide the operation with revised, stable pillar design criteria.

INTRODUCTION

Production at a bord and pillar mine situated on the Great Dyke of Zimbabwe recently suffered a serious setback when geotechnical conditions within the hangingwall, orebody, and footwall gave rise to widespread instability. Unfavourable rock mass behaviour included footwall heave, falls of ground (FoG), and bulking and spalling of pillars. The presence of a shear structure, subhorizontal and undulating in attitude, variable in thickness and continuity, was identified as a primary cause for the deterioration in rock mass conditions. Investigations into the contributing factors that gave rise to the situation were carried out, based on which it was concluded that the design and associated mining practices evidently needed to be revised. A new, robust, workable model was required that would allow the resumption of mining with confidence while at the same time trying to avoid a knee-jerk response of excessive conservativism.

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