A mining operation transitioning from open pit to underground in an area where the mine design does not allow caving through to surface normally requires a crown pillar for protection of the underground workings beneath the pit. Crown pillar stability is maintained to prevent inrushes of water and mud, air blasts, and collapse or large-scale movement of the pit walls or surrounding rock mass. This is important in providing a safe workplace for mine personnel, equipment, and infrastructure, and ensuring achievement of production goals. The Paboase underground mine at Chirano Gold Mines Limited in Ghana is situated beneath an existing and non-operating open pit. A crown pillar is required to separate the pit from the underground mine, where sublevel open stoping method was used to extract the upper mining block. During early studies, the crown pillar was designed based on preliminary data available at that time. With additional geotechnical data available from field mapping, core drilling, and laboratory testing, efforts were made to evaluate the stability of the crown pillar. Empirical and analytical methods were applied to determine the factor of safety for the crown pillars of various thicknesses under different stoping geometries. Numerical modelling was also utilized to evaluate the crown pillar performance under various in-situ stress conditions and mining sequences. In addition, stope stability in the upper mining block was analysed using the updated Matthews' stability graph. This paper presents the mine design, input data, and performance evaluation of the Paboase crown pillar and summarizes the results from the empirical, analytical, and numerical modelling analyses.

INTRODUCTION

When mining transitions from open pit to underground, it is usually necessary to leave a crown pillar for the underground mine beneath the bottom level of the pit. It is vital that the crown pillar remains stable throughout its planned service life to protect surface land users, and the underground mine and workers from inflows of water, and failures of soil and rock (Carter, 1989). The design of a stable and cost-effective crown pillar is one of the most challenging practical problems faced by mining engineers today. Surface subsidence, to a greater or lesser degree, is an inevitable consequence faced by many underground mining operations. The subsidence resulting from a crown pillar failure has always been a concern for economic and safety reasons.

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