A study was undertaken to investigate the potential to increase the overall extraction ratio through safe partial mining of the existing underground pillars on the No. 4 Seam at a colliery located in the Witbank Coalfields using a modified NEVID system. A bord and pillar mining layout on the No. 4 Seam will have a typical extraction ratio of approximately 60%. With the partial pillar extraction (stooping process), the overall extraction ratio would typically increase to approximately 75%, which makes it economically viable provided that it can be carried out safely. A numerical model was employed to assess the pillar and panel stability in a ‘high’ extraction stooping operation at various depths ranging between 20 m and 80 m below surface. A relationship for the stability of each of the remaining snooks is presented based on the axial pillar strain and the associated depth below surface, i.e. the depth at which goafing will occur may be inferred from these results. A novel approach to assessing pillar snook stability is proposed and employed in this study.

INTRODUCTION

A study was undertaken to investigate the potential to increase the overall extraction ratio through the safe partial mining of the existing underground pillars on the No. 4 Seam at a colliery in the Witbank coalfields. A decision was made that no pillar extraction would be carried out in areas where surface structures, streams, graves, or public roads were present (Pethö et al., 2012). Even so, the potential minable pillar area was sufficiently large to justify the economics for pillar extraction. Within these minable pillar areas, the only stipulation was to induce caving of the roof (referred to as goafing – van der Merwe and Madden, 2002), while ensuring stable conditions within the remainder of the stooping panel, i.e. where pillar extraction had not been carried out yet. The ground surface stability during the entire stooping process was not regarded as a determining criterion. This study therefore does not focus on the potential for subsidence, but mainly concentrates on the stability of the individual pillar snooks (post-extraction pillar remnants).

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