Factors affecting the undermining of surface structures above hard rock mines in the central Witwatersrand and the Western Bushveld Igneous Complex

Abstract

The Mines and Works Regulations allow the undermining of surface structures under conditions which are Itniformly applied in both the Central Witwatersrand and Bushveld Igneous Complex regions. which are different to There are, the Central however, factors Witwatersrand and Bushveld Igneous Complax and which affect the undermining of surface structures. Although the determination of the underground area affected by the restriction to mine under or near sut"face structures ("restricted area") is based on acoepted engineering principles t the determination of support requiremen'':.sfor the protection of the surfaoe is based on an extraction ratio or the conscruc+Lcn of waste packs and do not consider geotechnical differences. These requirements may re~ult in over- or underdesigned pillars, in spans bl'ilt.weenpillars either. being under- or overdesigned and in the lo-:::king up of valuable minerals. The depth cut-off of 240 m bel,ow surface at which the conditions are applied, Ls valld. There are similarities between the mining practices in the two environments based on the geometries of the orebodies and the strengths Of the oVerlying strata. However, pillar design and calcula'!:.ionof the mine spans between pillars differ fundamentally, as the strata immediately overlyin~ the Bushveld Ign~oue Complex on-reef exoavations are generally weaker than the rock masses in the Wi'l:.watel:'sranTdh.e pyroxenites oVerlying the UG2 re!llfare intersected by closely joints and to a depth spaced and moderately dipping of about 70 m the effects of weathering can be substantial. As a result the pyroxenites need to be partially or totally supported as a deadweight. The presence of geotechnical structures such nS dykes and faults further weakens the strata weathering along jointed zones on their margins 1S Severe and can extend to depths of 100 m. The de'.~erminationof safe mine spans depend on the competency of the norite/anorthosite series overlying the Merensky and UG2 reefs. Graphs based or; the Mohr-Coulomb failure index have been designed from which safe mine spans can be read off. The design of pillars is based on strength of material criteria and the orientation of the major jointing. The l1\iningon the Witwatersrand L. the area and depth range of interest has reSUlted in mainly supercriti(:lal mine spans, especially on the South reef horizon resulting in the subsidence of the overlying strata. Formulae have been designed to calculate the theoretical height Of caving in both the South and Main reef workings which, in conjunction with the graphs from which safe mine spans can be read off, can be used to determine maximum span widths.