Abstract

WATSON, B.P.  and  GERBER, R.. Determination of stable spans in UG2 excavations. J. S. Afr. Inst. Min. Metall. [online]. 2018, vol.118, n.5, pp.493-504. ISSN 2411-9717.  http://dx.doi.org/10.17159/2411-9717/2018/v118n5a6.

The hangingwall of the UG2 Reef is characterized by stratification in the form of thin and weakly cohesive chromitite stringers that can vary in number and height above a stope. These stringers, in conjunction with shallow-dipping thrust faulting, endemic across the platinum-bearing reefs of the Bushveld Complex, affect the maximum span that can be safely mined. The paper describes the research that was carried out to determine the interaction of support with the rock mass in both conventional and mechanized workings, and provides insights into stable span determination where excavations are intersected by a shallow-dipping thrust structure. Four important issues are highlighted. 1. Mine pole and pack support has a greater influence on stability than span in the context of the studied database for conventional mines. 2. Shallow-dipping discontinuities are a dominant feature in stability analyses. 3. The height of the vertical tensile zone does not restrict the fallout height if persistent shallow-dipping structures are present. 4. In the context of the numerical modelling shown in the paper, a span of 6 m is safe when 1.8 m long, full-column resin bolts are used at a support resistance of 48 kN/m², regardless of the k-ratio or height of the triplets (intersections must be dealt with separately). Several leading practices have been developed in recent years to identify and timeously support hazardous structures.

Keywords : thrust fault; dome structure; stable span; triplets; conventional mining; mechanized mining; UG2 Reef; vertical tensile zone.

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