The risks and potential influence of undermined areas in the Okiep Copper District with a specific focus on the Bruinhoek populated area of Concordia
Thesis (MEng)--Stellenbosch University, 2018.
ENGLISH ABSTRACT: The collapse and/or subsidence of overburden located above an undermined area is inevitably an impending consequence of underground mining activities across the world. One of the legacies left behind of approximately 150 years of mining in the Okiep Copper District is the presence of huge collapsed structures locally known as glory holes. Although some are remote, several holes are located close to populated areas and pose a hazard to the local inhabitants. The Bruinhoek populated area which forms part of the town of Concordia is undermined by the Wheal Julia East Mine. The potential formation of a glory hole within the undermined area poses a risk to local inhabitants of Bruinhoek. The aim of the study is to determine the likelihood of glory hole formation at Bruinhoek, as well as the inherent risk if this should happen. This aim was achieved by means of a data collection process that involved three reference mines which has glory holes, as well as the Wheal Julia East Mine. In order to determine the likelihood of the formation of a glory hole at the Bruinhoek populated area the main reasons causing the formation of the glory holes at the reference mines were determined first. This was done by the reviewing of previous investigations, studying survey and geological maps gathered from the O’okiep Copper Company (OCC), and field investigations that involved drone aerial surveys, site walkovers and the inspection of the sidewalls of the glory holes. The Wheal Julia East Mine was similarly studied and investigated. After the main reasons for glory hole formation at the reference mines were determined they were used as a proxy for the Wheal Julia East Mine. It was found that for a glory hole to form an underground cavity must have been created first. The main factor that contributed to the initiation of the failure of overburden at the reference mines was the presence of persistent discontinuities. Investigations proved that the Wheal Julia East Mine has created an underground cavity as well as that persistent discontinuities could occur following the investigations at the reference mines. Therefore it was determined that a glory hole at the Bruimhoek populated area could be expected in the future. The stope dimensions in plan view for the Wheal Julia East Mine where determined through the creation of overlay maps where a surface image of the Bruinhoek populated area overlies the geological maps of the Wheal Julia East Mine. It was found that the stope lies directly below two plots of the Bruinhoek populated area. The field investigations of the reference mines indicated that instabilities for example self-caving of the sidewalls and rim collapse are common after the formation of a glory hole. This means that in the case of the formation of a glory hole at Bruinhoek it is almost certain that it will self-cave and experience rim collapse, gradually becoming greater in size. The current situation at Bruinhoek has the potential to develop in a high risk situation endangering people’s lives.
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