Dewatering concepts at Zambian Copperbelt Mines
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During the financial year 1992/93, Zambian Consolidated Copper Mines Ltd (ZCCM) Ltd pumped a total of 263 million tonnes of water from its various mining operations. During the same period the Company produced 23 million tonnes of ore, giving a water to ore ratio of 11.4 tonnes of water per tonne of ore produced. Hydrostatic pressures interesected in underground boreholes ranged upto about 5MPa. Against this background the dewatering techniques that have been practised on the Copperbelt at ZCCM’s mines are reviewed. The methods include the surface exclusion of water, interception of water, simple drainage, breakthrough methods, dewatering drilling, grouting isotope analysis and computer modelling.
The surface exclusion of water includes the use of canals and pipelines to carry water over hydrological hazard zones, herringbone ditches to speed up run-off, stream gauging to locate hydrological hazard zones and weirs to quantify flow rates, and the judicious geological siting of dams and other surface water structures.
Interception methods basically revolve around the concept of interception of the potential mine drainage at the extremities of the mines in order to ensure that the cone of dewatering is lowered before it intercepts the main mining areas.
Simple drainage is the mining of drives into aquifers at reduced hydrostatic pressures in order to drain specific aquifers.
Breakthrough methods also involve the mining of drives into aquifers but in a more controlled manner than in simple drainage. In this instance drives are mined directly into aquifers utilising watertight doors or puddle pipes to protect the main mine workings.
Dewatering drilling is the most widely used method of dewatering used on the Copperbelt. It may be conveniently divided into surface and underground dewatering boreholes. Surface dewatering boreholes may be either pumped, utilising borehole pumps, used for piezometric measurements, or used in open pit situations to drain aquifers under hydrostatic pressure. Underground dewatering boreholes are the most widely practised method of dewatering on the Copperbelt and involve the drilling of boreholes into aquifers, in order to lower the hydrostatic head in a particular aquifer. A number of different techniques are discussed.
Grouting to exclude the inflow of water into mines has long been known as a method of groundwater exclusion. The uses of cementious grouts and resin grouts are discussed.
Isotope analysis has been used at Konkola Division to give indications of both the age and possible origins of the Konkola groundwaters.
Computer modelling utilising modflow software has been used at Konkola Division to predict drawdown of the hydrostatic head in a number of different mining scenarios.
A changeover from caving mining methods to mining methods involving the use of backfill should permit certain mines to effect major cost savings with regard to dewatering costs. The implications of this change in mining methods is discussed.
Environmental aspects of mine drainage from ZCCM’s mines is addressed and the lack of an acid mine drainage problem briefly discussed.
KeywordsFootwall Hanging Wall Hydrostatic Head Simple Drainage Surface Borehole
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