Book Extract: Mining and the environment

Mining can have an adverse impact on the environment and unfortunately this has frequently led to serious consequences. For example, the disposal of waste has led to unsightly spoils being left that disfigure the landscape, and to surface streams and groundwater being polluted. In addition, some urban areas have suffered serious subsidence damage due to undermining.
The impact of mining depends on many factors, especially the type of mining and the size of the operation. However, greater environmental awareness has led to tighter regulations being imposed by many countries to lessen the impact of mining. Rehabilitation of a site after mining operations have ceased has become entrenched in law and an environmental impact assessment is necessary prior to the development of any new mine. An environmental management programme is required to show how the mine will operate and the site be restored after mining operations have ended.
Although the adverse impacts on the environment must be minimised some environmental degradation is inescapable. Mines, however, are local phenomena, although they may impact beyond mine boundaries. They also account for only a small part of the land area of a nation (e.g. the mining industry accounts for less than 1 per cent of the total area of South Africa).
Land that has become derelict by past mining activity can be restored, at a cost. Rehabilitated spoil heaps frequently become centres of social amenity such as parklands, golf courses and even artificial ski slopes. Open pits, when filled with water, can be used as marinas, for fishing or as wildlife reserves. Even some underground mines can be used, such as those in limestone at Kansas City, Missouri, which are used as warehouses, cold storage facilities and offices. Mining therefore can be regarded as one of the stages in the sequential use of land.
Most surface mining methods are large scale, involving removal of massive volumes of material. Huge amounts of waste can be produced in the process. Important factors in surface mining include slope stability which is influenced by the nature of the rock mass(es), the geological structure and the hydrogeological conditions, and the stripping ratio, that is, the ratio of overburden to ore. This can determine the economic operation of a surface mine. A mineral occurring at a depth beyond the maximum stripping ratio will either have to remain unworked or be mined by underground methods.
Stratified mineral deposits that occur at or near the surface (such as coal or sedimentary iron ore) in relatively flat terrain generally are extracted either by strip mining or opencast mining. The deposit concerned is either horizontal or gently dipping and normally is within 60 metres of the surface. All the strata overlying the mineral deposit are removed and placed in stockpiles. When necessary, blasting is used to break rock above the mineral deposit. The mineral deposit itself may have to be drilled and blasted, once exposed, prior to its removal by conventional loading and haulage equipment. When the overburden is removed from the working face in long parallel strips and placed in long spoil heaps in the worked-out area, this is referred to as strip mining.
Open-pit mining is used to work ore bodies that occur at or near the surface where the ore body is steeply dipping or occurs in the form of a pipe. Initially, the overburden is stripped to expose the ore. The ore body is blasted in a series of benches, the walls of which are steeply dipping. Rock surrounding the ore body may have to be removed in order to maintain stable slopes in the pit. Because of the size of many open-pits, normally there is not enough waste material available to backfill them. Accordingly, the principal objectives after cessation of mining operations generally are to ensure that the walls of the pit are stable, and that the waste dumps and tailings dams are rehabilitated.

Extract from Basic Environmental and Engineering Geology by F. G. Bell
Whittles Publishing 2007 (RRP$150.00)
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Image: Tiered working at the Palabora open pit copper mine in South Africa. The host rock is very competent, allowing the pit walls to be cut steeper than normal. The open pit aspect of the mine ended in 2002 after almost 40 years of working, when it reached its economic depth (over 500m)