The Longwall Mining Process and its Effects
| the longwall mining process | |
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| The diagrams / maps used on this page are the property of BHP Billiton | |
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Longwall mining is form of underground coal mining which uses a massive machine called a longwall. The longwall equipment consists of a number of hydraulic jacks, called chocks, roof supports or shields, which are placed in a long line up to 400m in length in order to support the roof. An individual chock can extend to a maximum cutting height of up to 5m. The coal is cut by a rotating drum with bits called a shearer that moves along the length of the face in front of the chocks, disintegrating the coal. The coal is then removed from the coal face by a moving chain or AFC (articulated face conveyor) (also called armoured face conveyor) to the main gate. Here it is loaded onto a conveyor belt and transported to the surface. As the shearer removes the coal, the chocks move forward into the newly created cavity. As mining progresses and the entire longwall progresses through the seam, the cavity behind the longwall, known as the goaf, increases. This goaf collapses under the weight of the overlying strata. Sometimes longwall mining is called destructive or environmentally unsafe because it causes the land above the mined-out panel to sink. This can damage underground water tables, structures at the surface, and can cause erosion of the soil. Subsidence contours from LW301 and LW302 (blue rings) on the following map overlap Cataract River (light blue) over a considerable distance. This section of the Upper Cataract River has now been cracked and polluted. This river carries approximately 7% of Sydney's water supply. | |
| Subsidence contours from Appin Colliery - Area 3 | |
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| Environmental Effects of Longwall Mining | |
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The specific area or environment in which a plant or animal lives is called its habitat. A habitat provides all the basic requirements for survival. Alteration of habitat refers to a change in the structure or function of habitat, making it potentially unsuitable for the organism to live in. Subsidence due to longwall mining has been recognised as causing habitat alteration. Species and ecological communities that depend on aquatic and semi-aquatic habitats are particularly susceptible to the impacts of subsidence. Subsidence due to longwall mining can cause deformation of ground surfaces as well as cracking of valley floors and creek-lines. This can affect natural water flow regimes and water quality, depending on such factors as the width of the crack, riverbed steepness, the riverbed material and the presence of organic matter. Subsidence can also destabilise cliff faces. In turn, these impacts can lead to the alteration of species habitats and changes to the ecological function of communities. Effects can be temporary or long-term. When water flows are altered, there can be permanent effects on the functioning of ecosystems in localised areas, which may be exacerbated in drought conditions. For further information, see the Department of Environment and Climate Change's fact sheet on the Alteration of habitat following subsidence due to longwall mining as a key threatening process. |
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