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When material is removed from an underground mine, the earth above the mine will adjust to the altered landscape. This can make the ground surface fall or rise, move horizontally, tilt, or cause cracking, which can lead to impacts on built features (e.g. buildings, pipelines, dams and bridges) or environmental features (e.g. rivers, cliffs, aquifers, swamps and ecosystems). The various movements of the ground surface and subsurface following underground mining are collectively referred to as mine subsidence. Mine subsidence occurs for both coal and metalliferous mining.
The nature and extent of mine subsidence and any subsequent impacts depends entirely on local conditions and the mining methods being used. In some cases, mine subsidence will be hardly noticeable at the surface while in other cases there will be noticeable cracking in building walls or the grounds surface. In metalliferous mining often a hollow forms in the ground above the mining area. Some of the variables that affect mine subsidence include the geology of the area, the depth of mining, the amount of material extracted and the mining method used. Every situation is different, and requires a unique management approach depending on the local conditions.
There are many different techniques used to mine metalliferous minerals underground.
Longwall mining is now the primary form of underground coal mining because of its superior safety, productivity and cost effectiveness. During longwall mining, blocks of coal, known as ‘panels’, are extracted from a coal seam by a shearer moving along the face of the panel. A panel is typically 2-4 metres high, and can be up to 300m wide and several kilometres long. As mining progresses along the length of the panel, the overlying strata is collapsed behind the advancing longwall face.
Coal mine subsidence is tightly regulated in NSW. During the project assessment for new mines, the potential impacts of subsidence on surface features are assessed, including impacts on built features and environmental features. Subsidence Management Plans must be prepared and approved, which outline what management techniques will be used to keep impacts within acceptable levels. For older mines without modern approvals, Subsidence Management Plans play a bigger role in assessing subsidence and determining acceptable impacts.
Most residences that could potentially be affected by coal mine subsidence lie within Mine Subsidence Districts. Mine Subsidence Districts are areas where structures must be constructed subject to special building controls so they can withstand some ground movement. The Mine Subsidence Board ensures these controls are implemented.
Purchasers of land within Mine Subsidence Districts are notified of the Mine Subsidence District at the time of purchase. Any surface improvements authorised by the Mine Subsidence Board are protected through the Mine Subsidence Compensation Act 1961. Any cost of damage to authorised improvements as a result of subsidence is met by the Board, which is funded by a compulsory levy on the NSW mining industry. Buildings constructed before the Mine Subsidence District was declared or which are located outside a Mine Subsidence District are automatically covered for damage resulting from mine subsidence.
Mining can generate valid community concerns, as well as sparking inflated fears about potential and real impacts of operations. An example of a recent concern was that underground mining in the Southern Coalfield impacted the quality and quantity of Sydney’s drinking water. An Independent Expert Panel appointed by the NSW Government to conduct an inquiry into the impacts of mining in the Southern Coalfield found:
“The Panel is not aware of any scientific evidence supporting the view that subsidence impacts on rivers and significant streams, valley infill or headwater swamps, or shallow or deep aquifers have resulted in any measurable reduction in runoff to the water supply system operated by the Sydney Catchment Authority or to otherwise represent a threat to the water supply of Sydney or the Illawarra region.”
The minerals industry routinely works with government, scientists and researchers to understand the risks and potential impacts of any particular mining activity. But science alone does not always address community concerns, which is why companies work very closely with communities to help people understand the upsides and downsides of nearby mining operations.
Submissions
NSWMC Submission – Southern Coalfield Panel Independent Expert Panel
NSWMC Submission – Wyong Independent Expert Panel
External Links
Mine Subsidence Board
NSW Government Impacts of Underground Coal Mining on Natural Features in the Southern Coalfield – Strategic Review, July 2008
NSW Government Impacts of Potential Underground Coal Mining in the Wyong Local Government Area - Strategic Review, July 2008
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