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When material is removed from an underground mine, the earth above the mine can 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.
The nature and extent of mine subsidence and any subsequent impacts depends entirely on local conditions. 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. 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.
When underground mines that will cause subsidence are approved by the government, the government is accepting that subsidence impacts will occur. However, these impacts are deemed to be acceptable given the social and economic benefits that the project provides.
When there are low concentrations of minerals in the ore body (know as a low grade ore), large amounts of ore need to be extracted and processed for the mine to be economical. This can be done through open cut methods or underground methods with the appropriate method depending on local circumstances, such as the depth of the ore body.
Some of the underground methods used to extract large volumes of ore include block caving and panel caving. Each of these methods involves tunneling under the ore body, fracturing the overlying ore body and allowing it to cave down through gravity to underground draw points where it can be extracted for processing.
When the ore body caves down, the overlying rock subsides forming a subsidence zone at the surface. At the surface, the subsidence zone will typically take the shape of a cone of depression with steep slopes at the edges. There may also be some surface cracking around the edges of the subsidence zone.
The subsidence zone is contained to the area above the extracted ore body. The area and depth of the subsidence zone will depend on the local conditions, such as the amount of ore extracted and the nature of the local geology.
Access to the subsidence zone is restricted and any built or natural features (e.g. bushland) in this zone will generally be lost. Impacts caused by subsidence are fully assessed during the project approval process with appropriate measures implemented to minimise, mitigate or offset any impacts.
Longwall mining is now the primary form of underground mining because of its superior safety, productivity and cost effectiveness. Longwall mining is a method of underground coal mining whereby 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 collapse behind the advancing longwall face.
The ability to predict subsidence and any subsequent impacts is continually improving. Mining companies can now more accurately predict the potential surface outcomes on rivers, cliffs and valleys and ensure they are minimised by integrating geological and geophysical factors with appropriate mining engineering techniques in the design of the mine.
Mine subsidence is tightly regulated in NSW. During the project assessment for new mines, the potential impacts of subsidence on surface features is assessed, including impacts on man-made features (e.g. buildings, pipelines, dams and bridges) and environmental features (e.g. rivers, cliffs, aquifers, swamps and ecosystems). Subsidence Management Plans must be prepared and approved, outlining 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.
Many government agencies are involved in the Subsidence Management Plan process, which includes DTIRIS, Department of Planning & Infrastructure, Office of Environment and Heritage, Sydney Catchment Authority, the Dam Safety Committee, Mine Subsidence Board and Fishing & Aquaculture. This whole-of-government approach allows all subsidence-related issues to be considered in the process.
NSWMC believes there are a number of key factors to consider regarding the assessment of subsidence impacts:
- Each mining project is different and must be assessed on a case by case basis. Each project has different geology, subsidence behaviour, local environment and economic value, and must be assessed on its individual merits.
- Decisions should be made after assessing the full range of environmental, social and economic aspects of a project. Environmental impacts may acceptable if they are outweighed by the social and economic benefits. On the other hand, some environmental impacts may be unacceptable.
- The science of subsidence prediction is robust, with observed levels of subsidence within those predicted in the vast majority of cases.
- Mitigation and remediation of subsidence impacts can be successfully achieved through a number of methods including mine planning and design, and active and natural remediation.
- Excluding mining around significant features will not necessarily lead to the best outcomes for the community. The implementation of standard ‘buffer’ zones is not appropriate and will lead to significant opportunity costs with questionable benefits.
Subsidence impacts can be managed in a number of ways. Mine design can play a large role, including the width of longwall panels, the size of the coal pillars between longwall panels, the orientation of longwall panels and stopping mining within a certain distance of a particular feature. Remediation of subsidence impacts can also be achieved. For example, cracks in river beds can be filled with grout in order to restore surface water flows that have been diverted underground.
Most residences that are affected by subsidence lie within Mine Subsidence Districts. Mine Subsidence Districts are areas where buildings must be constructed subject to special controls so they can withstand some ground movement. The Mine Subsidence Board controls surface development approvals where development lies within declared Mine Subsidence Districts.
Purchasers of land within Mine Subsidence Districts are notified of the Mine Subsidence District at the time of purchase. Surface improvements authorised by the Mine Subsidence Board are protected through the Mine Subsidence Compensation Act 1961. Any cost of damage to such improvements, as a result of subsidence, is met by the Board, which 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.
Because subsidence varies according to the local conditions, impact assessments and development of impact minimisation measures can only be undertaken successfully on a case by case basis. This is a view supported by the Independent Expert Panel that conducted a Strategic Review of the Impacts of Underground Mining in the Southern Coalfield. The Strategic Review also acknowledged that further research is required to improve the understanding of ‘non-conventional’ subsidence (which occurs in areas with steep topography or geological irregularities) and the relationship between subsidence movements and swamps.
Submissions
NSWMC Submission – Southern Coalfield Panel Independent Expert Panel
NSWMC Submission – Wyong Independent Expert Panel
External Links
Mine Subsidence Board
Department of Planning Independent Strategic Review into Southern Coalfield
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