Longwall mining is a highly efficient form of underground coal mining which typically involves more subsidence than the traditional ‘bord and pillar’ method. Longwall mining removes a panel of coal and surrounding rock by working a face some 2 - 4m high, up to 300m wide and several kms long. The working area of the panel is supported by mechanical supports that are part of the longwall mining equipment. After the coal is mined, the supports are moved forward to the next mining section of the panel. This causes the overlying rocks to flex and fall into the vacant space. This downward movement of the rock strata causes subsidence at the surface.
Normal subsidence movements above a longwall mine are indicated (in simplistic form) in the diagram below. Rigorous planning prior to the mining of specific sites through Subsidence Management Plans and Rehabiltation and mine closure processes ensure that subsidence is minimised during mining and that the affected land is restored when the mine closes.
In 2005 the NSW Scientific Committee listed longwall mining as a key threatening process to the habitat of endangered species. The NSWMC considers this listing to be an unnecessary duplication of existing NSW Government planning and environmental compliance processes. Assessing the impact of a proposed underground mining operation on landscape and biodiversity and identifying specific mitigation/remediation measures associated with proposed longwall mining operations are integral to the development of Subsidence Management Plans, as is comprehensive community consultation. SMPs are also subject to set conditions governing ongoing environmental performance monitoring and communication to government, the community and other interested stakeholders.
Normal Mining Induced Movements above an Extracted Area
(after Whittaker, Reddish and Fitzpatrick, 1985)