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-41- <br />Subsidence <br />The impacts of subsidence due to mining can be expected to vary in <br />magnitude and extent. This variation will result from a combination of <br />numerous natural phenomenon and from the effects induced by mining. <br />Surface water and ground water can both be influenced by differences in <br />timing of mining and the physical environment. It is assumed that all <br />mining will result in subsidence at some future point in time, although <br />surface manifestation may require many years to occur. Therefore, <br />control and predication of subsidence is critical, necessary measures <br />to be implemented before and during any mining activity. <br />Natural phenomena which influence subsidence are: <br />structure of the overburden, and <br />Depth of overburden, <br />Thickness of the coal seam, <br />Stratigraphy, lithology and <br />Topography. <br />Mining activities which affect subsidence are: <br />- Size, shape, depth, location and <br />real extent of workings, <br />- Rate of development and extraction <br />of resource, and <br />- Mining methods. <br />The possible impacts of subsidence include: <br />- Modification of surface water hydrology - channel and watershed <br />geometry and sediment erosion rates, <br />- Podification of ground water hydrology - changes in spring <br />discharge, aquifer characteristics and recharge, <br />- Loss of water from surface impoundments due to disruption by <br />surface cracking, and <br />- Increase in surficial mass movement activity - landsliding and <br />rock falling. <br />Natural physical influences upon subsidence can be controlled to the <br />extent that mine plans can be designed to avoid obvious hazards (e .g. <br />shallow openings beneath perennial streams, landslide bodies, <br />impoundments, etc), or to limit subsidence in accordance with the <br />current state-of-the-art of ground control (engineered control of roof <br />and pillar failures in mines). Subsidence predication and ground <br />control engineering are not definitive sciences. Design engineering <br />concepts must be verified through continuous monitoring (surface <br />topography, ground water hydrology and surface water hydrology) and <br />observation. <br />