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-39- <br /> The impacts of subsidence due to mining can be expected to vary in magnitude <br /> and extent. This variation will result from a combination of numerous natural <br /> phenomenon and from the effects induced by underground mining. Surface water <br /> and ground water can both be influenced by differences in timing of mining and <br /> the physical environment. It is assumed that all mining will result in <br /> subsidence at some future point in time, although surface manifestation may <br /> require many years to occur. Therefore, control and prediction of subsidence <br /> are critical , necessary measures, which should be implemented before and <br /> during any mining activity. <br /> Natural phenomena which influence subsidence are: <br /> - Depth of overburden; <br /> - Thickness of the coal seam; <br /> - Stratigraphy, lithology and structure of the overburden; <br /> - Topography; and <br /> - Lithology of the floor strata. <br /> Mining activities which affect subsidence are: <br /> - Size, shape, depth, location, and areal extent of workings; <br /> - Rate of development and extraction of resource; <br /> - Mining methods; and <br /> - Number of overlying and underlying coal seams mined. <br /> The possible impacts of subsidence include: <br /> - Modification of surface water hydrology - channel and watershed <br /> geometry and sediment erosion rates; <br /> - Modification 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 - landslides and rock <br /> falls. <br /> Natural physical influences upon subsidence can be controlled to the extent <br /> that mine plans can be designed to avoid obvious hazards (e.g. , shallow <br /> workings beneath perennial streams, landslide bodies, impoundments, etc. ), or <br /> to limit subsidence in accordance with the current state-of-the-art of ground <br /> control (engineered control of roof and pillar failures in mines) . Subsidence <br /> prediction and ground control engineering are not definitive sciences. Design <br /> engineering concepts must be verified through continuous monitoring of surface <br /> topography, ground water hydrology and surface water hydrology. <br />