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-39- <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 underground <br /> mining. Surface water and ground water can both be influenced by <br /> differences in timing of mining and the physical environment. It is <br /> assumed that all mining will result in subsidence at some future point in <br /> time, although surface manifestation may require many years to occur. <br /> Therefore, control and prediction of subsidence are critical , necessary <br /> measures, which should be implemented before and during any mining <br /> activity. <br /> Natural phenomena which influence subsidence are: <br /> - Depth of overburden; <br /> - Thickness of the coal seam; <br /> - Stratiyraphy, 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 imparts of subsidence include: <br /> - Modification of surface water hydrology - channel and <br /> watershed 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 <br /> rock falls. <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 workings 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 nines) . Subsidence prediction and ground control <br /> engineering are not definitive sciences. Design engineering concepts <br /> must be verified through continuous nonitoring of surface topography, <br /> ground water hydrology and surface water hydrology. <br /> :'valuation of the impacts of subsidence upon hydrologic systems requires <br /> the collection and analysis of extensive Monitoring data consisting of <br /> surface 'water and ground 'water moni .oring ' quantity and quality) , <br /> precipitation caging, geolccical mapping, topographic surveying an,. <br />