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-37- <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 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 some degree of subsidence at some <br />future point in time, althouyh surface manifestation may require many <br />years to occur. Therefore, control and prediction of subsidence are <br />critical, necessary measures, which should be implemented before and <br />duriny any mining acitivity. <br />Natural phenomena which influence subsidence are: <br />- Uepth 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 <br />workings; <br />- Rate of development and extraction 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 />- Modification of ground water hydroloyy -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 mines). Subsidence prediction and ground control <br />engineering are not definitive sciences. Design engineering concepts <br />must be verified throuyh continuous monitoring (surface topography, <br />ground water hydrology and surface water hydrology) and observation. <br />