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-36- <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 are critical, necessary measures <br />to be implemented before and 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 <br />structure of the overburden <br />- Topography <br />Mining activities which affect subsidence are: <br />- Size, shape, depth, location and real extent of workings <br />- Rate of development and extraction of resource <br />- Mining methods <br />The possible impacts of subsidence includes: <br />- Modification of surface water hydrology <br />- channel and watershed geometry and <br />sediment erosion rates. <br />- Modification of ground water hydrology <br />- changes in spring discharge, aquifer <br />characteristics and recharge <br />- Loss of water from surface impoundments <br />due to disruption by surface cracking <br />- Increase in surficial mass movement <br />activity - landsliding and 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 />