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0 2.0 CONCLUSIONS AND RECOMMENDATIONS <br />A trough-type of subsidence is predicted over the full extraction area in the Dove <br />Gulch study area. Trough subsidence is associated with longwall mining and is much <br />less damaging than other types of subsidence. Sink-type subsidence is not expected over <br />the mains, where the minimum overburden is 600 ft. <br />The mechanism of subsidence at the surface is characterized by the formation of <br />tension (expansion) and compression zones. Tension results as the surface begins to <br />subside and compression as the surface begins to settle. Tension tends to open up <br />existing fractures and create new ones. Compression tends to close up these surface <br />fractures. Observations suggest closing and gradual healing of cracks in soils and <br />weathered bedrocks by natural caving of crack walls, in-filling by sediments and organic <br />material, and effects of storm events. Cracks remain more noticeable in brittle <br />sandstones. <br />is in the overburden above the coal seam, there are usually four zones of movement. <br />First, a fragmented zone of broken-up strata lies immediately above the coal seam and <br />can be as much as eight times thicker than mining height. Second, a fractured zone is <br />created that deforms and fractures, but maintains its continuity. This zone is estimated to <br />be between twenty to fifty times thicker than the minable thickness (240 to 600 ft). <br />Third, a deformation zone extends to the surface bedrock, where the strata deform <br />without significant formation of permanent fractures. Fourth, a soil zone forms that is an <br />extension of continuous deformation zone. Because of the less brittle nature of soils, <br />tensile cracks associated with transient subsidence may not be detected easily in front of <br />the face and any existing fractures tend to heal quickly. Tensile fractures forming at panel <br />boundaries last longer, but eventually close due to caving of fracture walls. <br />The overburden in the planned Dove Gulch study area consists of moderately <br />competent, laminated strata with favorable caving characteristics. Thus, the subsidence <br />process is expected to occur quickly after mining. Initial subsidence will occur within <br />weeks of longwall mining, with surface deformation becoming mostly complete within 2 <br />Maleki Technologies, Inc. Page 6