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3.0 SUBSIDENCE MECHANISM <br />Surface subsidence occurs because of downward rock mass movement caused by the <br />closure and collapse of mined-out excavations. Surface subsidence processes result in <br />both vertical and horizontal displacement of rocks. Two major mechanisms of surface <br />subsidence are associated with mining: formation of sinkholes and creation of troughs. <br />The type of subsidence mechanism predicted for the Dove Gulch study area is the <br />trough-type subsidence. It is characterized by the formation of a relatively smooth basin <br />and is much less damaging than sinkhole subsidence. Sinkholes result from sudden or <br />time-dependent collapse of overburden in localized areas, and these areas can be from <br />several feet to tens of feet in diameter. They are generally influenced by large geologic <br />discontinuities (faults) and sudden pillar failure. Sinkholes are not expected over the <br />North mains because main pillars are designed to remain stable (MTI 2005) and because <br />the overburden is thicker than 500 ft (mostly 600 to 1,100 ft). Based on long-term <br />measurements over the Hanna Basin, Wyoming (Karfakis 1987) and the Colorado Front <br />Range (Matheson and Bliss 1986), researchers have established a relationship between <br />the probabilities of sinkhole subsidence versus overburden depth. A great majority of <br />sinkholes (98% probability) form where depths are less than 160 ft. At typical cover <br />depths of 600 to 1,100 ft at the Dove Gulch project site, the probability of sinkhole <br />occurrence is negligible. <br />As longwall operations are initiated in the first panel, roof span increases behind the <br />longwall face until it caves. The roof span varies in mines, but typically ranges from 30 <br />to 200 ft, depending on the strength of the roof rocks. The remaining overburden rocks <br />will remain stable, transferring their load to the face and gate pillars. At some critical <br />face position, the arching and load transfer mechanism collapses, and ground movement <br />expands toward the surface, causing subsidence. <br />. The caving process is associated with fracturing of near-seam strata and settling of <br />Maleki Technologies, Inc. Page 9