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and Tandanand and Powell (U.S. Bureau of Mines, Twin Cities <br />• Research Center), an angle of break of 27 degrees was calculated <br />from our typical formation parameters. By allowing flexural beam <br />failures in the layers to successfully propagate up through the <br />geologic column above the 600-foot wide mine panels it was <br />determined that Formation 10 at a height of 240 feet above the <br />seam was the first layer theoretically able to span the caving <br />zone in the center of the ground arch and also support the <br />overlying formations. However, since this particular layer <br />(Formation 10) was an interbedded material. The possibility was <br />considered that this layer might not act as a single unit <br />allowing caving to propogate still higher in the profile. <br />Therefore, Formation 8, a massive sandstone at a height of 360 <br />feet above the C seam, was found to be the next layer <br />• theoretically able to span the caving. This means that wherever <br />overburden thickness remains below 360 feet rubblization would <br />not extend to the surface and trough rather than chimney <br />subsidence would take place. <br />• <br />20 <br />