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The potential iFor future roof or pillar failures is greatest where overburden stress is <br />greatest, which is in the deeper parts of the workings. Most of the workings are at depths <br />greater than 500 feet. Fluctuating water levels in the workings could promote roof or <br />pillar failures. Pillars in the Eagle #5 workings are typically more than 100 feet long on <br />all sides, and roof spans between pillars are typically 20 feet wide. If subsidence occurs <br />in a 12-foot high mine void that is 100 feet long on all sides, at a depth of 500 feet, the <br />vertical subsidence on the land surface can be predicted to be approximately 6 inches, <br />based on the relationship of vertical subsidence, mining depth, and coal seam thickness <br />determined four Mesaverde Formation coal in a mine at Somerset, Colorado (Figure 53 in <br />Dunrud, 1998„ Engineering Geology Applied to the Design and Operation of <br />Underground Coal Mines, U.S. Geological Survey Bulletin 2147). A 6-inch vertical <br />displacement distributed over a distance of 100 feet on the land surface is a slope of 0.3 <br />degrees, an imperceptible amount. If the 6-inch displacement is concentrated over a 20- <br />foot distance on the land surface, the resulting slope is 1.5 degrees. <br />The observed subsidence effects in the bond release area support a finding that the major <br />subsidence effects have occurred and subsidence-caused material damage has been <br />prevented. <br />