1987-12-01_PERMIT FILE - C1981017 (194)

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2 - 2. Description of worst possible consequences which subsid- ence could have on structures and renewable resource lands. There are no and will not be any structures in the area in question except those that have been or might be erected by MCR in the course of mining operations. No damage to grazing land has been detected to date in spite of nearly 30 years of mining activities, and as no known aquifers exist above the coal beds being mined, no damage to aquifers or aquifer recharge areas is anticipated. 3 . Description of worst case subsidence-related phenomena that could occur. The worst case subsidence-related phenom- ena that could occur in the Coal Basin area involve physical displacement of the surface and related phenomena, such as tension and compression cracks , subsidence pits (hummocked profiles) , subsidence-triggered slumps or landslides, and so on. Inspections of the surface over mined-out areas by representatives of Willard Owens Associates (9/12/80) , CMLRD (8/9/85) , and Mr. C. R. Dunrud, U.S . Geological Survey and Mr . Vatt McComb of the Bureau of Land Management (9/20/85) , as well as by the writer (8/9/85) and numerous MCR mining engineers and environmental specialists (various dates) have detected no such features. Therefore, the worst-case subsid- ence-related phenomena are limited to actual but visually undetectable vertical lowering of the land surface over mined-out areas and the horizontal distance away from the mined-out areas but within the angle of draw, and to the possible development of again visually undetectable compres- sion arches over large barriers in the mines. Vertical movement is a function of the thickness of coal removed, depth of cover and the lithology, strength and other characteristics of the overburden; the latter two factors are commonly ignored in calculating subsidence effects, since there are little data tying these characteristics to actual subsidence cases. However, it is logical to assume that the strength of rock materials, for example the two thick sand- stones between the lowest coal bed mined by MCR and the sur- face, has a significant effect on the ultimate total subsid- ence that may occur; that whether rock units break or bend as underlying support is removed determines to some extent total subsidence and the nature of its surface expression; and that the overall nature of the rock units involved determines the extent to which bulking, or swelling of the collapsed material over the mined-out area, may decrease the subsidence ratio (ratio between measured subsidence and mined thickness) . According to MCR' s permit (Vol . 7 , Chapter IV, page 26) , pub- lished subsidence ratios generally range from 0.25 to 0.70 , with limited data from Mesaverde Group mines averaging about 0 .40, with a high value of 0.70; virtually all data are from mines shallower than those of concern here. A "worst Case"