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West Elk Mine <br />The maximum height of fracturing above longwall panels in the South of Divide mining <br />area is estimated to range from about 10 to 20 times the extraction thickness. This is near <br />the mid-range of 9 to 30 times coal extraction thickness as reported by Peng (1992, p. 7). <br />This estimate may be conservative for rocks above the E Seam. <br />Drainage, however, may cease after mining is complete and any water-bearing zones <br />present may be restored. This is particularly likely in the upper part of the fractured zone <br />in shale sequences between sandstone layers, once subsidence is completed and the <br />separated beds re-compress and close in response to overburden load (see Exhibit 60B, <br />Figure 2). Evidence of restored water levels has been measured and reported in some wells <br />in the West Elk Mine subsidence monitoring area after mining and subsidence were <br />complete. <br />Continuous Deformation Zone and Near-Surface Zone <br />These two zones aze discussed together because the ground surface is where nearly all <br />measurements are made that monitor subsidence processes active in the zone of continuous <br />deformation. <br />Near-Surface Zone <br />The neaz-surface zone, which typically consists of weathered bedrock, colluvium, and soil <br />• ranging in depth from a few feet to a few tens of feet, may deform differently than the underlying <br />bedrock. Field studies by Mr. Dunrud indicate that near-surface colluvium and alluvium, <br />which consist of predominantly clay and silt, can undergo significantly more extension <br />without rupturing than can the underlying material. In both the Somerset, Colorado and <br />Sheridan, Wyoming areas colluvium and alluvium 5 to 10 feet thick were observed to cover <br />cracks as much as 10 to 14 inches wide so that there was no indication of the underlying <br />ruptures. Mr. Dunrud's observations in the Bear Creek area in 1976 are discussed in the <br />Final Environmental Impact Statement for the Iron Point Coal Lease Tract and Elk Creek <br />Coal Lease Tract (2000). <br />The zone of continuous deformation, which is transitional to the overlying near-surface zone <br />and to the underlying zone of fracturing, undergoes differential vertical lowering and <br />flexure as laterally-constrained plates (in three dimensions) or beams (in two dimensions). <br />With flexure, shear occurs at the boundaries of rock units with different strength and <br />stiffness, characteristics, such as sandstones and shales. Zones of tension above the neutral <br />surfaces of a rock unit, for example, become compressive above the boundary with another <br />rock unit and below its neutral surface (Figure 2, Enlargement 2 of Exhibit 60B). Any <br />cracks, therefore, which occur in the tension zone of a rock unit, terminate at the neutral <br />surface, because the unit is in compression below this point. <br />Maximum Vertical Displacement, Tilt, Horizontal Strain, and Depth of Surjace Cracks <br />• <br />1.05-1 /1 <br />Revised November 1004 PRI G <br />