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West Elk Mine <br />• underlying bedrock, especially on steep slopes. The zone of continuous deformation, which is <br />transitional to the underlying zone of fracturing, consists of differential vertical lowering and <br />flexure of the overburden rocks above the zone of caving and fracturing. <br />Near-Surface Zone <br />Field studies by Mr. Dunrud indicate that near-surface cotluvium and alluvium, which consist of <br />predominantly clay and silt, can undergo significantly more extension without rupturing than can <br />the underlying material. In both the Somerset, Colorado and Sheridan, Wyoming azeas <br />colluvium and alluvium 5 to 10 feet thick were observed to cover cracks as much as 10 to 14 <br />inches wide so that there was no indication of the underlying ruptures. Mr. Dunrud's <br />observations in the Bear Creek area in 1976 are discussed in the Final Environmental Impact <br />Statement for the Iron Point Coal Lease Tract and Elk Creek Coal Lease Tract (2000). <br />The zone of continuous deformation, which is transitional to the overlying near-surface zone and <br />to the underlying zone of fracturing, undergoes differential vertical lowering and flexure as <br />laterally-constrained plates (in three dimensions) or beams (in two dimensions). With flexure, <br />sheaz occurs at the boundazies of rock units with different strength and stiffness, characteristics, <br />such as sandstones and shales. Zones of tension above the neutral surfaces of a rock unit, for <br />example, become compressive above the boundary with another rock unit and below its neutral <br />surface (Figure 2, Enlargement 2 of Exhibit 60B). Any cracks, therefore, which occur in the <br />tension zone of a rock unit, terminate at the neutral surface, because the unit is in compression <br />below this point. <br />Maximum Vertical Displacement. Tdlt. Horizontal Sbain. and Depth of Surface Cracks <br />Differential vertical lowering of the continuous deformation and near surface zones causes <br />vertical displacement (S), horizontal displacement (Sh), filt (M), and horizontal strain (E). Each of <br />these parameters is graphically illustrated in Figure 2, Exhibit 60B. In flat or gently sloping terrain <br />(slopes less than about 30 percent), surface profiles of subsidence depressions are similar to flexure <br />of fixed-end, laterally constrained beams. Tensile stresses aze present in areas of positive curvature <br />decreasing to zero at the neutral surface before which they reverse to become compressive stresses <br />(see Figure 1, Exhibit 60B). <br />In flat or gently sloping terrain, vertical displacement typically increases inwazd from the limit of <br />the subsidence depression, is half the maximum value at the point of inflection, and is at its <br />maximum in the middle of the depression (also called subsidence basin or subsidence trough). <br />Horizontal displacement and tilt increase inward from the margin of the depression to a <br />maximum at the point of inflection and become zero again at the point of maximum vertical <br />displacement (Exhibit 60B, Figure 3). Maximum values of tilt, curvature, and strain, discussed <br />herein, apply only to slopes less than about 30 percent; values may be greater on slopes steeper <br />than 30 percent. <br />Positive curvature (convex upwazd) and horizontal tensile strain increase inwazd from the margin <br />of the depression to a maximum about midway between the depression mazgin and the point of <br />inflection and decrease to zero again at the point of inflection. Negative curvature (concave <br />upward) and compressive horizontal strain increase inward from the point of inflection to a <br />1.05-126 Revised June 1005 PRIG, Rev. March 1006; May 1006 PRIO <br />