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Subsidence Evaluation For <br />Exhibit 60 The Apache Rocks And The Box Canyon Mining Areas Page 12 <br />• about 750 feet (Table 3). Maximum tensile strain in both azeas of discussion may be <br />considerably greater than the predicted range along slopes steeper than 30 percent and on the <br />cliffs and ridges that flank West Flatiron. <br />Maximum horizontal compressive strain (-Em) in the current subsidence monitoring network <br />ranges from about -0.2 to -0.8•(Sm/d) above longwall panel boundaries and about 3 to 4•(Sm/d) in <br />the centers of the longwall panels between chain pillars (Table 1 and Figure 4). The range in <br />four mining azeas of the Western United States studied by the author is from - l.5 to -4 (S/d) and <br />averages -2.5•(S/d). Using an average value of -2.0•(Sm/d), maximum compressive strain <br />estimates above the centers of the longwall panels range from -0.003 on West Flatiron to -0.026 <br />above the northernmost longwall panel beneath Box Canyon. <br />4.3.2 Maximum Projected Depths of Surface Cracks <br />Curvature, or differential tilt (the second derivative of vertical displacement with respect to <br />horizontal distance) of subsided rock layers causes horizontal strain. Comparison of calculated <br />curvature values and horizontal tensile strain derived from horizontal displacement <br />measurements, therefore, provides a means of calculating the depth of the neutral surface, and <br />hence the maximum depth of tension cracks from the surface. <br />• The depth of the neutral surface can be estimated by dividing the maximum horizontal strain <br />values by the maximum curvature values at a given location. The calculated depth of the <br />possible tensile zone ranges from about 50 to 100 feet. Below the neutral surface is a zone of <br />compression. Crack depth may be much less, but not more than these maximum values. An <br />unpublished study for the U. S. Bureau of Mines (Engineers International) indicated that surface <br />crack depth razely is greater than about 50 feet. Also, cracks will likely terminate at boundaries <br />of shales and claystones. <br />• <br />Based on this analysis in the current mine area, it is estimated that the maximum depth of the <br />tension cracks would range from 50 feet to as much as 100 fee[ in the Apache Rocks and the Box <br />Canyon mining azeas. Also, because of reduced or lacking lateral constraint, cracks may locally <br />range from 75 feet to perhaps as much as 150 feet deep along the precipitous slopes and cliffs on <br />the Hanks of West Flatiron and north of Dry Fork. Cracks may be as much as 200 feet deep <br />when both the E- and B-Seams are mined. <br />Very little evidence of cracking above the longwall panels due to the downwarping process was <br />observed by the author. One northeast-trending crack along a bedrock joint was observed above <br />the east chain pillars of 1 NW longwall panel. Mine personnel have observed that this crack has <br />almost completely closed. <br />831-032.181 Wright Water Engineers, Inc. <br />