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Subsidence Evaluation For Exhibit 608 South of Divide Mining Area Pape 10 Maximum tensile and compressive strain is significantly greater above lazge barrier pillars and rigid chain pillars and mine boundaries than it is above longwall mining faces. This is because tensile strains caused by mining the two adjacent panels aze additive above the common rigid chain pillazs or unyielding mine panel boundary pillars. Cracks tend to be wider and deeper above barrier pillars or lease boundary barrier pillazs than chain pillazs because of their greater rigidity (for example, the large tension crack on the north side of Lone Pine Cnilch). The tensile strains obtained from the curves in Figure 5 aze believed to be conservative for the South of Divide mining azea. Maximum horizontal tensile strains, measured by the author in bedrock during annual observations in the Apache Rocks azea (in hazd brittle sandstone, where the only strain is revealed by cracks), were 0.0031 to 0.0062 (0.31 to 0.62 percent). These values are about 35 to 45 percent less than those shown in Table 1. The tensile strain is considered to be close to a maximum value for those observed by the author in the Apache rocks azea because 1) the features are located above the azea if influence of a lazge solid coal pillaz, and 2) no greater strain was observed in the Apache Rocks mining area. South o~ Divide Mining Area: Maximum tensile and compressive horizontal strains aze calculated, using the values obtained from the West Elk Mine azea, and as projected in Figure 5 (Table 2). These values aze believed to be conservative, based on the author's annual observations over the last six years in the Apache Rocks mining azea. • Panels EI to E8: For these eight panels, projected horizontal tensile strain ranges from 0.004 to 0.022 (0.4 to 2.2 percent) where the planned coal extraction thickness is 8 feet; from 0.007 to 0.027 (0.7 to 2.7 percent) where the extraction thickness equals 12 feet; and from 0.007 to 0.039 (0.7 to 3.9 percent) where the extraction thickness equals 14 feet. Horizontal compressive strain ranges from -0.004 to -0.024 (-0.4 to -2.4 percent) where the extraction thickness equals 8 feet; -0.007 to -0.029 (-0.7 to -2.9 percent) where the thickness equals 12 feet; and -0.007 to -0.042 (-0.7 to -4.2 percent) where it equals 14 feet (Table 2). • Panel E9: Predicted horizontal tensile strain in this single panel ranges from 0,018 to 0.027 (1.8 to 2.7 percent) where the coal extraction thickness equals 12 feet and 0.021 to 0.031 (2.1 to 3.1 percent) where the extraction thickness equals 14 feet. Compressive strain this panel is predicted to range from -0.018 to -0.029 (-1.8 to -2.9 percent) where the extraction thickness equals 12 feet and -0.021 to-0.034 (-2.1 to -3.4 percent) where it equals 14 feet (Table 2). 5.3.2 Maximum Projected Depths of Surface Cracks Curvature, or differential tilt (curvature is the second derivative of vertical displacement with respect to horizontal distance) of subsided rock layers causes horizontal strain. Comparison of calculated curvature values and horizontal tensile strain derived from horizontal displacement 831-032.820 Wright Water Engineers, Inc.