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Subsidence Evaluation For <br />Exhibit 60 The Apache Rocks And The Box Canyon Mining Areas Page 9 <br />• feet and chain pillar dimensions similaz to the current West Elk mining area, maximum <br />subsidence (Sm=a•t) is projected as follows (Table 3): <br />The first two northern panels will range from supercritical to critical at points of minimum <br />overburden depth beneath the canyons to subcritical beneath ridges on the flanks of West <br />Flatiron. Maximum subsidence (Sm) above panel centers is projected to range from 7.2 to 9.6 <br />feet (O.bt to 0.8t) beneath the canyons to about 6.0 to 8.4 feet (O.St to 0.7t) beneath areas of <br />greatest overburden depth. Maximum subsidence above chain pillars (S~p) is projected to <br />range from 1.2 to 3.6 feet (0.1 t to 0.3t). <br />2. The third panel southwazd will be subcritical beneath Box Canyon based on width-to-depth <br />ratios [W/d] ranging from 0.5 to l.l. Maximum subsidence is projected to range from as <br />much as 8.4 feet (0.7t) in aeeas of shallower overburden to about 6.0 feet (O.St) under deepest <br />cover; SAP is projected to range from 2.4 feet (0.2t) to as much as 4.8 feet (0.4t) in areas of <br />deepest cover. <br />3. Panels 4 through 6 range from critical to subcritical (W/d = 1.3 to 0.5), assuming that the <br />chain pillars support the overburden load. However, maximum subsidence is expected to <br />range from 3.6 to 7.2 feet (0.3t to 0.6t) because the subsidence factor above the chain pillars <br />(SAP) is projected to range from 3.6 to 7.2 feet (0.3t to as much as 0.6t). Subsidence of as <br />• much as 7.2 feet (0.6t) is therefore projected because the chain pillars and caved material in <br />the caved zone (gob) aze projected to compress about equally. With the increased overburden <br />loads under West Flatiron and its flanking cliffs and steep slopes, these three southern panels <br />are expected to locally subside as a super panel (i.e., multiple panels and adjacent chain <br />pillars that behave as a single panel) of supercritical width beneath areas of greatest <br />overburden depth. <br />4. The subsidence factor is projected to be less where the B-Seam roof rocks (sandstones, <br />shales, and siltstones) cave into dry mining conditions than it will be where the roof rocks <br />cave into wet mining conditions. This variance is expected because the final bulking factor <br />of caved rocks under dry conditions is greater than it is for caved rocks under wet conditions. <br />4.3.1.3 Maximum Tilt <br />Maximum tilt (Mm) was calculated from differential vertical displacements at the West Elk Mine <br />monitoring network in terms of the ratio of maximum vertical displacement to overburden depth <br />(Sm/d in dimensionless units L/L) (Table 1 and Figure 4). Tilt values at the West Elk ivfine range <br />from 0.014 [0 0.021 dimensionless units (L/L). Maximum calculated tilt ranges from about two <br />to three times Sm/d (2 to 3~(Sm/d)) at the West Elk Mine. Maximum tilt in four different mining <br />areas of the Western United States ranges from 2.5 to 5•(Sm/d). <br />• <br />831-032.181 Wright Water Engineers, lnc. <br />