2014-10-10_PERMIT FILE - C1996083 (8)

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So in single seam mining the maximum possible subsidence for a super - critical width panel, Smax =V(H), is 8.6 feet (0.9 X 9.5), and for double seam mining, Smax =VD(H) + VB(H), is 8.6 +9.5 =18.1 feet (0.9 X 9.5 + 1.0 X 9.5) They go further to estimate the maximum subsidence for a panel at sub - critical width, Smax with the expression; Smax = S /M(Smax) Where; S/m is obtained from Figure 15 -2 Figure 15 -2 produces a subsidence reduction factor for the fact that the panel is sub - critical of 0.52 for a 1,500 feet deep single seam mining panel, so smax is 4.5 feet. Under the current mine plan the maximum amount of subsidence anticipated will be experienced in the center of longwall panels D -2 and B -1 in double seam mining where the overburden depth reaches approximately 800 feet to the B -seam and 500 feet to the D- seam. Figure 15 -2 produces panel correction factors of 0.85 at the B -seam depth and 0.73 at the D -seam depth, so Smax is 7.3 + 6.9 = 14.2 feet. Where full extraction zones overly or underlay partial extraction zones, i.e. gateways, the maximum subsidence is less and the subsidence profile is affected. Gentry and Abel (1978) report values of subsidence reduction factor ranging between 0.51 and 0.71, with the higher values corresponding to subsidence on the ridge line. Their data were measured at the York Canyon Mine, a longwall operation, in New Mexico, where the Raton Formation, which overlies the mined seam is of a somewhat similar character to the Mesaverde Formation in the North Fork of the Gunnison River Valley. Maximum Subsidence Over Mined Areas The maximum subsidence over mined areas (S) would equal the maximum possible subsidence (Smax = 8.6 feet in single seam mining or 18.1 feet in double seam mining) if the total width of the extracted zone exceeded the critical width, the width of the extracted zone exceeded 1.4 times the overburden height. Such a case may not arise at the proposed mine. PR -14 - 6 - 03/14