2008-01-16_APPLICATION CORRESPONDENCE - C2008086 (3)

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DRAFT The McClane Canyon Mine map, Figure 6. McClane Canyon Mine Workings, indicates room- and-pillar advance mining, without pillar robbing on the retreat, to overburden depths just over 1,500 feet (generally 80 by 80-foot pillars on 100-foot centers for 36% recovery). Pillars were robbed on the retreat from other panels at depths of very nearly 1,300 feet. Total coal extraction (recovery) appears to have been as much as 78% within two small (roughly 520-foot maximum width) irregular shaped panels that were retreat mined to depths of 1,100 feet. The mine map indicates that robbing the 100 by 70-foot advance pillars using a method called "christmas treeing" was incomplete and erratic, with many 100 by 70-foot advance pillars and occasional 100 by 30-foot stump pillars left within the panels. This method is no longer permitted by MSHA because of safety concerns. In general, however, panel recovery was approximately 64% when robbing pillars on the retreat, apparently leaving different shaped stump pillars when retreating from different panels. The width of those panels ranged from 350 to 570 feet. No observations have been reported of surface subsidence effects over the McClane Canyon Mine. Estimates of maximum subsidence (Smex), tensile and compressive strains and maximum slope changes were made over the five selected panels and are indicated by number on Figure 7. Subsidence Predicted for Five Selected Panels, McClane Canyon Mine. The method used combined the British National Coal Board (NCB, 1975) method for longwall subsidence prediction and the room-and-pillar adaptation by Abel and Lee (1984) presented on Figure 8. Subsidence Over Room-And-Pillar Workings After Failure. Table 1. Predicted Maximum Subsidence For Selected Panels, McClane Canyon Mine presents the estimated super-critical subsidence over each of the selected room-and-pillar panels and the predicted NCB corrected subsidence for the panel width with respect to depth. The predicted maximum surface subsidence for the five panels ranged from 1.52 feet to 2.56 feet. Table 2. Predicted Maximum Strains and Tilt for Selected Panels, McClane Canyon Mine presents the maximum tensile and compressive strains and slope angle changes In addition, a rough estimate of potential open surtace fracture widths was made for each selected panel. This rough estimate was based on the relationship between the vertical subsidence measured over York Canyon Mine longwall panels with known widths and overburden depths, measured surface fracture widths and the NCB predicted maximum surface tensile strains for these known conditions permitted an estimate of the potential width of surface tension fractures. The magnitude of the maximum predicted surface extension strains presented in Table 2 would develop roughly over the perimeters of the selected retreat mined panels, which ranged from 1780 µE to 2860 µE. Extension strains in the predicted range from approximately 1800 µE to 2900 µE on the surface would cause repairable damage, ranging from cracking of single story brick walls to cracking of reinforced concrete frames. The predicted tensile strains would result in estimated 1-inch to 2-inch wide tensile cracks at the ground surface. The predicted magnitude of the maximum surface compressive strains ranged from 1940 µE to 3000µE, as Page 8 of 57