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Memo to Tony Waldron Bowie No. 2 Geotechnical Adequacy page 4 1.2 MINING SCHEDULE AND RATE OF SUBSIDENCE The preparer states; "It is reasonable to assume that the initial subsidence is negligible compared to the final, maximum subsidence." The report further states; "While subsidence of small magnitude will occur during mining of entries and rooms, the majority of the subsidence will occur during and after pillar recovery. These statements are generally true, but not necessarily certain; particularly if discontinuities in the rock mass, such as faults, are encountered during development. These discontinuities can become the locus of dramatic short-term ground mass response. Instances of this phenomena occurred within the East workings of the Bowie No. 1 mine (previously the Orchard Valley minel. For this reason it is important to anticipate the possible subsidence implications of faults within the mine plan area. The preparer relates a general conceptual characterization of subsidence response above retreating longwall workings, as developed by Gentry and Abel above the York Canyon mine's longwall workings. The application of longwall subsidence experience to room and pillar extraction assumes that efficient and relatively complete pillar recovery will be accomplished. In instances where pillar recovery, due to bad ground conditions, equipment problems, or water problems is less efficient than projected, the distribution of strain, the magnitude of ultimate subsidence, and the timing of subsidence will be effected. Therefore, it will be important to establish the effectiveness of pillar recovery in retreat early in the operation of the Bowie No. 2 mine. Further, if recovery proves to be less than projected, it may be necessary to place high resolution monuments above early panels to determine the actual character of ground subsidence manifested. 1.3 SUBSIDENCE MAGNITUDE AND PROFILE ~ he preparer references an NCB method for projecting the maximum subsidence above a panel of sub-critical width. Figure 15-3, a reproduction of Figure 3 of the Subsidence Engineer's Handbook (SERI, is used to determine the projected maximum subsidence above a partially extracted super-critical panel. Exhibit 15 determines a subsidence factor ISmax1 of 0.472 for this situation, which results in a maximum projected vertical subsidence of for the 12.0 feet extracted height of 0.472 x 12.0 feet = 5.7 feet. The preparer next applies figure 4 of the SEH, which corrects maximum projected subsidence for the effects of sub-critical panel width. Exhibit 15 determines an s/S (subvsidence correction factor) of 0.23. In accordance with SEH methodlogy the super-critical panel maximum subsidence is multiplied by the susbsidence correction factor to determine the projected subsidence above the sub-critical panel. In this case the preparer projects a maximum expected vertyical subsidence above the sub-critical panel of 0.23 x 5.7 feet = 1.3 feet.