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Sanborn Creek Project Page 17 August 21, 1992 • "C" AND "E" SEAM INTERACTION ANALYSIS The "E" Seam workings in the Hawk's Nest East Portal and West Portal Mines are approximately 340 feet above the planned "C" Seam workings and 380 feet above the planned "B" Seam workings in the Sanborn Creek Mine, Table 1. The Hawk's Nest Mines overly most of the planned panels in the Sanborn Creek Mine. The possibility of chimney collapse penetrating through 34o feet of interburden is extremely remote and would be unprecedented. Piggott and Eynon's (1977) geometric calculation, Figure 1 and Table 2, indicate the maximum height that a collapse could progress upward is approximately 182 feet above the "C" Seam. This calculation conservatively assumes worst-case conical chimney collapse, 20 feet of coal extracted at the "C" Seam elevation and the entire interburden composed of lower swelling, 33 percent, shale. Gray et al (1977) found no chimney collapses reaching a height of over 200 feet above the Pittsburgh Seam, Table 3. The increased permeability accompanying planned pillar crushing during pillar robbing during panel retreat mining in the "C" and "B" Seams will probably progressively drain any water that may be impounded in overlying Hawk's Nest Mine workings. The maximum calculated "E" Seam tensile strain, 16300 micro-strain under low-cover and 12900 micro-strain under high-cover as shown on Table 4, is sufficient to open up any bedding plane and joint • weaknesses present. If water is impounded in the Hawk's Nest Mine workings it must be anticipated that it will find a path to slowly drain into the Sanborn Creek Mine. Pre-drainage of impounded water, if found in the Hawk's Nest Mines, is recommended. OTHER POTENTIAL WATER PROBLEMS Four exploratory NX, nominal 3-in. outside diameter, holes, WSC DH 5, 6, 7 and 8, were apparently drilled downward from Hawk's Nest Mine "E" Seam workings through the "B" Seam. These holes present a restricted potential path for water that may be impounded in the Hawk's Nest Mine to drain into the planned Sanborn Creek Mine workings. The rate at which water can enter the Sanborn Creek Mine will be limited by the 3-in. diameter of the boreholes. If water is found to be impounded in the Hawk's Nest Mines and the head present measured it will be possible to calculate the rate at which such a borehole would release water into the Sanborn Creek Mine workings. It is recommended that if water found to be impounded in the abandoned Hawk's Nest Mine workings that it be pre-drained to eliminate this potential problem. Standby pumping capacity could be maintained to accommodate such a water inflow. It is also possible to leave protective pillars around the drillhole locations if they can be accurately located. The USBM guidelines recommend leaving 150-foot radius pillars between oil • and gas wells and mine workings. These later courses of action are not recommended, because of the cost of standby pumping capacity in the first case and the disruption of mining plans in the second case.