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Terry Tunnel Bulkhead #2 Page 2 August 17, 2000 <br /> The recommended bulkhead location is at the dry rock zone <br /> closest to the portal, between stations 3+05 and 3+12 . This <br /> location will impound the greatest possible quantity of the water <br /> currently entering the Terry Tunnel, more than 90% of the current <br /> inflow. The recommended location is at a dry solid latite tunnel <br /> section that appears to be impermeable. It is at a depth <br /> sufficient to prevent hydrofracturing when subjected to the design <br /> 203 psi pressure head. <br /> Prior to driving the Terry Tunnel the watertable was probably <br /> in the more fractured and weathered near surface latite and <br /> alluvial soil cover. The original water table, which was <br /> depressed by drainage into the Terry Tunnel, will rise as water is <br /> impounded behind the near-portal bulkhead. As the water head on <br /> the near-portal bulkhead increases and the water level rises, the <br /> permeable fractures will progressively fill, the groundwater <br /> presently entering the Terry Tunnel will decrease and more of the <br /> groundwater will remain in the original near-surface groundwater <br /> regime. <br /> The water prevented from moving to and through the Terry <br /> Tunnel will reenter the near-surface groundwater system, the <br /> original more fractured and more permeable near-surface latite <br /> alluvium. When the water head behind the bulkhead reaches <br /> approximately 200 feet the depression of the original watertable <br /> should be reestablished directly over the bulkhead, as indicated <br /> on Figure 1 . The area of reestablished original watertable will <br /> increase as the water head behind the bulkhead increases . <br /> The water level will continue to rise, recharging the <br /> permeable fracture system, until the decreasing inflow and <br /> increasing outflow rates reach equilibrium. The indication from <br /> Table 1 is that equilibrium will be reached at or before the time <br /> the water head acting against the bulkhead reaches 469 feet, 203 <br /> psi pressure head. The easternmost partially water filled <br /> Benjamin Franklin stope that breaches the ground surface <br /> approximately 800 feet southwest of station T-9 in the Terry <br /> Tunnel plunges at 710. The Benjamin Franklin Mine does not connect <br /> with the Terry Tunnel . However, the vein structure appears to be <br /> permeable. The projection of the Benjamin Franklin vein crosses <br /> the Terry Tunnel at station T-9. The 71' plunge of the stope was <br /> used to conservatively estimate the design head for the bulkhead, <br /> as indicated on Figure 1 . Previous outcrop structure mapping of <br /> the Benjamin Franklin Vein indicate that its dip decreases to 550 <br /> to the southwest. As indicated on Figure 1, a flatter vein dip <br /> would reduce the conservative design head utilized in the bulkhead <br /> design presented in Appendix A. <br /> It is unlikely that water in the Sunnyside Mine pool is <br /> bypassing the existing Terry Tunnel bulkhead, as indicated by the <br />