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k. <br />The hydralic head that was used in designing the No. 5 mine portal seals was <br />estimated from the pre-mining piezometric surface in the F-seam. It was <br />estimated that the pre-mining hydralic head in the F seam was approximately 90 <br />to 140 feet above ground surface in the vicinity of the portal. This is equiv- <br />alent to a pressure of 40 to 60 pounds per square inch. <br />The above calculated values were based on a portal elevation of 6160 feet msl and <br />an estimated pre-mining piezometric surface in the F seam near the mine portal <br />of 5250 to 6300 feet msl. The pre-mining piezometric surface was estimated from <br />projections of present observed piezometric declines in the overlying Middle <br />Sandstone and on the present piezometric surface of the overlying White Sandstone. <br />While the White Sandstone does not overlie the portal area, the estimated pre- <br />mising piezometric levels in the Middle Sandstone were similar to the present <br />levels in the llhite Sandstone where the White Sandstone occurs. Also, the 'White <br />Sandstone does not appear to have been significantly impacted by mining. <br />The design that follows is based upon current engineering practices. However, <br />Empire will maintain the ability to modify the design and obtain MLR approval <br />for the modification. This flexibility is needed so that the most efficient seal <br />can be constructed at the time the portals are sealed. The life of the Eagle <br />No. 5 mine is at a minimum five years and it would be prudent to be able to take <br />advantage of any technological progress that may be made in the design and construction <br />of seals during that time. <br />