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• 2.05.4(2)(e) <br />The design of the concrete water -tight seal was taken from Bureau of Mines <br />Information authored by Cheken, Garrett, and Pitt. They developed an <br />equation used for design after work done in Africa on seals which where able to <br />withstand hydrostatic pressure in excess of 6,000 psi. The equation is as <br />follows; <br />L = (p'a`b) /((a +b)'fc) <br />where; <br />L = Length of plug, ft. <br />a = width of entry, ft. <br />b = Height of entry, ft. <br />p = Hydrostatic pressure, psi. <br />fc= Allowable compressive strength of rock or concrete, whichever is the <br />lesser, psi. <br />The height and width, a and b, of the portal entries is 8 -feet and 20 -feet, <br />respectfully. The roof, ribs and floor will be cleaned to competent material but <br />will not be recessed into solid rock or coal. The maximum calculated <br />hydrostatic pressure, p, for the seal will be 60 -feet of water or 26 psi (measured <br />from the seal elevation to the piezometric surface of the upper perched water <br />• bearing zone). Coal will have the lowest compressive strength, and triaxial <br />tests preformed on the coal typically average 1000 psi. Garrett Campbell and <br />Pitt recommend the use of a factor of safety of 4 to 5. The allowable <br />compressive strength will be (1/5) "(1000)= 200 psi. Using this data, the length <br />of the plug required would be 0.75'. The applicant would like additional <br />certainty added to the plug length and will propose that a minimum plug length <br />should be 3 -feet, or 4 times that calculated. A sketch of the water tight seal is <br />shown on Map -30 of Volume ll. <br />Gob Vent Boreholes (GBVs) present unconventional circumstances for <br />abandonment procedure to assure methane does not leak to the surface. The <br />Mine is using a pressurized ventilation system to "push" methane up through <br />the GBV's to cause exhausting of methane gas to the surface. <br />GVB's are constructed by drilling a hole to within 30 ft. above the seam then <br />placing a string of 5.5 or 6.5 inch casing in the hole with the open bottom of <br />casing suspended approximately 230 feet above the seam leaving 200 ft. of <br />open hole. The bottom 200 ft. of casing is perforated, at the top of the <br />perforations a cement basket is placed and cement is pumped into the annulus <br />above the basket completely grouting the casing in- place. This provides at <br />least 200 ft. of fully grouted casing, in the worst case scenario (GVB- 4 -D -A), <br />where the total depth of he hole is 500 ft. and the amount of perforated casing <br />was adjusted to 100 ft. Typically hole depths as in excess of 1000 feet which <br />results in the grouted interval (fully cemented to the formation) in excess of 600 <br />• <br />ft. <br />PR-06 2.05-58- 08/01 <br />A PPRa�6D <br />//5/02 <br />