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• 2.05.4(2)(e) <br />• <br />C <br />PR -12 <br />A PPRov6b <br />[1/ 16 //o <br />and triaxial tests preformed on the coal typically average 1000 psi. Garrett <br />Campbell and Pitt recommend the use of a factor of safety of 4 to 5. The <br />allowable compressive strength will be (1/5) *(1000)= 200 psi. Using this data, <br />the length of the plug required would be 0.75'. The applicant would like <br />additional certainty added to the plug length and will propose that a minimum <br />plug length should be 3 -feet, or 4 times that calculated. A sketch of the water <br />tight seal is shown on Map -30 of Volume II. <br />Gob Vent Boreholes (GVBs) 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 Apush@ 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 <br />annulus above the basket completely grouting the casing in- place. This <br />provides at least 200 ft. of fully grouted casing, in the worst case scenario <br />(GVB- 4 -D -A), where the total depth of he hole is 500 ft. and the amount of <br />perforated casing was adjusted to 100 ft. Typically hole depths are in excess <br />of 1000 feet which results in the grouted interval (fully cemented to the <br />formation) in excess of 600 ft. <br />GVB's will be sealed as follows: <br />1. The condition of borehole will be verified by sounding the total depth of <br />the borehole. <br />2. A bridge plug will be placed at the lowest point possible in the blank <br />casing. The bridge plug will always be placed above the perforated <br />casing. <br />3. The placement of the bridge plug will be verified. <br />4. The cessation of methane gas flow will be confirmed. Placement of a <br />hole plug or other suitable media may be required to stop methane <br />flow. <br />5. The theoretical well volume will be calculated. <br />6. The borehole will be filled with cement to within three feet of the ground <br />surface or a minimum height of 50 feet above the bridge plug or both. <br />Cement will be placed utilizing a tremmie. The top of the cement plug <br />will be sounded. <br />7. The actual volume of cement used will be compared to the theoretical <br />well volume to assure the cement did not bridge. <br />2.05-81 06/10 <br />