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• feet and the mining method room and pillar. The surface cracking has been <br />limited to two to three inches, The combination of a 1/3 increase in mining <br />height, a minimum tripling of the depth of cover and the switching from <br />room and pillar to longwall mining should result in very similar cracking <br />of the surface. Increasing mining height linearly increases surface crack- <br />ing, a 33% increase in this case. The tripling of the depth linearly reduces <br />surface cracking by 67%. ~S. a om .o n pa a• o o w ~h~i~g <br />s?h'o'fl-13~iinc~iea~se~Yra~t}eMIC~ARdsu~bff~el~itelthea~ma~gn~.fade=of=surs5ae~etire~'ac~ki~ng. <br />The resulting relative surface cracking prediction is: <br />(1 + 0.33) (1 - 0,67) 2 = 0.88 <br />I predict essentially the same surface effects we have seen in the past. <br />• I hope this letter report meets your requirements. <br />Sincerely, <br />~.~,~ <br />ohn F. Abel, Jr. <br />Colorado P.E. 5642 <br /> <br />