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A <br />• <br />,. <br />JOHN F. ABEL. JR. <br />MINING ENGINEER <br />qG~~ <br />9~ <br />310 LOOKOUT VIEW COURT <br />OOLOE N. CO BD601 <br />•e~enROr.e za- z»-aTOs <br />SfY-aa01 <br />Mr. Dale L. Fenwick <br />Powderhorn Coal Co. <br />P.O. Box 1430 <br />Palisade, Colorado 81526 <br />Dear Dale: <br />August 5, 1985 <br />As you requested, I have predicted the "worst-case" potential subsidence <br />• <br />effects for the planned Colorado Ute transmission line. The longwall, induced <br />trough subsidence (Figure 1) predictions were based on the method presented <br />in the Subsidence Engineers Handbook published by the British National Coal <br />Board (NCB), Mining Department in 1975. The prediction of vertical trough <br />subsidence of the surface above planned room and pillar panels was based <br />on the method presented by Abel and Lee (1980). Once the vertical subsidence <br />over room and pillar workings was predicted the NCB method was used to predict <br />the maximum surface strain and tilt. <br />Chimney, or sinkhole, subsidence has not been predicted for the area <br />• <br />where the Colorado Ute transmission line crosses the leases. As Gray, et. <br />al. (1977, p. 34) point out, chimney subsidence over the Pittsubrgh seam is <br />rare at depths over 120 ft and does not occur at depths greater than 200 ft. <br />Similarly, Piggott and Eynon (1978, p. 764) point out the relationship bet- <br />ween the bulking of rock upon collapse and the plugging of a collapse chimney <br />by its own debris. Figure 2 shows that the potential height of a chimney <br />collapse feature should not exceed ten times the mining height, in this <br /> <br />s <br />