2002-05-13_PERMIT FILE - C1981041 (4)

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~~ PublicService® November 19, 1996 Mr. Larry Reschke POWDERHORN COAL COMPANY P.O. Box 1430 Palisade, CO 81526 Public Service Company of Colorado Electric Transmission and Substation Engineering 550 15th Streel, Suite 600 Denver, CO 60202-0256 Subject: Subsidence of PSCo's Rifle-San Juan 345kV Transmission Line Transmission Line Structures Dear Mr. Reschke: I apologize for not responding sooner to your question concerning the effect of undermining the Rifle-San Juan 345kV transmission line lattice tower structures. PSCo, together with Tri-State Generation and Transmission Association, Inc., will first evaluate the structures using the data provided by Powderhorn Coal Company for the 'bvorst case" subsidence scenario for longwall mining with 76% extraction ratio. If this method cannot determine the effect on the structures, a subsidence study may be conducted using Powderhorn Coal Company's actual room r3 pillar mining plan, with 50% extraction ratio, to predict the • subsidence. As your letter of March 11, 1996 stated, the 50% extraction ratio is used for safety zone under creeks that are to be protected from subsidence. So we expect that the lattice tower can withstand the relative minor subsidence and horizontal movement. We will have a good indication of how much displacement the lattice tower structures can withstand soon, as Twentymile Coal Company has just undermined the Hayden-Gore Pass 345kV transmission line on 11/12196. Both transmission lines uses the same, or very similar, structures, so meaningful comparison can be made. As I understand it, the first undermining of any structure is at least two years away. This would provide more than adequate time to do our evaluation and study, if necessary. If you have any questions or comments, please feel free to call me at (303)571-7289. Sincerely, ~ ~~ Cary H. an Civil En sneer cc: Pat Dille, Tri-State Doug Hanson, WAPA • LETfER1.DOC, N°vgnM, r9, i998