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