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ENVIROCON, INC. <br />101 INTERNATIONAL WAY <br />POST OFFICE BOX 8247 <br />MIBSOIILA, MONTANA 5900] <br />TELEPHONE. ~a06) 023~1t00 <br />FAX: p061 523~n02 <br />ENCiIROCON, INC. <br />February 6, 1992 <br />Ms. Kathleen Welt <br />Mountain Coal Company <br />West F,lk Mine <br />P.O. Box 591 <br />Somerset, Colorado 81434 <br />RE: RESPONSE TO CMLRD JANUARY 1992 LETTER <br />Dear Kathy: <br />This letter summarizes the response to the CMLRD letter dated January 23, <br />199Q concerning the data for the slope stability analysis for the stack tube analysis. <br />THe CMLRD requested more specific geotechnical information on the gob, collu~rium <br />and clay backfill than what was provided in the December 11, 1991 letter which <br />summarized the stack tube stability analysis. <br />The data used in the analysis was previously presented and discussed in the <br />"Lower Waste Pile Permit Application" prepared by Rocky Mountain Geotechrrica.! in <br />August, 1985. Section 5 of this application summarized the shear strength data that <br />~A~as used in the stability analysis for the refuse pile design. The materials that T.vere <br />tasted are the same materials that were analyzed in the stack tube project. <br />The 1985 report recommended a shear strength friction angle of 30 degrees and <br />a cohesion of 288 psf be used as the shear strength parameter for the gob material. <br />Due to the loading factor of the coal at the stack tube site, the shear strength <br />parameter used for the stack tube stability analysis was a slightly more conservative <br />28 degree friction angle and a 200 psf cohesion. <br />The colluvium shear strength value of 30 degrees friction angle and 300 psf <br />used in the stack tube analysis was obtained from Table 5.3 of the pennit application. <br />This table is attached to this letter. The value used represented a very conservative <br />strength value for the clayey colluvium which is found at this site. <br />A7Un~1L! A't Company ~~ <br />