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= <br />September 7, 2102 <br />Tamme Bishop, PE <br />JE Stover & Associates <br />23532 North 7th ST., Unit B <br />Grand Junction, CO 81501 <br />RE: BOWIE# 2 MINE, CWDA PILE #4 <br />Civil, Structural & Geotechnical Engineers <br />SUBJ: STABILTY ANALYSIS, AS -BUILT CONDITIONS <br />Dear Tamme: <br />222 South Park Ave. a Montrose, CO 81401 <br />Ph.: (970) 249 -682B • FAX: (970) 249 -0945 <br />Per your recent request, we have analyzed the Bowie Mine Coal Waste Disposal Area <br />(CWDA) #4 for slope stability based on the as -built conditions you provided in AutoCAD files <br />dated August 22, 2012. Based on the recent as -built topographic survey and the stability <br />analysis presented in our May 2009 report for both the native and waste materials, we <br />performed analyses in SLIDE (version 5.0) to confirm the factor of safety for the current <br />conditions. <br />Assumptions for As -Built Stability Analysis <br />• Finished height of the CWDA pile #4 is 165 feet <br />• Material properties are the same as originally modeled in our 2009 stability analysis: <br />Gob has a internal angle of friction (0) of 34 degrees, and a cohesive strength of 0 <br />psf; Native soil has a cp of 34 degrees and a cohesive strength of 100 psf <br />• No phreatic surface was modeled in the as- built model since the pile is no longer <br />receiving wet gob and, as demonstrated by the on -going monitoring results of the <br />adjacent CWDA pile #2, the coal waste dewaters over time and will eventually have <br />no phreatic surface associated with it. <br />The minimum global factor of safety (F.S.) for the current configuration of CWDA #4 at the <br />Bowie #2 Mine was determined to be 1.686 for the conservative Janbu simplified method <br />used for analysis. To confirm this calculation, the slope was also analyzed using three other <br />methods as indicated in Table Z.O. <br />Table 1.0 Analvses Methods and Results from SLIDE modelina <br />Method <br />Minimum Factor <br />of Safe <br />Comments <br />Janbu Simplified <br />1.686 <br />Most conservative <br />Bishop simplified <br />1.686 <br />= Spencer <br />1,686 <br />GLE /Mot enstern -Price <br />1.686 <br />Bowie Mine #2 — CWDA Pile # 4 PN 05- 126 -GEO <br />REVISED As -Built Stability Analysis <br />