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11BUCKHOR <br />LM sm <br />Civil, Structural & Geotechnical Engineers <br />Iti <br />November 9, 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 #2 EXPANSION <br />SUBJ: STABILTY ANALYSIS OF PROPOSED SLOPES <br />Dear Tamme: <br />Per your recent request, we have analyzed the proposed expansion of the Bowie Mine Coal <br />Waste Disposal Area (CWDA) #2 for slope stability based on the AutoCAD topography you <br />provided dated October 19, 2012. Based on the recent topographic survey and the material <br />properties from the stability analysis presented in our December 2008 report for both the <br />native and waste materials, we performed analyses in SLIDE (version 5.0) to confirm the <br />factor of safety for the proposed expansion. <br />Assumptions for CWDA #2 Expansion Stability Analysis <br />• Average depth to native soils was modeled as 29 feet per our bore logs in 2005. No <br />additional drilling was done in the 2008 stability analysis. <br />• Material properties are the same as originally modeled in our 2008 Gob Pile #2 <br />stability analysis: Gob has an average internal angle of friction, phi,((D) of 34 <br />degrees and an average unit weight of 110 pcf, and an average cohesive strength of <br />0 psf; Native soil has a 0 of 34 degrees, an average unit weight of 125 pcf and a <br />cohesive strength of 0 psf <br />• No phreatic surface was modeled in our analysis since the geotechnical investigation, <br />performed in our December 2008 CWDA pile #2 report, encountered no <br />groundwater in the native soils and the ultimate pile stability assumes dewatering <br />which is reasonable given the decreasing pore pressures in the vibrating wire <br />piezometer results over the past 3 years. <br />The minimum global factor of safety (F.S.) for the proposed configuration of the expanded <br />CWDA #2 at the Bowie #2 Mine was determined to be 1.640 for section F -F' using the <br />simplified Janbu method used for analysis. To confirm this calculation, the slope was also <br />analyzed using three other methods: Spencer, Bishop simplified and GLE /Morgenstern -Price <br />with the global minimum for each section as indicated in Table 1.0. In each of the analyses, <br />the Janbu Simplified method produced the lowest, and therefore, most conservative Factor <br />of Safety. For that reason, the Janbu Simplified F.S. was reported in Table 1.0. <br />Bowie Mine #2 — CWDA Pile # 2 Expansion PN 05- 126 -GEO <br />REVISED Stability Analysis <br />