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the southeast. Small watercourses traverse the footprint. It is assumed that these watercourses <br />will be diverted around the gob pile. <br />According to Dunrud (1989), geology of the proposed new gob pile site consists of <br />undifferentiated Quaternary alluvium and colluvium (Qac) on the north side of the site, and <br />grades to Quarternary alluvium (Qa) on the southern portion of the site. Bedrock in the area <br />consists of nearly flat Cretaceous sedimentary bedrock units. Mancos Shale is mapped near the <br />bottom of the valley slope with the Mesaverde Formation overlying the Mancos Shale. The <br />Rollins Sandstone member of the Mesaverde Formation outcrops can be observed above the <br />existing coal waste stockpile. <br />3.0 SUBSURFACE CONDITIONS <br />As previously indicated, Buckhorn has not conducted a site specific geotechnical investigation at <br />the proposed new site. Lambert and Associates (2001) conducted a geotechnical engineering <br />study along the railroad track alignment south of the proposed site. In that study, 24 borings <br />were advanced to depths up to 55 feet. Soils encountered were generally described as "clay, <br />silty, sandy, gravelly, cobbles, medium stiff to stiff, moist to very moist (CL) ". "Sand and <br />gravel, clayey with cobbles, medium dense (SP -GP)" were encountered in two borings at <br />approximate depths of 45 feet. The upper clayey soils are interpreted to be alluvial/colluvial <br />soils originating from slopes on the north side of the valley. The cleaner sand and gravels <br />encountered at approximate depths of 45 feet are likely alluvial deposits from the North Fork of <br />the Gunnison River. No formational bedrock was encountered in any of the boreholes. <br />Groundwater was encountered at approximate depths of 45 feet in two boreholes. The <br />foundation soils at the proposed new gob wastepile site should be verified with a site <br />investigation. <br />6.0 STABILITY EVALUATIONS <br />Slope stability of the proposed south coal waste stockpile was evaluated using two - dimensional <br />limiting equilibrium stability analyses with the computer software SLIDE developed by <br />Rocscience (2004). Critical circular failure surfaces were determined using total and effective <br />stress analyses using the simplified Bishop and Spencer procedures. Only static loading <br />conditions were evaluated. <br />6.1 Cross - Sections and Geometry <br />Stability of the proposed gob pile was evaluated for three cross - sections, as indicated on <br />Drawing 2. Cross - section A -A' is a northwest - southeast trending section at the maximum <br />section of the proposed gob pile. Cross - section B -B' is transverse to Section A -A'. Section C- <br />C', roughly parallel to Section A -A', represents the southern portion of the gob pile with a <br />smaller crest area and smaller cross - sectional area. In general, the geometry of the gob pile <br />evaluated in the stability analyses consisted of 311: IV (horizontal:vertical) slopes with one 20- <br />Bowie No 2 South Gob Pile preliminary stability evaluation <br />Project #05- 126- GEO -08 <br />Page 5 of 10 <br />