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~J <br />• height of 280 feet, and a soil layer ranging in thickness from 20 feet at the back of <br />the fill to 30 feet at the toe. <br />Plans indicate the ground slope at the sites represented by the low fill <br />analysis section ranged from 12 to 20 percent and proposed fill heights from about <br />135 to 160 feet. The low analysis section represents conservative assumptions with <br />a slope of 20 percent, fill height of 160 feet, and soil layer thickness of 20 feet. <br />Fill 19 represents an unusual condition. The topography is steep (over 20 <br />percent) and the fill is relatively high (210 feet). Fill 19 was added to the project after <br />stability analyses typical sections were developed. The fill will require additional <br />analysis after site specific investigation. For now, Fill 19 is considered to be <br />represented by the low analysis section. The increased height (210 feet compared <br />to 160 feet in the model) will need to be checked. <br />Material Properties <br />• <br />A three layer model was selected to represent the spoil, soil layer and <br />underlying bedrock. Our experience indicates the sedimentary bedrock possesses <br />sufficient strength so that failure through the bedrock is not a consideration. <br />Strength values for the clay assume a sandy clay with low plasticity based upon the <br />geologic conditions, soil mapping and classification testing from previous <br />investigation at a nearby site. Research of technical publications and strength <br />testing we have performed on similar clays were also considered. The previous <br />strength testing was primarily on colluvial/alluvial clays derived from sandstone/shale <br />bedrock in northwest Colorado. Conservative effective and total stress parameters <br />were estimated such that site specific investigations and testing are expected to <br />exceed the values utilized in the analysis. Normal values were selected and then <br />varied to show the sensitivity of the calculated Factor of Safety to the strength. The <br />normal strength values used for the clay layers are: <br />C J <br />LORENCITO COAL COMPANY <br />CM 75,451 8 <br />