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-20- <br />• <br />was cut cross -dip with the bedrock dip slope out of the cut face. The re- <br />sults of the analysis and experience during mining indicated lower bound <br />strengths of 0=15 and cohesion of 2,000 psf for the highwall rock. These <br />values applied for the shales. Based upon a boring done by Colorado Yampa <br />Coal it appears that bedrock below the old pit predominantly consists of <br />sandstone. We believe that the strength values used in our previous inves- <br />tigation would be somewhat conservative for this investigation. Cohesion of <br />5,000 psf and an angle of 20 degrees is reasonable in our opinion, consider- <br />ing past experience in the specific area. <br />The spoil materials vary in overall size, gradation and rock type through- <br />out the spoil bank partially due to the placement and grading procedure. We <br />believe that the spoil tends to be soil -like near surface while the spoils <br />• at depth are larger diameter, contain little soil size material and are es- <br />sentially non - cohesive. We have observed angles of respose between 34 degrees <br />and 37 degrees for the spoils at different pit locations at the mine over <br />the course of several years. The observed angles vary less than 10 percent. <br />We used an angle of internal friction of 34 degrees, or the lower bound ob- <br />served angle of repose, in our analysis to evaluate the stability of the exist- <br />ing spoil. The angle was varied between 34 and 37 degrees in our stability <br />analysis for recommended slope configurations. <br />Our laboratory testing indicates a residual angle of internal friction <br />of about 24 degrees and a cohesion of 350± psf for the fire clay. Atterberg <br />limits testing and gradation analyses indicate that the material is a moderately <br />plastic clay. The material is relatively soft and presently has moisture <br />L J <br />