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• -10- <br />The gradation tests are only valid for the materials obtained from the <br />field density tests and are not typical of the overall spoil. Extremely <br />large sample sizes would be necessary to obtain representative gradations <br />of the spoil. Based upon our observations during mining we believe that the <br />gradation of the spoil varies with depth and probably varies with horizontal <br />location as well. In our opinion, the materials are much coarser than <br />indicated by our gradation tests and is predominantly cobble and boulder size <br />with depth. <br />Direct shear tests cannot be performed on full size samples of the spoil <br />using conventional testing equipment due to the large grain sizes. The spoil <br />was modeled in our tests by replacing the portion of the material from grada- <br />tion tests larger than the No. 4 sieve with an equal weight of material be- <br />tween the No. 4 and No. 16 sieve sizes. This results, in relatively coarse <br />grain samples at a size which can be tested using standard laboratory direct <br />shear equipment. The samples tested contained about 30 percent clay and /or <br />silt size material as did, the samples on which gradation tests were performed. <br />The results of direct shear testing (Fig. A -5) indicate a peak angle of <br />internal friction of 37.5 degrees and a cohesion of about 400 psf. Residual <br />values were somewhat lower with an angle of internal friction of 22 degrees <br />and no cohesion. Most spoil slope angles measured in the field varied between <br />34 and 37 degrees. The field angles are within the range we have observed at <br />this mine over a period of years and are, in our opinion, indicative of the <br />loose spoils angle of internal friction. We used angles of internal friction <br />of 34 and 37 degrees in our stability analysis to model the strength of the <br />spoil. <br />