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© MONTGOMERY WATSON <br />• <br />October 18, 2000 <br />Karl Koehler <br />Trapper Mining, Inc. <br />PO Box 187 <br />Craig, CO 81626 <br />RE: Proposed West Horse Embankment Dam -Preliminary Slope Stability Analysis <br />Dear Karl, <br />This letter report details the results of the preliminary slope stability analysis performed on <br />the West Horse Embankment Dam. Included in this report is a summary of the properties of the <br />soil found in the mine area at and around the location of the proposed pond, and the stability <br />analysis performed on the proposed Typical Embankment cross section (Figure TR 86-2). <br />1. Soil Characterization <br />• <br />In August 2000 Montgomery Watson performed a geotechnical investigation of the area. The <br />geotechnical investigation included disturbed sampling of the subgrade material at the bottom of <br />a test pit positioned approximately in the location of the proposed West Horse Embankment Dam. <br />During the on site geotechnical investigation disturbed and Shelby tube samples were also <br />retrieved from additional sediment pond locations (Horse, East Flume, and Deal). During <br />excavation of the test pit at West Horse loose black silt was excavated to a depth of <br />approximately 8'. At approximately 8' the soil transitioned into a medium dense black and brown <br />clay with some sand. Samples were recovered from this deeper denser clay material. This <br />material and the material recovered from the soil-sampling program at the other sediment pond <br />sites was then subjected to laboratory testing. A summary of the results is located at the beginning <br />of Attachment 1(Soil Testing). The USCS classification of the material that will constitute the <br />foundation material for the dam is CL (low plasticity clay with some sand). <br />The soil samples from the various dam locations were compared based on USCS <br />classification, Atterberg Limits, and grain size distribution. The results of all the soil testing aze <br />included in Attachment 1. Since the properties of the sub-grade material at the West Horse site <br />correspond to the CL material that makes up the subgrade material at the East Flume site, the soil <br />strength and density.properties from the East Flume Keyway were used (see Table 1). As shown <br />on the table, effective strength parameters for the East Flume Keyway material are internal <br />friction angle 33.4 degrees with cohesion of 14.4 psf. Since at this preliminary stage no further <br />data is currently available describing the material that will make up the compacted embankment <br />at the West Horse site, soil was estimated to have the same properties of the borrow material [hat <br />made up the dam at East Flume Pond Dam. The values of wet unit weight selected for the <br />compacted and existing conditions were selected to match the properties measured for the East <br />Flume pond materials and were 117 pcf and 102.5 pcf, respectively. <br />• <br />~~_ 8G DEC 5 2000 <br />Ifi6 South Union Blvd. <br />Suite 410 <br />Lakewood, Colorado <br />00228 <br />Te I :303 763 5140 <br />Fax: 303 763 8003 <br />Serving the World's En vrronmentel Needs <br /> <br />