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~ <br />u <br /> <br /> <br />U <br />MONTGOMERY WATSON <br />October 18, 2000 <br />Karl Koehler <br />Trapper Mining, Inc. <br />PO Box 187 <br />Craig, CO 81626 <br />RECEIVED <br />DEC 18 2000 <br />Division of Minerals and Geology <br />1tE: Horse Embankment Dam - As-built Slope Stability Analysis <br />Dear Karl, <br />This letter report details the results of the slope stability analysis performed on the Horse <br />Embankment Dam. Included in this report is a sununary of the properties of the soil found in the <br />mine area at and around the location of the Horse Embankment Dam, and the stability analysis <br />performed on the as-built Horse Embankment cross section. <br />1. Soil Characterization <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 />the Horse sediment pond and from the Horse embankment dam. During the on site geotechnical <br />investigation disturbed and Shelby tube samples were also retrieved from additional sediment <br />pond locations (West Horse, East Flume, and Deal). During sampling at Horse cohesive brown <br />clay with some sand was encountered throughout the bottom of the pond and throughout the <br />sampling points on the embankment. Samples were recovered from both in the pond footprint and <br />from the embankment. This material and the material recovered from the soil-sampling program <br />at the other sediment pond sites was then subjected to laboratory testing. A summary of the <br />results is located at the beginning of Attachment 2 (Soil Testing). The USCS classification of the <br />material that will constitute the foundation material for the dam is CL (low plasticity clay with <br />some sand). The material that the compacted embankment itself is constructed of was also CL <br />(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 are <br />included in Attachment 2. Since the properties of the sub-grade material at the Horse site is <br />similar to the CL material that makes up the subgrade at the East Flume site, the soil strength and <br />density properties from the East Flume Keyway were used (see Table 1). As shown on the table, <br />effective strength parameters for the East Flume Keyway material are internal friction angle 33.4° <br />with cohesion of 14.4 psf. The values of wet unit weight selected for the existing conditions was <br />selected to match the properties measured for the East Flume pond materials and is 102.5 pcf. A <br />published relationship between plasticity index (IP or PI) and friction angel was used to confirm <br />the values used for the foundation material at Horse and to determine what values to use for the <br />strength properties of the remolded clay in the embankment. The strength of remolded clay is <br />lower than undisturbed clay (as indicated on attached figure 2-35 from Bowles (1996)), therefore; <br />a reduced friction angel must be utilized when modeling the stability of the recompacted <br />embankment material. The friction angel determined from the attached figure (Bowles, 1996) is <br />approximately 25°. This value was utilized to model the stability of the embankment dam. <br />165 Saurh Union Blvd <br />Suite 410 <br />Lakewood, Colorado <br />aozze <br />Te I :303 763 5140 <br />Fax' 303 763 6003 <br />M~-/~£S DEC 2 1 2000 <br />Seirmo the Worltl's Environmental Nees <br />~ <br />