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CIVIL RES~URCES,LLC <br />' ENGINEERS & PLANNERS <br />May 9, 2005 <br />Colorado Department of Transportation <br />Mr. Joe Intermill <br />43379 Highway 51 <br />Ault, CO 80610 <br /> <br />MAC' i 0 2GC5 <br />~ivisioa of fu"iaerais & Gaoia(3y <br />RE: SW TKO Joint Venture, LLC, Division of Minerals and Geology Application M-200407, <br />Lupton Lakes Resource; CDOT Objection Letter- Stope Stability <br />Dear Mr. Intermill: <br />This letter is being written to present new and previously presented information regarding your concern with <br />respect to slope stability for project slopes that parallel Highway 85. Following is the excerpt from your letter <br />regarding your concerns related to slope stability: <br />CDOT Comment: CDOT would like to have additional setback from the right-of-way such that if the mining <br />slope failed to the angle of repose, CDOT's right-of-way would not be affected. Groundwater seepage <br />affects should be evaluated and a final factor of safety of 1.35 is desired. (Paraphrased) <br />Aaalicant Response: The angle of repose for the natural soil formation is approximately 36 degrees based <br />on the intemal friction strength of the material. This is equal to a 1.4 horizontal to 1 vertical slope. <br />Therefore, approximately 30 feet would remain between the top of the failed slope to the CDOTrght-of-way. <br />A wetted excavation face height of five feet was conservatively assumed for the XSTABL analysis that <br />indicates a factor of safety of 1.57 at the CDOT right-of-way line. Refer to the attached analysis in ,4ppendix <br />A and the proposed mining drawing shown on Figure 1. <br />The alluvium at the Site is awell-graded, free-draining deposit. A pump test was performed in September, <br />2004 for the Groundwater Model Study and a hydraulic conductivity value range of 165 - 225 fUday was <br />determined. In addition, the alluvium was analyzed for sieve size to evaluate the free draining characteristics <br />of the material. The alluvium has an average minus Sieve #200 of less than three percent, as shown on <br />Table 1. The alluvium will produce a relatively flat cone of depression during the mining phase (time when <br />highwall is not reclaimed). As such, the phreatic surface was conservatively modeled to be discharging five <br />feet above the toe of the highwall with a hydraulic gradient of 0.03 feet per foot. <br />Shear strength for the alluvium was investigated on May 7, 2005 by advancing SPT tests through the course <br />of the deposit. The average blow count (N) for the two holes was 31 {corresponding to an intemal friction <br />angle of 36.2 degrees). For purposes of modeling, the N value was conservatively estimated to be 26, which <br />corresponds to the previously modeled intemal friction angle of 35 degrees per the attached chart in <br />Appendix B (Terzhagi & Peck,1948). The maximum cross section was modeled as a 50-foot thick deposit <br />near the intersection of Highway 85 and Weld County Road 8. The top of the highwall begins 50 feet from <br />the CDOT ROW (property line) with a 1:1 (H:V) slope. The resulting factor of safety was evaluated to be <br />1.57 to the CDOT ROW. <br />451 ^AK STREET, SUITE 209 ~ P.O. BaX 6B0 ~ FREDERICK, CO 80530 ~ (3O3I 833-1 416 ~ (3031 B33-ZB SO - Fnx <br />