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GeotechNcL I Emlronmennl EnalneeNna • Materlaa Te•Nna <br />July 9, 1997 <br />Senora Coal Cotttpany <br />P.O. Drawer D <br />Hayden, CO 81639 <br />Atm: Mr. Michael A1taviUa, P.E. <br />Job Number: 97-3000 <br />Subject: Additional Stability Analysis & <br />Embankment Constuction Recommendations, Yoast <br />Mine Sedimentation Pond 011, Sec, l6, TSN, <br />R87W, Routt County, Colorado. <br />Gentlertten: <br />As requested, Northwest Colorado Consultants, Inc. has prepared this letter to present additional stability <br />analysis and embankment construction recommendations for the proposed Yoast Mine Sedimentation <br />Pond 011 to be constructed within the SE ]!4 of Section 16, TSN, R87W in Routt County, Colorado. <br />T1tis fetter is a supplement to the geotechnical investigation ead stability analysis previously prepared for <br />Pond Oll under this job Dumber and dated May 30, 1997. We understand that Colorado Division of <br />Minerals and Geology has received our report and a review of the stability analysis revealed a potential <br />failure surface with a factor of safety less than the required L.30. <br />Based on our review of the data, it appears that a deficient factor of safety may result if excessive pore <br />water pressure is allowed to build beneath the downstream side of the embankment as shown in our <br />original analysis. Therefore, it appears that a drainage blanket will be required beneath the embankment <br />fills from the cutoff trench downstream to the toe of the embankment to intercept the potential seepage. <br />The attached stability analyses indicate that a theoretical factor of safety in the range of 1.35 to 1.39 will <br />result with the instailatioa of an effective drainage bladcet beneath the embankment from the cutoff <br />trench to the toe of the embankment. The drainage blanket should be constructed from the proposed <br />cumff trench elevation and maintain a consistent positive slope to the tce of the embattkmwt. <br />Our recommendations for the toe drain/drainage blanket ate included in the original report and we <br />believe these details should be sufficient to provide the required subsurface drainage and adequate factor <br />of safety against failure. We also believe that construction of the cumff trench will require the additional <br />efforts described in the report to stabilize the base of the excavation. These efforts, as well as proper <br />. subgrade preparation beneath the embankment should also contribute to higher factors of safety for the <br />embankment. <br />(970) 879.7888 • Fez (970) 879.7891 <br />29585 U.S. 40 West, Box 775226 • Steamboat Springs, CO 80477 <br />