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Denmber 2001 Sencca Coa! Campaery * Seneca 77 W Minc Sedimen[atiox Pond 0017 DerlBn Surbibty Anafynr ~ 1 <br />1.0 INTRODUCTION <br />1.1 PURPOSE <br />The purpose of this stability analysis is to calculate the factor of safety (FOS) under static conditions for the <br />design conditions of Sedimentation Pond 017 embankment in order to document design compliance with <br />applicable regulatory standazds under Rule 4.05.6(11)(j) or (k). <br />1.2 METHODOLOGY <br />The static FOS for Sedimentation Pond 017 was calculated using a basic, two-dimensional equilibrium <br />analysis program called SLOPE/W (Version 4.24). The calculated FOS is the ratio of those resisting forces <br />that would prevent embankment movement or failure to the corresponding driving forces. Resisting forces <br />include the internal strength parameters for both the soil materials used in the constmction of the <br />embankment and the underlying base materials on which the facility will be constructed. Potential driving <br />forces include gravitational forces on [he embankment and foundation materials and the gravitational and <br />lateral forces associated with the water volume that could be impounded behind the embartlortent. For this <br />analysis, a circular failure mode was assumed, and the FOS was calculated using the Bishop's method. <br />1.3 PROCEDURE <br />The design embankment cross-section used to evaluate embankment stability was determined from the <br />digitized topography shown on Drawing 13-8B, Attachment 13-SB of the Seneca II W Permit (2001). A <br />water surface elevation behind the embankment of 7,391.9 feet above mean sea level (amsl) was used for <br />the analysis. This water surface elevation is equivalent to the elevation of the invert to the principal <br />spillway. <br />The embankment geometry analyzed is based on the design topography cut perpendicular to the dam at the <br />tallest point relative to the downstream topogaphy. The embankment slopes used in the design aze 3H:1 V <br />and the crest length is 15 feet. The potential phreatic surface for the stability analysis is based on a water <br />surface elevation of 7,391.9 ft-amsl, three fee[ below [he crest of [he embankment. The phreatic surface <br />through the embankment was estimated based on Darcy's law and observation of similaz homogenous pond <br />embankments at the mine site. At these locations seepage does not occur above the embankment toe. The <br />resulting surface is shown on the SLOPE/W computer printouts contained in Appendix B. <br />The closest site to Pond A where soil samples have been retrieved is Pond 016. Pond 016 soils were used [o <br />approximate conditions at Pond 017 since pond 016 is relatively close and is likely founded on similaz <br />materials to [hose which will be found at Pond 017A. An undisturbed sample was collected from the <br />foundation area of Pond O16. The foundation soil sample was obtained from a Shelby tube sample. <br />Triaxial (with pore pressure), grain size and A[[erberg limits tests were run on [he sample and [he results <br />can be found in Appendix A, and are summarized on Table 1. The results of these tests were used to <br />approximate the properties of the Pond 017 foundation conditions due to the relatively close proximity of <br />the two embankments. <br />During construction of Pond 016 embankment, till material was sampled and subjected to sieve analysis <br />and Atterberg limit testing. The strength properties for the embankment fill were obtained from tests on a <br />sample from Pond 010. The Pond 010 embankment strength results were selected to represent the strength <br />properties of the Pond 017 embankment fill due to the similarity in index and sieve properties between the <br />sample from the Pond 016 embankment fill located near Pond 017 and the Pond 010 embankment fill (see <br />Table 1). The Pond 010 embankment shear strength test was performed on a remolded sample at 95% of <br />the maximum dry density from the standard Proctor test (ASTM D698). The results can be found in <br />Appendix A, and aze summarized on Table 1. <br />The parameters described in Table 1 were input into the SLOPE/W program to evaluate static stability of <br />the Pond 017 structure and are shown in the attached program printouts with associated results. <br />MomlBamery Wadon Hama, 0165 Soutb Umox Boulevard, Suiu 4010 * Lakewood, Colorado 80228 * (303) 763-5140 <br />J:\2470108 Senem Cmt\PR-03\Pond 17Rev\POND 17 Dengn STABII]7Y.drc <br />12/12/01 djm <br />