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April2000 Srnera Coa! Company * Senrm /1 W Mine Srdlmertlation Pond 016 Srahl67y Analyrir ~ 1 • 1.0 INTRODUCTION 1.1 PURPOSE The purpose of [his stability analysis is to calculate the factor of safety (FOS) under static conditions for the as-built conditions of Sedimentation Pond Ol6 embankment in order to document design compliance with applicable regulatory standards under Rule 4.05.6(1 t)(j) or (k). 1.2 METHODOLOGY The static FOS for Sedimentation Pond 016 was calculated using a basic, two-dimensional equilibrium analysis program called SLOPF/W (Version 3). The calculated FOS is the ratio of those resisting forces that would prevent embankment movement or failure to the corresponding driving forces. Resisting forces include the internal strength pazame[ers for both the soil materials used in the construction of the embankment and the underlying base materials on which the facility will be conswcted. Potential driving forces include gravitational forces on the embankment and foundation materials and the gravitational and lateral forces associated with the water volume that could be impounded behind the embankment. For this analysis, a circular failure mode was assumed, and the FOS was calculated using the Spenser method. 1.3 PROCEDURE The as-built embankment cross-section shown on Exhibit 13-9A of the Seneca Il W Perini[ (Leidich, 2000) was used to evaluate embankment stability. A water surface elevation behind the embankment of 7,462.8 feet above mean sea level (amsq was used for the analysis. This water surface elevation is equivalent to the elevation of the invert to the principal spillway. • The embankment geometry analyzed is based on the as-built survey. The potential phreatic surface for the stability analysis is based on a water surface elevation of 7,462.8 ft-amsl corresponding to the invert elevation of the principal spillway. The phreatic surface through the embankment was estimated based on Darcy's law and observation of similar homogenous pond embankments at the mine site. A[ these locations seepage does no[ occur above the embankment toe. The resulting surface is shown on the SLOPE/W computer printouts contained in Appendix B. An undisturbed sample was collected from the Foundation area of the proposed Pond 16. The foundation soil sample was obtained from a Shelby tube sample taken from a test pit at the location of the proposed embankment. Triaxial (with pour pressure), grain size and Atterberg limits tests were run on the sample and the results can be found in Appendix A, and are summarized on Table 1. • During construction the embankment till material was sampled and subjected to sieve analysis and Atterberg limit testing. The strength properties for the embankment fill were obtained from tests on a sample from Pond 010. The Pond 10 embankment strength results were selected to represent the strength properties of the Pond l6 embankment Fill due to the similarity in index and sieve properties between the sample from the Pond 16 embankment fill and the Pond 10 embankment fill (see Table t). The Pond IO embankment shear strength test was performed on a remolded sample at 95% of the maximum dry density from the standard Proctor test (ASTM D698). The results can be found in Appendix A, and are summarized on Table 1. TABLE 1 . r .,`' .. - INDEX AND STRENGTH PROPERTIES OF SOILS FOR PONDS SAMPLE ATTERBERG LIMITS (LL - PI) PARTICLE SIZE DATA (°/ passing No. 200) EFFECTIVE FRICTION ANGLE, ~• EFFECTIVE COHESION, c' (psQ Pond IG Foundauon 49 - 20 96 29.9 0 Pond 1G Embankment Ft0 37 22 83 - - Pand 10 Embankment Fill 38 - 32 86 27.5 }38 hlomtgomrry It%ahon Ammrar, Ln. 165 Sau7h Union Boulruurd, Surer {IO * Lakeurood, CoGrmlo 30773 * (303J 7G3-Sl{0 ~:\IJ'OI//B Semm Cwl~"rriS\POIVD IG A~Am/1.rC4P/LI/Y.drc I1 /?8/P7 mG