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Ap>r! 2002 Senem Caal Company * Seneca II W Mine Se&mcntatiox Aond 016A Design StabiGtyAnalyrir ~ 1 1.0 INTRODUCTION 1.1 PURPOSE The purpose of this stability analysis is to calculate the factor of safety (FOS) under static conditions for the design conditions of Sedimentation Pond 016A embankment in order to document design compliance with applicable regulatory standards under Rules 4.05.9 (6) and 4.05.9 (8) (b). 1.2 METHODOLOGY The static FOS for Sedimentation Pond 016A was calculated using a basic, two-dimensional equilibrium analysis program called SLOPE/W (Version 4.24). 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 parameters for both the soil materials used in the construction of the embankment and the underlying base materials on which the facility will be constructed. 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 Bishop's method. 1.3 PROCEDURE The design embankment cross-section used to evaluate embankment stability was determined from the digitized topography shown on Drawing 13-8C, Attachment 13-SB of the Seneca II W Permit (2001). A water surface elevation behind the embankment of 7,525.0 feet above mean sea level (amsl) 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 design topography cu[ perpendiculaz to the dam a[ the tallest point relative to [he downstream topography. The embankment slopes used in [he design are 3H:1 V and the crest length is IS feet. The potential phreatic surface for the stability analysis is based on a water surface elevation of 7,525.0 ft-amsl. The phreatic surface through the embankment was estimated based on Darcy's law and observation of similar homogenous pond embankments at the mine site. At these locations seepage does not occur above the embankment tce. 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 Pond 016. The foundation soil sample was obtained from a Shelby tube sample taken from a test pit at the location of the embankment. Triaxial (with pore pressure), grain size and Atterberg limits tests were run on the sample and the results can be found in Appendix A, and are summazized on Table 1. The results of these tests were used to approximate the properties of the Pond 016A foundation conditions due to the close proximity of the two embankments. During consruction of Pond 016, embankment t-ill 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 010 embankment strength results were selected to represent the strength properties of the Pond 016A embankment fill due to the similarity in index and sieve properties between the sample from the Pond 016 embankment fill located near Pond 016A and the Pond 010 embankment fill (see Table 1). The Pond 010 embankment shear srength test was performed on a remolded sample at 95%a of the maximum dry density from the standazd Proctor test (ASTM D698). The results can be found in Appendix A, and are summarized on Table I. The parameters described in Table 1 were input into the SLOPE/W program to evaluate static stability of the Pond 016A structure and are shown in the attached program printouts with associated results. Montgomery Watron Hama, 0165 Soutb Union Boulevard, Suite 910 * lnkevrood, Colorado 80278 * (303) 763-5140 J:\2470108SamroCoal~PR-03\PoudiGAEV\Wmd~POND 16A DerrgnSTAB1I.ITYnmpanddor W/77/02 din