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March 2004 Seuera Coa! Company * Seneca TI W Mine Sedimentation Pand 016A Ar-built Stability Analyrir ~ 1 • 1.0 INTRODUCTION l.] PURPOSE The purpose of this stability analysis is to calculate the factor of safety (FOS) under static conditions for [he as-built 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). L2 METHODOLOGY The static FOS for Sedimentation Pond 016A was calculated using a basic, two-dimensional equilibrium analysis program called SLOPE/W (Version 5.1). The calculated FOS is [he 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 [he underlying base materials on which [he facility was constructed. Potential driving forces include gravitational forces on the embankment and foundation materials and the gravitational and laG:ral 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. l.3 PROCEDURE The design embankment cross-section used to evaluate embankment stability was determined from the digitized topography shown on Drawing 13-9C, Pond 16A As-built Drawing for [he Seneca II W Permit. A water surface elevation behind [he embankment of 7,525.5 fee[ above mean sea level (amsl) was used for the analysis. This water surface elevation is equivalent to the surveyed elevation of the invert to the principal spillway. • The embankment geometry analyzed is based on the as-built topography cut perpendicular the downstream topography. The potential phreatic surface for [he stability analysis is based on a water surface elevation of 7,525.5 ft-amsl. The phreatic surface through the embankment was estimated based on Dazcy's law and ob:;ervation of similar homogenous pond embankments at the mine site. A[ these locations seepage does not occur above [he embankment toe. The resulting surface is shown on [he 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 [he sample and the results can be found in Appendix A, and are summazized on Table 1. The results of these tests were used [o approximate the properties of the Pond 016A foundation conditions due to the close proximity of [he two embankments. During construction of Pond 016A, embankment fill material was sampled and subjected to sieve analysis and Atterberg limit testing. The strength properties for [he embankment fill were obtained from tests on a sample from Pond 010. The Pond 010 embankment strength results were selected [o represent the strength properties of the Pond 016A embankment fill. This was judged appropriate due to the similazity in index and sieve properties between [he sample from the Pond 016A embankment fill and the pond 10 material (see Table I ). The Pond 010 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. 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 [he attached program printouts with associated results. MWH, 7807 Cal fornia .Street Suite 2900 * Denver, Colorado 80202 * (303) ?97-2222 f:\7f70108 ,fcnem CoalSPvndr ArbL\Wara\POND 16A ASbuiltSTABIIJTY.dor 03/07/04 djm