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Deromber 2001 Sareea Coa! Campauy * Seneeu ]I W Mrne Sedlmentakon Pond 0017 Design S/,~brGty Analydt Dt 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 017 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 017 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 [hat would prevent embankment movement or failure to the corresponding driving forces. Resisting forces include the internal svength paaameters for both the soil materials used in the conswction 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-SB, Attachment 13-SB of [he Seneca II W Permit (2001). A water surface elevation behind the embankment of 7,391.9 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[ perpendicular to [he dam at the tallest point relative to the downsveam topography. The embankment slopes used in the design are 3H:IV and the crest length is 15 feet. The potential phreatic surface for the stability analysis is based on a water surface elevation of 7,39].9 ft-amsl, three feet below the crest of the embankment. 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 not occur above the embankment toe. The resulting surface is shown on the SLOPE/W computer printouts contained in Appendix B. The closest site to Pond A where soil samples have been revieved is Pond 016. Pond 016 soils were used to approximate conditions at Pond 017 since pond 016 is relatively close and is likely founded on similar materials to those which will be found at Pond 017A. An undisturbed sample was collected from the foundation area of Pond 016. The foundation soil sample was obtained from a Shelby tube sample. 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 summarized on Table 1. The results of these tests were used to approximate the properties of the Pond 017 foundation conditions due to the relatively close proximity of the two embankments. During construction of Pond 016 embankment, fill material was sampled and subjected to sieve analysis and Atterberg limit testing. The svength 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 svength properties of the Pond Ol7 embankment fill due to the similarity in index and sieve properties between the sample from the Pond O16 embankment fill located near Pond 017 and the Pond Ol0.embankment fill (see Table 1). 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 017 structure and are shown in [he attached program printouts with associated results. Momtgomrp~ WaUon NarZa, 0765 Soath Unian Bonlevarel, Suilr 4010 +Zlrkewaod, Co/arada 80218 * (303J 763-5740 C\7F.h1P\PON17 17 Drngn STABIIII'Y.dor 05/02/01 eyin