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Ayri12000 Seneca Coa(Comf~arry * Seneca II W Mine Sedimentation Pond 076 Stability Analyri.r ~ 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 as-built conditions of Sedimentation Pond 016 embankment in order to document design compliance with applicable regulatory standards under Rule 4.05.6(11)(j) or (k). 1.2 METHODOLOGY The static FOS for Sedimentation Pond 016 was calculated using a basic, two-dimensional equilibrium analysis program called SLOPE/W (Version 3). The calculated FOS is [he ratio of those resisting forces that would prevent embankment movement or failure to the corzesponding driving forces. Resisting forces include the internal svength pazameters for both the soil materials used in the consvuction of the embankment and the underlying base materials on which the faci]ity 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 [hat 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 1I W Permit (Leidich, 2000) was used to evaluate embankment stability. A water surface elevation behind the embankment of 7,462.8 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 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 phrea[ic surface through the embankment was estimated based on Darcy's law and observation of similar homogenous pond embankments a[ [he 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 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 I. • During consvuction the embankment fill 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 svength results were selected to represent the svength properties of the Pond 16 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 1). The Pond 10 embankment shear svength test was performed on a remolded sample at 95%n of the maximum dry density from the standard Proctor [es[ (ASTM D698). The results can be found in Appendix A, and are summarized on Table 1. ±..„~~,~ :.u._.. ,~s~Ih1D E-7Cx ,$'1'1tEN ~ i;`I- ,+PIt'~PERTIE ,t,O F 5 _- Ils` ~1FOR~P6'... S~ v, ,,,,,v. _{.. <. y SAMPLE ATTERBERG LIMITS (LL - PI) PARTICLE SIZE DATA (°/ passing No. 200) EFFECTIVE FRICTION ANGLE, ~• EFFECTIVE COHESION, c' (ps1) Pond 16 Foundation 49 - 20 96 29.9 0 Pond lG Embankment Fill 37-22 83 Pond ]0 Embankment Fill 38 - 22 86 27.5 338 Montgomery Wauon Amenra{ Inc 165 Smdh Unian Boulevard, Suite 410 * Lakewood, Calosada 80228 * (303J 763-5740 /:\2J70/08 Senrm Cca(~Tr-35\POND 1G Ailmi!lSTABILt7Y.da' 71/18/00 mb